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Staphylococcal Infections

Chapter 152 | Part 5: Infectious Diseases · Part 5 – Infectious Diseases: Bacterial

Detailed clinical reference synthesised from Harrison's Principles of Internal Medicine, 22nd Edition

🔑 Key Clinical Points

    • S. aureus is a pyogenic pathogen known for its capacity to induce abscess formation at both local and distant sites (i.e., metastatic infections).
    • Approximately 20–40% of healthy persons are colonized with S. aureus, with a smaller percentage (~10%) persistently colonized with the same strain.
    • CA-MRSA strains have been responsible for an increasing number of nosocomial infections and have enhanced capacity to cause disease in immunocompetent individuals.
    • MRI is the most reliable imaging modality to help establish the diagnosis of osteomyelitis; findings may be normal for up to 14 days after the onset of symptoms.
    • S. aureus bacteremia may be complicated by sepsis, endocarditis, vasculitis, or metastatic seeding; the frequency of metastatic seeding during bacteremia has been estimated to be as high as 31%.
    • TSST-1 and enterotoxins bind directly to the invariant region of MHC—outside the MHC class II groove, resulting in a dramatic overexpansion of T-cell clones (up to 20% of the total T-cell population).
    • S. aureus is the most common cause of surgical wound infections and is second only to NSaS as a cause of primary bacteremia.
    • In prosthetic-valve endocarditis, medical therapy alone is often not sufficient and urgent valve replacement is necessary.
    • S. aureus is rarely a blood culture contaminant; uniformly positive cultures of blood collected over time suggest an endovascular infection such as endocarditis.
    • The regulatory gene agr is part of a quorum-sensing signal transduction pathway that senses and responds to bacterial density.

📑 Table of Contents

📋 Figures in This Chapter

# Type Description
1 🖼 Figure Global distribution of community-associated MRSA
2 🖼 Figure S
3 🖼 Figure Staphylococcal scalded skin syndrome in a 6-year-old boy
4 🖼 Figure Gram’s stain of S
5 🖼 Figure Computed tomography scan illustrating septic pulmonary emboli in a patient with methicillin-resistant...

1. DEFINITION & OVERVIEW

📖 Harrison's defines this as:

S. aureus is a pyogenic pathogen known for its capacity to induce abscess formation at both local and distant sites (i.e., metastatic infections).

  • Staphylococci, gram-positive cocci in the family Micrococcaceae, form grapelike clusters on Gram's stain.
  • These organisms (~1 μm in diameter) are catalase-positive (unlike streptococcal species), nonmotile, aerobic, and facultatively anaerobic.
  • They are capable of prolonged survival on environmental surfaces under varying conditions.
  • S. aureus is generally distinguished from other staphylococcal species by coagulase production, a surface enzyme that converts fibrinogen to fibrin.
  • The 'other' staphylococci, coagulase-negative staphylococci, are less virulent than S. aureus but remain important pathogens in select settings, such as infections involving prosthetic devices.
  • S. aureus ferments mannitol, is positive for protein A, and produces DNAse.
  • On blood agar plates, S. aureus forms golden β-hemolytic colonies; in contrast, most NSaS form small, white nonhemolytic colonies.
  • Latex kits that detect both protein A and clumping factor can distinguish S. aureus from most other staphylococcal species.

1.1 Microbiology and Taxonomy

  • Staphylococci are catalase-positive, nonmotile, aerobic, and facultatively anaerobic.
  • S. aureus is distinguished from other staphylococcal species by coagulase production.
  • S. aureus ferments mannitol, is positive for protein A, and produces DNAse.
  • Newer methods such as matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF) are increasingly being used for staphylococcal speciation.
  • Determining whether multiple staphylococcal isolates from different patients are the same or different is often relevant when there is concern that a nosocomial outbreak of staphylococcal infections is due to a common point source.
  • Molecular typing methods, such as pulsed-field gel electrophoresis and sequence-based techniques (e.g., staphylococcal protein A [SpA] typing), have been used for this purpose.
  • Whole-genome sequencing has emerged as the gold standard for discrimination among different isolates.

2. EPIDEMIOLOGY

  • S. aureus is both a commensal and an opportunistic pathogen.
  • Approximately 20–40% of healthy persons are colonized with S. aureus, with a smaller percentage (~10%) persistently colonized with the same strain.
  • The rate of colonization is elevated among type 1 diabetics, HIV-infected patients, patients undergoing hemodialysis, injection drug users, and individuals with damaged skin.
  • The anterior nares and oropharynx are frequent sites of human colonization, although the skin (especially when damaged), axilla, vagina, and perineum also are often colonized.
  • Most individuals who develop S. aureus infections become infected with a strain that is already a part of their own commensal flora.
  • Breaches of the skin or mucosal membrane allow S. aureus to initiate infection.
  • Person-to-person transmission of S. aureus also occurs, most frequently from direct personal contact with an infected body site.
  • In the past three decades, there has been a dramatic change in the epidemiology of infections due to methicillin-resistant S. aureus (MRSA).
  • In addition to its major role as a nosocomial pathogen, MRSA has become an established community-based pathogen.
  • Numerous outbreaks of community-associated MRSA (CA-MRSA) infections have been reported in both rural and urban settings in widely separated regions throughout the world.
  • Risk factors common to these outbreaks include poor hygienic conditions, close contact, contaminated material, and damaged skin.
  • In some geographic regions of the world, the infections have been caused by a single CA-MRSA strain, while in others, a variety of CA-MRSA strains have been responsible.
  • In the United States, strain sequence type 8 (PFGE type USA300) has been the predominant clone.
  • Although most infections caused by these strains have involved the skin and soft tissue, 5–10% have been invasive and potentially life-threatening.
  • Of concern has been the enhanced capacity of CA-MRSA to cause disease in immunocompetent individuals.
  • S. aureus is a leading cause of health care–associated infections.
  • It is the most common cause of surgical wound infections and is second only to NSaS as a cause of primary bacteremia.
  • These isolates are often resistant to multiple antibiotics; thus, available therapeutic options may be limited.
  • In the community, S. aureus remains an important cause of SSTIs, respiratory infections, and, especially among injection drug users, infective endocarditis.
  • The increasing use of home infusion therapy also poses a risk of community-acquired staphylococcal infections.

