Acinetobacter Infections¶
Chapter 167 | Part 5: Infectious Diseases
KEY CLINICAL POINTS¶
- Acinetobacter species are gram-negative, oxidase-negative, nonmotile coccobacilli with high antimicrobial resistance and environmental persistence.
- A. baumannii is the most clinically relevant species, causing multidrug-resistant infections in ICUs, trauma patients, and disaster settings.
- Treatment is challenging due to carbapenem resistance, biofilm formation, and limited therapeutic options requiring combination regimens.
1. DEFINITION & OVERVIEW¶
Acinetobacter species are gram-negative, oxidase-negative, nonmotile coccobacilli first described in 1911. They are now classified as Acinetobacter, with A. baumannii being the most clinically significant species. These organisms are naturally found in water, soil, and on human skin, with environmental persistence due to lipid A modification and desiccation resistance.
Table 167-1 Therapeutic Options for Multidrug-Resistant Acinetobacter baumannii¶
| ANTIBIOTIC | DOSINGa | COMMENTS |
|---|---|---|
| Sulbactam | 6–9 g/d | Unavailable as single drug in many countries; dosing strategies proposed with ampicillin |
| Ampicillin-sulbactam | 3 g q4h (infuse 30 min)/9 g q8h (infuse 4 h)/27 g q24h (continuous infusion) | Preferred for carbapenem-resistant isolates |
| Sulbactam-durlobactam | 1 g/1 g q6h (infuse 3 h) | Combination with carbapenems or polymyxins |
| Meropenem | 2 g q8h (infuse 3 h) | For carbapenem-susceptible isolates |
| Imipenem-cilastatin | 500 mg q6h (infuse 3 h) | For carbapenem-susceptible isolates |
| Cefiderocol | 2 g q8h (infuse 3 h) | Siderophore cephalosporin with in vitro synergy against carbapenemases |
| Colistin | Dosing per international consensus guidelines | Preferred for urinary tract infections; used with ampicillin-sulbactam |
| Tigecycline | 200-mg loading dose followed by 100 mg q12h | Used in combination therapy |
| Minocycline | 200 mg q12h (IV/PO) | Used in combination therapy |
1.1 Taxonomy and Classification¶
Acinetobacter species were previously named Micrococcus calcoaceticus but renamed multiple times. Since 1950, they have been known as Acinetobacter. Molecular methods like MALDI-TOF-MS and PCR are required for species identification, particularly for A. baumannii.
1.2 Environmental Persistence¶
A. baumannii can survive desiccation for weeks, enabling persistence in hospital environments. This contributes to nosocomial transmission via contaminated equipment and poor hand hygiene.
2. EPIDEMIOLOGY¶
Acinetobacter infections are predominantly healthcare-associated, with A. baumannii being the most common pathogen. CDC estimates 12,000 infections annually in the U.S., 7300 caused by multidrug-resistant strains. Outbreaks are common in ICUs, LTACHs, and disaster settings (e.g., tsunamis, earthquakes).
2.1 Risk Factors¶
Prolonged ICU stays, central venous catheters, mechanical ventilation, tracheostomy, and prior antibiotic exposure increase risk. Trauma patients, burn victims, and immunocompromised individuals are particularly vulnerable.
2.2 Geographic Distribution¶
Historically linked to hot/humid climates, but now reported globally. Outbreaks in temperate regions (e.g., U.S., Europe) are increasingly common due to multidrug-resistant clones.
3. ETIOLOGY & PATHOPHYSIOLOGY¶
A. baumannii employs virulence mechanisms including biofilm formation, phospholipase activity, and iron-acquisition systems. Resistance is mediated by intrinsic β -lactamases (AmpC), acquired carbapenemases (OXA-23-like, OXA-51-like), and efflux pumps.
3.1 Virulence Factors