2.1 Risk Factors and Colonization Sites

  • Diabetes combines an increased rate of S. aureus colonization and the use of injectable insulin with the possibility of impaired leukocyte function.
  • Individuals with congenital or acquired qualitative or quantitative defects of polymorphonuclear leukocytes (PMNs) are at increased risk of S. aureus infections.
  • This group includes neutropenic patients (e.g., those receiving chemotherapeutic agents), those with cystic fibrosis or osteomyelitis.
  • Other groups at risk include individuals with end-stage renal disease, HIV infection, skin abnormalities, or prosthetic devices.
  • The anterior nares and oropharynx are primary sites of staphylococcal colonization.
  • In the nares, colonization appears to involve the attachment of S. aureus to keratinized epithelial cells.
  • Other factors that contribute to colonization include the influence of other resident nasal flora and their bacterial density, host factors, and nasal mucosal damage (e.g., that resulting from inhalational drug use).
  • Other colonized body sites, such as damaged skin, the groin, and the oropharynx, may be particularly important reservoirs for CA-MRSA strains.

3. ETIOLOGY & PATHOPHYSIOLOGY

  • S. aureus Genome: The complete genomes of S. aureus strains are now readily available.
  • Among the interesting revelations are (1) the high degree of nucleotide sequence similarity of the core genomes of different strains; (2) the acquisition of a relatively large amount of genetic information by horizontal transfer from other bacterial species; and (3) the presence of unique 'pathogenicity' or 'genomic' islands—mobile genetic elements that contain clusters of enterotoxin and exotoxin genes and/or antimicrobial resistance determinants.
  • Among the genes in these islands is mecA, the gene responsible for methicillin resistance.
  • Methicillin resistance–containing islands have been designated staphylococcal cassette chromosome mec (SCCmec).
  • There are different SCCmec types that range in size from ~20 to 60 kb.
  • Among the more common SCCmec types, types 1–3 are traditionally associated with nosocomial MRSA isolates, whereas types 4–6 have been associated with epidemic CA-MRSA strains.
  • A relatively limited number of MRSA clones have been responsible for most community- and hospital-associated infections worldwide.
  • Regulation of Virulence Gene Expression: In both toxin-mediated and non-toxin-mediated diseases due to S. aureus, the expression of virulence determinants associated with infection depends on a series of regulatory genes (e.g., accessory gene regulator [agr] and staphylococcal accessory regulator [sar]) that coordinately control the expression of many virulence genes.
  • The regulatory gene agr is part of a quorum-sensing signal transduction pathway that senses and responds to bacterial density.
  • Staphylococcal surface proteins are synthesized during the bacterial exponential growth phase in vitro.
  • In contrast, many secreted proteins, such as α toxin, the enterotoxins, and assorted enzymes, are released during the post–exponential growth phase in response to transcription of the effector molecule of agr, RNAIII.
  • These regulatory genes appear to serve a similar function in vivo.
  • Successful invasion requires the sequential expression of these different bacterial elements.
  • Bacterial adhesins are needed to initiate colonization of host tissue surfaces.
  • The subsequent release of various enzymes enables the colony to obtain nutritional support and permits bacteria to spread to adjacent tissues.
  • Studies with strains in which these regulatory genes are inactivated show reduced virulence in several animal models of S. aureus infection.
  • Pathogenesis of Invasive S. aureus Infection: Staphylococci are opportunists.
  • For these organisms to invade the host and cause infection, some or all of the following steps are necessary: contamination and colonization of host tissue surfaces, breach of cutaneous or mucosal barriers, establishment of a localized infection, invasion, evasion of the host response, and metastatic spread.
  • Colonizing strains or strains transferred from other exposures are introduced into damaged skin, a wound, or the bloodstream.
  • Recurrences of S. aureus infections may be common, apparently because of the capacity of these pathogens to persist in a quiescent state in various tissues, and then to cause recrudescent infections when suitable conditions arise.
  • Staphylococcal cell wall—consisting of alternating N-acetyl muramic acid and N-acetyl glucosamine units in combination with an additional cell wall component, lipoteichoic acid—can initiate an inflammatory response that includes the sepsis syndrome.
  • Staphylococcal alpha (α) toxin is a critical staphylococcal toxin.
  • It causes pore formation in various eukaryotic cells and can also initiate an inflammatory response with findings suggestive of sepsis.
  • The S. aureus toxin Panton-Valentine leukocidin is cytolytic to PMNs, macrophages, and monocytes.
  • Strains elaborating this toxin have been epidemiologically linked with cutaneous and more serious infections (i.e., pneumonia) caused by strains of CA-MRSA.
  • Evasion of Host Defense Mechanisms: Staphylococci have many host immune evasion strategies that are crucial to their survival.
  • They possess an antiphagocytic polysaccharide microcapsule.
  • Most human S. aureus infections are due to strains with capsular types 5 and 8.
  • The zwitterionic (both negatively and positively charged) S. aureus capsule also plays a critical role in the induction of abscess formation.
  • Protein A, an MSCRAMM unique to S. aureus, acts as an Fc receptor, binding the Fc portion of IgG subclasses 1, 2, and 4 and preventing opsonophagocytosis by PMNs.
  • Both chemotaxis inhibitory protein of staphylococci (CHIPS, a secreted protein) and extracellular adherence protein (EAP, a surface protein) interfere with PMN migration to sites of infection.
  • There are several cytolytic toxins, including α toxin and Panton-Valentine toxin, that are secreted by staphylococci that cause lysis of different host cells and contribute to host tissue damage.
  • An additional potential mechanism of S. aureus evasion is its capacity for intracellular survival.
  • Both professional and nonprofessional phagocytes internalize staphylococci.
  • Internalization by these cells may provide a sanctuary that protects bacteria against the host's defenses.
  • This phenomenon appears to be especially relevant for hepatic Kupffer cells during staphylococcal bacteremias.
  • The intracellular environment favors the phenotypic expression of S. aureus small-colony variants, which are found in patients receiving antimicrobial therapy (e.g., with aminoglycosides) and in those with cystic fibrosis or osteomyelitis.
  • These variants, whether intra- or extracellular, may facilitate prolonged staphylococcal survival in different tissue sites and enhance the likelihood of recurrences.
  • Finally, S. aureus can survive within PMNs and may use these cells to spread and seed other tissue sites.
  • Pathogenesis of Community-Acquired MRSA Infections: A number of specific virulence determinants contribute to the pathogenesis of CA-MRSA infections.
  • A strong epidemiologic association links the presence of the gene for the Panton-Valentine leukocidin with SSTIs and with necrotizing postinfluenza pneumonia.
  • Other determinants that play a role in the pathogenesis of these infections and contribute to the unique virulence of these clones include the arginine catabolic mobile element (ACME), a cluster of unique genes that may facilitate evasion of host defense mechanisms; phenol-soluble modulins, a family of cytolytic peptides; and α toxin.

3.1 Pathogenesis of Toxin-Mediated Disease

  • S. aureus produces three types of toxins: cytotoxins, pyrogenic toxin superantigens, and exfoliative toxins.
  • Both epidemiologic data and studies in animals suggest that antitoxin antibodies are protective against illness in TSS, staphylococcal food poisoning, and staphylococcal scalded-skin syndrome.
  • TSST-1 and enterotoxins bind directly to the invariant region of MHC—outside the MHC class II groove.
  • TSST-1 and the enterotoxins can then bind T-cell receptors via the vβ chain; this binding results in a dramatic overexpansion of T-cell clones (up to 20% of the total T-cell population).
  • The consequence of this T-cell expansion is a cytokine storm, with the release of inflammatory mediators that include interferon γ, IL-1, IL-6, TNF-α, and TNF-β.
  • The resulting multisystem disease produces a constellation of findings that mimic those found in endotoxin shock; however, the pathogenic mechanisms differ.
  • A different region of the enterotoxin molecule is responsible for the symptoms of food poisoning.
  • The enterotoxins are heat stable and can survive conditions that kill the bacteria.
  • Illness results from the ingestion of preformed toxin; as a result, the incubation period is short (1–6 h).
  • The toxin stimulates the vagus nerve and the vomiting center of the brain.
  • It also appears to stimulate intestinal peristaltic activity.
  • The exfoliative toxins are responsible for SSSS, most commonly seen in newborns.
  • The toxins that produce disease in humans are of two serotypes: ETA and ETB.
  • These toxins are serine proteases that cleave desmosomal cadherins in the superficial layer of the skin, triggering exfoliation.
  • The result is a split in the epidermis at the granular level, which is responsible for the superficial desquamation of the skin that typifies this illness.

3.2 Host Response to S. aureus Infection

  • The primary host response to S. aureus infection is the recruitment of PMNs.
  • These cells are attracted to infection sites by bacterial components such as formylated peptides or peptidoglycan as well as by the cytokines tumor necrosis factor (TNF) and interleukins (ILs) 1 and 6, which are released by activated macrophages and endothelial cells.
  • Although most individuals have antibodies to staphylococci, it is not clear that antibody levels are qualitatively or quantitatively sufficient to protect against infection.
  • Anticapsular and anti-MSCRAMM antibodies facilitate opsonization in vitro and have been protective against infection in several animal models; however, vaccines with these components have not yet successfully prevented staphylococcal infections in clinical trials.

4. CLINICAL FEATURES

  • Constitutional findings may result from either localized or systemic infections.
  • Illness develops after toxin synthesis and absorption and the subsequent toxin-initiated host response.
  • Skin and Soft Tissue Infections: S. aureus causes a variety of cutaneous infections.
  • Common factors predisposing to S. aureus cutaneous infection include chronic skin conditions (e.g., eczema), skin damage (e.g., insect bites, minor trauma), injections (e.g., in diabetes, injection drug use), and poor personal hygiene.
  • These infections are characterized by the formation of pus-containing blisters, which often begin in hair follicles and spread to adjoining tissues.
  • Folliculitis is a superficial infection that involves the hair follicle, with a central area of purulence (pus) surrounded by induration and erythema.
  • Furuncles (boils) are more extensive, painful lesions that tend to occur in hairy, moist regions of the body and extend from the hair follicle to become a true abscess with an area of central purulence.
  • Carbuncles are most often located in the lower neck and are even more severe and painful, resulting from the coalescence of other lesions that extend to a deeper layer of the subcutaneous tissue.
  • In general, furuncles and carbuncles are readily apparent, with pus often expressible or discharging from the abscess.
  • Other cutaneous S. aureus infections include impetigo.
  • Mastitis develops in 1–3% of nursing mothers.
  • This infection of the breast, which generally presents within 2–3 weeks after delivery, is characterized by findings that range from cellulitis to abscess formation.
  • Systemic signs, such as fever and chills, are often present in more severe cases.
  • Musculoskeletal Infections: S. aureus is a common cause of bone infections—both those resulting from hematogenous dissemination and those arising from contiguous spread from a soft tissue site.
  • Hematogenous osteomyelitis in children most often involves the long bones.
  • Infections present with fever and bone pain or with a child's reluctance to bear weight.
  • The white blood cell count and erythrocyte sedimentation rate are often elevated.
  • Blood cultures are positive in ~50% of cases.
  • When necessary, bone biopsies for culture and histopathologic examination are usually diagnostic.
  • In adults, hematogenous osteomyelitis involving the long bones is less common.
  • However, vertebral osteomyelitis is among the more common clinical presentations.
  • Vertebral bone infections are most often seen in patients with endocarditis, those undergoing hemodialysis, diabetics, and injection drug users.
  • These infections may present with intense back pain and fever but may also be clinically occult, presenting as chronic back pain with low-grade fever.
  • S. aureus is the most common cause of epidural abscess, a complication that can result in neurologic compromise.
  • Patients report difficulty voiding or walking and radicular pain in addition to the symptoms associated with their osteomyelitis.
  • Surgical intervention in this setting often constitutes a medical emergency.
  • In adults, septic arthritis may result from trauma, surgery, or hematogenous dissemination.
  • The most commonly involved joints include the knees, shoulders, hips, and phalanges.
  • Infection frequently develops in joints previously damaged by osteoarthritis or rheumatoid arthritis.
  • S. aureus is one of the most common causes of surgical wound infections.
  • S. aureus is the most common cause of septic arthritis in native joints.
  • If left untreated, this infection is rapidly progressive and may be associated with extensive joint destruction.
  • It presents with intense pain on motion of the affected joint, swelling, and fever.
  • Aspiration of the joint reveals turbid fluid, with >50,000 PMNs/μL and gram-positive cocci in clusters seen on Gram's stain.
  • Pyomyositis is an unusual infection of skeletal muscles that is seen primarily in tropical climates but also occurs in immunocompromised (e.g., HIV-infected) patients.
  • It is believed to arise from occult bacteremia.
  • Pyomyositis presents as fever, swelling, and pain overlying the involved muscle.
  • Aspiration of fluid from the involved tissue yields pus.
  • Although a history of trauma may be associated with the infection, its pathogenesis is poorly understood.
  • Respiratory Tract Infections: Respiratory tract infections caused by S. aureus occur in selected clinical settings.
  • S. aureus is a cause of serious respiratory tract infections in newborns and infants; these infections present with shortness of breath, fever, and respiratory failure.
  • Chest x-ray may reveal pneumatoceles (shaggy, thin-walled cavities).
  • Pneumothorax and empyema are recognized complications.
  • In adults, nosocomial S. aureus pulmonary infections are common among intubated patients in intensive care units.
  • Nasally colonized patients are at increased risk of these infections.
  • The clinical presentation is no different from pulmonary infections caused by other bacterial pathogens.
  • Patients produce increased volumes of purulent sputum and develop respiratory distress, fever, and new pulmonary infiltrates.
  • Distinguishing bacterial pneumonia from respiratory failure or other causes of new pulmonary infiltrates in critically ill patients is difficult and relies on a constellation of clinical, radiologic, and laboratory findings.
  • Community-acquired respiratory tract infections due to S. aureus often follow viral infections—most commonly influenza.
  • Patients may present with fever, bloody sputum production, and midlung-field pneumatoceles or multiple, patchy pulmonary infiltrates.
  • Diagnosis is made by sputum Gram's stain and culture.
  • Blood cultures, although useful, are usually negative.
  • Bacteremia, Sepsis, and Infective Endocarditis: S. aureus bacteremia may be complicated by sepsis, endocarditis, vasculitis, or metastatic seeding (establishment of suppurative collections at other tissue sites).
  • Among the more commonly seeded tissue sites are bones, joints, kidneys, and lungs.
  • The frequency of metastatic seeding during bacteremia has been estimated to be as high as 31%.
  • The incidence of these complications increases with the duration of the bacteremia.
  • Comorbid conditions that are frequently seen in association with S. aureus bacteremia and that increase the risk of complications include diabetes, HIV infection, and renal insufficiency.
  • Other host factors that increase the risk of complications include presentation with community-acquired S. aureus bacteremia, lack of an identifiable primary focus of infection, and the presence of prosthetic devices or material.
  • Clinically, S. aureus sepsis presents in a manner similar to that documented for sepsis due to other bacteria.
  • The well-described progression of hemodynamic changes—beginning with respiratory alkalosis and clinical findings of hypotension and fever—is commonly seen.
  • The microbiologic diagnosis is established by positive blood cultures.
  • S. aureus endocarditis is encountered in four clinical settings: (1) right-sided endocarditis in association with injection drug use; (2) left-sided native-valve endocarditis; (3) prosthetic-valve endocarditis; and (4) nosocomial endocarditis.
  • In each of these settings, the diagnosis is suspected from the patient's history and the recognition of physical signs suggestive of endocarditis.
  • These findings include cardiac manifestations, such as new or changing cardiac valvular murmurs; cutaneous evidence, such as vasculitic lesions, Osler's nodes, or Janeway lesions; evidence of right- or left-sided embolic disease; and a history suggesting a risk for S. aureus bacteremia.
  • In the absence of antecedent antibiotic therapy, blood cultures are almost uniformly positive.
  • Transthoracic echocardiography, while less sensitive than transesophageal echocardiography, is less invasive and often identifies valvular vegetations.
  • The Duke criteria (Chap. 133) are commonly used to help establish this diagnosis.
  • Individuals with antecedent cardiac valvular damage more commonly present with left-sided native-valve endocarditis involving the damaged valve.
  • These patients tend to be older than those with right-sided endocarditis, their prognosis is worse, and their incidence of complications (including peripheral emboli, cardiac decompensation, cerebrovascular events, and metastatic seeding) is increased.
  • S. aureus is one of the more common causes of prosthetic-valve endocarditis.
  • This infection is especially fulminant in the early postoperative period and is associated with increased morbidity and mortality.
  • In most instances, medical therapy alone is not sufficient and urgent valve replacement is necessary.
  • Patients are prone to develop valvular insufficiency or myocardial abscesses originating from the region of valve implantation.
  • The increased frequency of nosocomial endocarditis (15–30% of cases, depending on the series) reflects in part the increased use of intravascular devices.
  • This form of endocarditis is most commonly caused by S. aureus.
  • These patients are often critically ill, are receiving antibiotics for various other indications, and have comorbid conditions.
  • As a result, blood cultures may be negative, and the diagnosis is missed.
  • Prosthetic Device–Related Infections: S. aureus accounts for a large proportion of prosthetic device–related infections.
  • These infections include intravascular and peritoneal catheters, prosthetic valves, orthopedic devices, pacemakers, left ventricular assist devices, or vascular grafts.
  • In contrast with the more indolent presentation of NSaS infections, S. aureus device-related infections are often acute, have both local and systemic manifestations, and tend to progress more rapidly.
  • It is relatively common for a pyogenic collection to be present at the device site.
  • Aspiration of these collections and performance of blood cultures are important components in establishing a diagnosis.
  • S. aureus infections tend to occur more commonly soon after implantation unless the device is used for access (e.g., intravascular or hemodialysis catheters).
  • In the latter instance, infections can occur at any time.
  • As in most prosthetic-device infections, successful therapy requires removal of the device.

4.1 Skin and Soft Tissue Infections

  • Folliculitis is a superficial infection that involves the hair follicle, with a central area of purulence (pus) surrounded by induration and erythema.
  • Furuncles (boils) are more extensive, painful lesions that tend to occur in hairy, moist regions of the body and extend from the hair follicle to become a true abscess with an area of central purulence.
  • Carbuncles are most often located in the lower neck and are even more severe and painful, resulting from the coalescence of other lesions that extend to a deeper layer of the subcutaneous tissue.
  • In general, furuncles and carbuncles are readily apparent, with pus often expressible or discharging from the abscess.
  • Other cutaneous S. aureus infections include impetigo.
  • Mastitis develops in 1–3% of nursing mothers.
  • This infection of the breast, which generally presents within 2–3 weeks after delivery, is characterized by findings that range from cellulitis to abscess formation.
  • Systemic signs, such as fever and chills, are often present in more severe cases.

4.2 Musculoskeletal Infections

  • S. aureus is a common cause of bone infections—both those resulting from hematogenous dissemination and those arising from contiguous spread from a soft tissue site.
  • Hematogenous osteomyelitis in children most often involves the long bones.
  • Infections present with fever and bone pain or with a child's reluctance to bear weight.
  • The white blood cell count and erythrocyte sedimentation rate are often elevated.
  • Blood cultures are positive in ~50% of cases.
  • When necessary, bone biopsies for culture and histopathologic examination are usually diagnostic.
  • In adults, hematogenous osteomyelitis involving the long bones is less common.
  • However, vertebral osteomyelitis is among the more common clinical presentations.
  • Vertebral bone infections are most often seen in patients with endocarditis, those undergoing hemodialysis, diabetics, and injection drug users.
  • These infections may present with intense back pain and fever but may also be clinically occult, presenting as chronic back pain with low-grade fever.
  • S. aureus is the most common cause of epidural abscess, a complication that can result in neurologic compromise.
  • Patients report difficulty voiding or walking and radicular pain in addition to the symptoms associated with their osteomyelitis.
  • Surgical intervention in this setting often constitutes a medical emergency.
  • In adults, septic arthritis may result from trauma, surgery, or hematogenous dissemination.
  • The most commonly involved joints include the knees, shoulders, hips, and phalanges.
  • Infection frequently develops in joints previously damaged by osteoarthritis or rheumatoid arthritis.
  • S. aureus is one of the most common causes of surgical wound infections.
  • S. aureus is the most common cause of septic arthritis in native joints.
  • If left untreated, this infection is rapidly progressive and may be associated with extensive joint destruction.
  • It presents with intense pain on motion of the affected joint, swelling, and fever.
  • Aspiration of the joint reveals turbid fluid, with >50,000 PMNs/μL and gram-positive cocci in clusters seen on Gram's stain.
  • Pyomyositis is an unusual infection of skeletal muscles that is seen primarily in tropical climates but also occurs in immunocompromised (e.g., HIV-infected) patients.
  • It is believed to arise from occult bacteremia.
  • Pyomyositis presents as fever, swelling, and pain overlying the involved muscle.
  • Aspiration of fluid from the involved tissue yields pus.
  • Although a history of trauma may be associated with the infection, its pathogenesis is poorly understood.

4.3 Respiratory Tract Infections

  • S. aureus is a cause of serious respiratory tract infections in newborns and infants; these infections present with shortness of breath, fever, and respiratory failure.
  • Chest x-ray may reveal pneumatoceles (shaggy, thin-walled cavities).
  • Pneumothorax and empyema are recognized complications.
  • In adults, nosocomial S. aureus pulmonary infections are common among intubated patients in intensive care units.
  • Nasally colonized patients are at increased risk of these infections.
  • The clinical presentation is no different from pulmonary infections caused by other bacterial pathogens.
  • Patients produce increased volumes of purulent sputum and develop respiratory distress, fever, and new pulmonary infiltrates.
  • Distinguishing bacterial pneumonia from respiratory failure or other causes of new pulmonary infiltrates in critically ill patients is difficult and relies on a constellation of clinical, radiologic, and laboratory findings.
  • Community-acquired respiratory tract infections due to S. aureus often follow viral infections—most commonly influenza.
  • Patients may present with fever, bloody sputum production, and midlung-field pneumatoceles or multiple, patchy pulmonary infiltrates.
  • Diagnosis is made by sputum Gram's stain and culture.
  • Blood cultures, although useful, are usually negative.

4.4 Bacteremia and Infective Endocarditis

  • S. aureus bacteremia may be complicated by sepsis, endocarditis, vasculitis, or metastatic seeding (establishment of suppurative collections at other tissue sites).
  • Among the more commonly seeded tissue sites are bones, joints, kidneys, and lungs.
  • The frequency of metastatic seeding during bacteremia has been estimated to be as high as 31%.
  • The incidence of these complications increases with the duration of the bacteremia.
  • Comorbid conditions that are frequently seen in association with S. aureus bacteremia and that increase the risk of complications include diabetes, HIV infection, and renal insufficiency.
  • Other host factors that increase the risk of complications include presentation with community-acquired S. aureus bacteremia, lack of an identifiable primary focus of infection, and the presence of prosthetic devices or material.
  • Clinically, S. aureus sepsis presents in a manner similar to that documented for sepsis due to other bacteria.
  • The well-described progression of hemodynamic changes—beginning with respiratory alkalosis and clinical findings of hypotension and fever—is commonly seen.
  • The microbiologic diagnosis is established by positive blood cultures.
  • S. aureus endocarditis is encountered in four clinical settings: (1) right-sided endocarditis in association with injection drug use; (2) left-sided native-valve endocarditis; (3) prosthetic-valve endocarditis; and (4) nosocomial endocarditis.
  • In each of these settings, the diagnosis is suspected from the patient's history and the recognition of physical signs suggestive of endocarditis.
  • These findings include cardiac manifestations, such as new or changing cardiac valvular murmurs; cutaneous evidence, such as vasculitic lesions, Osler's nodes, or Janeway lesions; evidence of right- or left-sided embolic disease; and a history suggesting a risk for S. aureus bacteremia.
  • In the absence of antecedent antibiotic therapy, blood cultures are almost uniformly positive.
  • Transthoracic echocardiography, while less sensitive than transesophageal echocardiography, is less invasive and often identifies valvular vegetations.
  • The Duke criteria (Chap. 133) are commonly used to help establish this diagnosis.
  • Individuals with antecedent cardiac valvular damage more commonly present with left-sided native-valve endocarditis involving the damaged valve.
  • These patients tend to be older than those with right-sided endocarditis, their prognosis is worse, and their incidence of complications (including peripheral emboli, cardiac decompensation, cerebrovascular events, and metastatic seeding) is increased.
  • S. aureus is one of the more common causes of prosthetic-valve endocarditis.
  • This infection is especially fulminant in the early postoperative period and is associated with increased morbidity and mortality.
  • In most instances, medical therapy alone is not sufficient and urgent valve replacement is necessary.
  • Patients are prone to develop valvular insufficiency or myocardial abscesses originating from the region of valve implantation.
  • The increased frequency of nosocomial endocarditis (15–30% of cases, depending on the series) reflects in part the increased use of intravascular devices.
  • This form of endocarditis is most commonly caused by S. aureus.
  • These patients are often critically ill, are receiving antibiotics for various other indications, and have comorbid conditions.
  • As a result, blood cultures may be negative, and the diagnosis is missed.
  • Prosthetic Device–Related Infections: S. aureus accounts for a large proportion of prosthetic device–related infections.
  • These infections include intravascular and peritoneal catheters, prosthetic valves, orthopedic devices, pacemakers, left ventricular assist devices, or vascular grafts.
  • In contrast with the more indolent presentation of NSaS infections, S. aureus device-related infections are often acute, have both local and systemic manifestations, and tend to progress more rapidly.
  • It is relatively common for a pyogenic collection to be present at the device site.
  • Aspiration of these collections and performance of blood cultures are important components in establishing a diagnosis.
  • S. aureus infections tend to occur more commonly soon after implantation unless the device is used for access (e.g., intravascular or hemodialysis catheters).
  • In the latter instance, infections can occur at any time.
  • As in most prosthetic-device infections, successful therapy requires removal of the device.

5. DIFFERENTIAL DIAGNOSIS

  • Distinguishing bacterial pneumonia from respiratory failure or other causes of new pulmonary infiltrates in critically ill patients is difficult and relies on a constellation of clinical, radiologic, and laboratory findings.
  • Distinguishing osteomyelitis from overlying soft tissue infection with underlying osteitis is a diagnostic challenge.
  • Distinguishing S. aureus bacteremia from other causes of bacteremia requires consideration of metastatic seeding.
  • Distinguishing S. aureus endocarditis from other causes of endocarditis requires consideration of the Duke criteria and echocardiographic findings.
  • Distinguishing CA-MRSA infections from other SSTIs requires consideration of PVL status and strain typing.

5.1 Distinguishing Features

  • Distinguishing bacterial pneumonia from respiratory failure or other causes of new pulmonary infiltrates in critically ill patients is difficult and relies on a constellation of clinical, radiologic, and laboratory findings.
  • Distinguishing osteomyelitis from overlying soft tissue infection with underlying osteitis is a diagnostic challenge.
  • Distinguishing S. aureus bacteremia from other causes of bacteremia requires consideration of metastatic seeding.
  • Distinguishing S. aureus endocarditis from other causes of endocarditis requires consideration of the Duke criteria and echocardiographic findings.
  • Distinguishing CA-MRSA infections from other SSTIs requires consideration of PVL status and strain typing.

6. INVESTIGATIONS & DIAGNOSIS

  • Staphylococcal infections are readily diagnosed by Gram's stain and microscopic examination of abscess contents or of infected tissue.
  • Routine cultures of infected material usually are positive; blood cultures are sometimes positive even when infections are localized to extravascular sites.
  • S. aureus is rarely a blood culture contaminant.
  • Polymerase chain reaction (PCR)–based assays are now often used for the rapid diagnosis of S. aureus infection.
  • A number of point-of-care tests are available to screen patients for colonization with MRSA.
  • Determining whether patients with documented S. aureus bacteremia also have infective endocarditis or a metastatic focus of infection remains a diagnostic challenge.
  • Uniformly positive cultures of blood collected over time suggest an endovascular infection such as endocarditis.
  • Routine x-rays are an appropriate first step for osteomyelitis, but findings may be normal for up to 14 days after the onset of symptoms.
  • If an MRI is not possible, computed tomography (CT) is an acceptable alternative.
  • Magnetic resonance imaging (MRI) is the most reliable imaging modality to help establish the diagnosis of osteomyelitis.
  • Bone involvement is established by bone culture and histopathologic examination (revealing evidence of PMN infiltration).
  • Samples obtained during surgery are the most reliable for osteomyelitis diagnosis.
  • An MRI is the most reliable radiologic test to distinguish between osteomyelitis and overlying soft tissue infection with underlying osteitis.
  • Transthoracic echocardiography, while less sensitive than transesophageal echocardiography, is less invasive and often identifies valvular vegetations.
  • The Duke criteria (Chap. 133) are commonly used to help establish the diagnosis of endocarditis.
  • Diagnosis of respiratory tract infections is made by sputum Gram's stain and culture.
  • Blood cultures, although useful, are usually negative in community-acquired respiratory tract infections due to S. aureus.

6.1 Diagnostic Criteria and Algorithms

  • Staphylococcal infections are readily diagnosed by Gram's stain and microscopic examination of abscess contents or of infected tissue.
  • Routine cultures of infected material usually are positive; blood cultures are sometimes positive even when infections are localized to extravascular sites.
  • S. aureus is rarely a blood culture contaminant.
  • Polymerase chain reaction (PCR)–based assays are now often used for the rapid diagnosis of S. aureus infection.
  • A number of point-of-care tests are available to screen patients for colonization with MRSA.
  • Determining whether patients with documented S. aureus bacteremia also have infective endocarditis or a metastatic focus of infection remains a diagnostic challenge.
  • Uniformly positive cultures of blood collected over time suggest an endovascular infection such as endocarditis.
  • Routine x-rays are an appropriate first step for osteomyelitis, but findings may be normal for up to 14 days after the onset of symptoms.
  • If an MRI is not possible, computed tomography (CT) is an acceptable alternative.
  • Magnetic resonance imaging (MRI) is the most reliable imaging modality to help establish the diagnosis of osteomyelitis.
  • Bone involvement is established by bone culture and histopathologic examination (revealing evidence of PMN infiltration).
  • Samples obtained during surgery are the most reliable for osteomyelitis diagnosis.
  • An MRI is the most reliable radiologic test to distinguish between osteomyelitis and overlying soft tissue infection with underlying osteitis.
  • Transthoracic echocardiography, while less sensitive than transesophageal echocardiography, is less invasive and often identifies valvular vegetations.
  • The Duke criteria (Chap. 133) are commonly used to help establish the diagnosis of endocarditis.
  • Diagnosis of respiratory tract infections is made by sputum Gram's stain and culture.
  • Blood cultures, although useful, are usually negative in community-acquired respiratory tract infections due to S. aureus.

7. MANAGEMENT & TREATMENT

  • In most instances, medical therapy alone is not sufficient for prosthetic-valve endocarditis and urgent valve replacement is necessary.
  • As in most prosthetic-device infections, successful therapy requires removal of the device.
  • S. aureus infections tend to occur more commonly soon after implantation unless the device is used for access (e.g., intravascular or hemodialysis catheters).
  • In the latter instance, infections can occur at any time.
  • As in most prosthetic-device infections, successful therapy requires removal of the device.
  • S. aureus is a leading cause of health care–associated infections.
  • It is the most common cause of surgical wound infections and is second only to NSaS as a cause of primary bacteremia.
  • These isolates are often resistant to multiple antibiotics; thus, available therapeutic options may be limited.
  • In the community, S. aureus remains an important cause of SSTIs, respiratory infections, and, especially among injection drug users, infective endocarditis.
  • The increasing use of home infusion therapy also poses a risk of community-acquired staphylococcal infections.
  • Other Prevention Strategies: Pneumococcal disease can be averted through the prevention of illnesses that predispose individuals to pneumococcal infections.
  • Relevant measures include smoking cessation and influenza vaccination, as well as improved management and control of diabetes, HIV infection, heart disease, and lung disease.
  • Finally, the reduction of antibiotic misuse is a strategy for the prevention of pneumococcal disease in that antimicrobial resistance directly and indirectly perpetuates organism transmission and disease in the community.

7.1 General Principles

  • In most instances, medical therapy alone is not sufficient for prosthetic-valve endocarditis and urgent valve replacement is necessary.
  • As in most prosthetic-device infections, successful therapy requires removal of the device.
  • S. aureus infections tend to occur more commonly soon after implantation unless the device is used for access (e.g., intravascular or hemodialysis catheters).
  • In the latter instance, infections can occur at any time.
  • As in most prosthetic-device infections, successful therapy requires removal of the device.
  • S. aureus is a leading cause of health care–associated infections.
  • It is the most common cause of surgical wound infections and is second only to NSaS as a cause of primary bacteremia.
  • These isolates are often resistant to multiple antibiotics; thus, available therapeutic options may be limited.
  • In the community, S. aureus remains an important cause of SSTIs, respiratory infections, and, especially among injection drug users, infective endocarditis.
  • The increasing use of home infusion therapy also poses a risk of community-acquired staphylococcal infections.

7.2 Prevention Strategies

  • Other Prevention Strategies: Pneumococcal disease can be averted through the prevention of illnesses that predispose individuals to pneumococcal infections.
  • Relevant measures include smoking cessation and influenza vaccination, as well as improved management and control of diabetes, HIV infection, heart disease, and lung disease.
  • Finally, the reduction of antibiotic misuse is a strategy for the prevention of pneumococcal disease in that antimicrobial resistance directly and indirectly perpetuates organism transmission and disease in the community.

8. PROGNOSIS & COMPLICATIONS

  • Despite the availability of effective antibiotics, mortality rates from prosthetic device–related infections continue to range from 20 to 40%, depending on both the host and the nature of the infection.
  • The frequency of metastatic seeding during bacteremia has been estimated to be as high as 31%.
  • The incidence of these complications increases with the duration of the bacteremia.
  • Comorbid conditions that are frequently seen in association with S. aureus bacteremia and that increase the risk of complications include diabetes, HIV infection, and renal insufficiency.
  • Other host factors that increase the risk of complications include presentation with community-acquired S. aureus bacteremia, lack of an identifiable primary focus of infection, and the presence of prosthetic devices or material.
  • S. aureus endocarditis is encountered in four clinical settings: (1) right-sided endocarditis in association with injection drug use; (2) left-sided native-valve endocarditis; (3) prosthetic-valve endocarditis; and (4) nosocomial endocarditis.
  • Individuals with antecedent cardiac valvular damage more commonly present with left-sided native-valve endocarditis involving the damaged valve.
  • These patients tend to be older than those with right-sided endocarditis, their prognosis is worse, and their incidence of complications (including peripheral emboli, cardiac decompensation, cerebrovascular events, and metastatic seeding) is increased.
  • S. aureus is one of the more common causes of prosthetic-valve endocarditis.
  • This infection is especially fulminant in the early postoperative period and is associated with increased morbidity and mortality.
  • In most instances, medical therapy alone is not sufficient and urgent valve replacement is necessary.
  • Patients are prone to develop valvular insufficiency or myocardial abscesses originating from the region of valve implantation.
  • The increased frequency of nosocomial endocarditis (15–30% of cases, depending on the series) reflects in part the increased use of intravascular devices.
  • This form of endocarditis is most commonly caused by S. aureus.
  • These patients are often critically ill, are receiving antibiotics for various other indications, and have comorbid conditions.
  • As a result, blood cultures may be negative, and the diagnosis is missed.

8.1 Mortality and Morbidity

  • Despite the availability of effective antibiotics, mortality rates from prosthetic device–related infections continue to range from 20 to 40%, depending on both the host and the nature of the infection.
  • The frequency of metastatic seeding during bacteremia has been estimated to be as high as 31%.
  • The incidence of these complications increases with the duration of the bacteremia.
  • Comorbid conditions that are frequently seen in association with S. aureus bacteremia and that increase the risk of complications include diabetes, HIV infection, and renal insufficiency.
  • Other host factors that increase the risk of complications include presentation with community-acquired S. aureus bacteremia, lack of an identifiable primary focus of infection, and the presence of prosthetic devices or material.

9. SPECIAL CONSIDERATIONS

  • Other groups at risk include individuals with end-stage renal disease, HIV infection, skin abnormalities, or prosthetic devices.
  • Individuals with congenital or acquired qualitative or quantitative defects of polymorphonuclear leukocytes (PMNs) are at increased risk of S. aureus infections.
  • This group includes neutropenic patients (e.g., those receiving chemotherapeutic agents), those with cystic fibrosis or osteomyelitis.
  • Pyomyositis is an unusual infection of skeletal muscles that is seen primarily in tropical climates but also occurs in immunocompromised (e.g., HIV-infected) patients.
  • It is believed to arise from occult bacteremia.
  • Pyomyositis presents as fever, swelling, and pain overlying the involved muscle.
  • Aspiration of fluid from the involved tissue yields pus.
  • Although a history of trauma may be associated with the infection, its pathogenesis is poorly understood.
  • S. aureus is a cause of serious respiratory tract infections in newborns and infants; these infections present with shortness of breath, fever, and respiratory failure.
  • Chest x-ray may reveal pneumatoceles (shaggy, thin-walled cavities).
  • Pneumothorax and empyema are recognized complications.
  • In adults, nosocomial S. aureus pulmonary infections are common among intubated patients in intensive care units.
  • Nasally colonized patients are at increased risk of these infections.
  • The clinical presentation is no different from pulmonary infections caused by other bacterial pathogens.
  • Patients produce increased volumes of purulent sputum and develop respiratory distress, fever, and new pulmonary infiltrates.
  • Distinguishing bacterial pneumonia from respiratory failure or other causes of new pulmonary infiltrates in critically ill patients is difficult and relies on a constellation of clinical, radiologic, and laboratory findings.
  • Community-acquired respiratory tract infections due to S. aureus often follow viral infections—most commonly influenza.
  • Patients may present with fever, bloody sputum production, and midlung-field pneumatoceles or multiple, patchy pulmonary infiltrates.
  • Diagnosis is made by sputum Gram's stain and culture.
  • Blood cultures, although useful, are usually negative.

9.1 Immunocompromised Hosts

  • Other groups at risk include individuals with end-stage renal disease, HIV infection, skin abnormalities, or prosthetic devices.
  • Individuals with congenital or acquired qualitative or quantitative defects of polymorphonuclear leukocytes (PMNs) are at increased risk of S. aureus infections.
  • This group includes neutropenic patients (e.g., those receiving chemotherapeutic agents), those with cystic fibrosis or osteomyelitis.
  • Pyomyositis is an unusual infection of skeletal muscles that is seen primarily in tropical climates but also occurs in immunocompromised (e.g., HIV-infected) patients.
  • It is believed to arise from occult bacteremia.
  • Pyomyositis presents as fever, swelling, and pain overlying the involved muscle.
  • Aspiration of fluid from the involved tissue yields pus.
  • Although a history of trauma may be associated with the infection, its pathogenesis is poorly understood.

9.2 Pediatric Considerations

  • S. aureus is a cause of serious respiratory tract infections in newborns and infants; these infections present with shortness of breath, fever, and respiratory failure.
  • Chest x-ray may reveal pneumatoceles (shaggy, thin-walled cavities).
  • Pneumothorax and empyema are recognized complications.
  • The exfoliative toxins are responsible for SSSS, most commonly seen in newborns.
  • The toxins that produce disease in humans are of two serotypes: ETA and ETB.
  • These toxins are serine proteases that cleave desmosomal cadherins in the superficial layer of the skin, triggering exfoliation.
  • The result is a split in the epidermis at the granular level, which is responsible for the superficial desquamation of the skin that typifies this illness.

10. KEY PEARLS & CLINICAL TRAPS

  • S. aureus is a pyogenic pathogen known for its capacity to induce abscess formation at both local and distant sites (i.e., metastatic infections).
  • Approximately 20–40% of healthy persons are colonized with S. aureus, with a smaller percentage (~10%) persistently colonized with the same strain.
  • CA-MRSA strains have been responsible for an increasing number of nosocomial infections and have enhanced capacity to cause disease in immunocompetent individuals.
  • MRI is the most reliable imaging modality to help establish the diagnosis of osteomyelitis; findings may be normal for up to 14 days after the onset of symptoms.
  • S. aureus bacteremia may be complicated by sepsis, endocarditis, vasculitis, or metastatic seeding; the frequency of metastatic seeding during bacteremia has been estimated to be as high as 31%.
  • TSST-1 and enterotoxins bind directly to the invariant region of MHC—outside the MHC class II groove, resulting in a dramatic overexpansion of T-cell clones (up to 20% of the total T-cell population).
  • S. aureus is the most common cause of surgical wound infections and is second only to NSaS as a cause of primary bacteremia.
  • In prosthetic-valve endocarditis, medical therapy alone is often not sufficient and urgent valve replacement is necessary.
  • S. aureus is rarely a blood culture contaminant; uniformly positive cultures of blood collected over time suggest an endovascular infection such as endocarditis.
  • The regulatory gene agr is part of a quorum-sensing signal transduction pathway that senses and responds to bacterial density.

10.1 Board Exam Favorites

  • S. aureus is a pyogenic pathogen known for its capacity to induce abscess formation at both local and distant sites (i.e., metastatic infections).
  • Approximately 20–40% of healthy persons are colonized with S. aureus, with a smaller percentage (~10%) persistently colonized with the same strain.
  • CA-MRSA strains have been responsible for an increasing number of nosocomial infections and have enhanced capacity to cause disease in immunocompetent individuals.
  • MRI is the most reliable imaging modality to help establish the diagnosis of osteomyelitis; findings may be normal for up to 14 days after the onset of symptoms.
  • S. aureus bacteremia may be complicated by sepsis, endocarditis, vasculitis, or metastatic seeding; the frequency of metastatic seeding during bacteremia has been estimated to be as high as 31%.
  • TSST-1 and enterotoxins bind directly to the invariant region of MHC—outside the MHC class II groove, resulting in a dramatic overexpansion of T-cell clones (up to 20% of the total T-cell population).
  • S. aureus is the most common cause of surgical wound infections and is second only to NSaS as a cause of primary bacteremia.
  • In prosthetic-valve endocarditis, medical therapy alone is often not sufficient and urgent valve replacement is necessary.
  • S. aureus is rarely a blood culture contaminant; uniformly positive cultures of blood collected over time suggest an endovascular infection such as endocarditis.
  • The regulatory gene agr is part of a quorum-sensing signal transduction pathway that senses and responds to bacterial density.

Figures & Illustrations

Reproduced from Harrison's 22nd Edition.

Figure 1

Global distribution of community-associated MRSA

Caption: FIGURE 152-2 Global distribution of community-associated MRSA. Dotted lines indicate with the main strains—i.e., ST1 (green), ST8 (red), ST30 (blue), and ST80 (gray PVL-negative strains; ±, PVL-positive and -negative strains. (Reproduced with methicillin-resistant Staphylococcus aureus. Lancet 375:1557, 2010.) — Gram's stain of S. aureus in a sputum sample, illustrating staphylococcal clusters.

Figure 2

S

Caption: FIGURE 152-3 S. aureus vertebral osteomyelitis and epidural abscess involving the thoracic disk between T9 and T10. Sagittal postcontrast magnetic resonance is imaging of the spine illustrates destruction of the T9–T10 intervertebral space with enhancement (long arrow). There is impingement on the thoracic cord and an epidural collection extending from T9 through T11 (short arrows). — Global distribution of community-associated MRSA. Dotted lines indicate possible route of dissemination. Estimates of the areas are shown where infection with the main strains—i.e., ST1 (green), ST8 (red), ST30 (blue), and ST80 (gray hatched)—have been reported. +, Panton-Valentine leukocidin (PVL)-positive strains; –, PVL-negative strains; ±, PVL-positive and -negative strains.

Figure 3

Staphylococcal scalded skin syndrome in a 6-year-old boy

Caption: FIGURE 152-5 Staphylococcal scalded skin syndrome in a 6-year-old boy. Nikolsky’s sign, with separation of the superficial layer of the outer epidermal layer, is visible. (Adapted from LA Schenfeld: Staphylococcal scalded skin syndrome: N Engl J Med 342:1178, 2000.) as — S. aureus vertebral osteomyelitis and epidural abscess involving the thoracic disk between T9 and T10. Sagittal postcontrast magnetic resonance imaging of the spine illustrates destruction of the T9–T10 intervertebral space with enhancement (long arrow). There is impingement on the thoracic cord and an epidural collection extending from T9 through T11 (short arrows).

Figure 4

Gram’s stain of S

Caption: FIGURE 152-1 Gram’s stain of S. aureus in a sputum sample, illustrating staphylococcal clusters. (From ASM MicrobeLibrary.org. © Pfizer, Inc.) — Computed tomography scan illustrating septic pulmonary emboli in a patient with methicillin-resistant Staphylococcus aureus bacteremia.

Figure 5

Computed tomography scan illustrating septic pulmonary emboli in a patient...

Caption: FIGURE 152-4 Computed tomography scan illustrating septic pulmonary emboli in a patient with methicillin-resistant Staphylococcus aureus bacteremia. — Clinical photograph of staphylococcal scalded-skin syndrome showing superficial desquamation of the skin at the granular level, which is responsible for the superficial desquamation of the skin that typifies this illness.

Generated from Harrison's Principles of Internal Medicine, 22nd Edition.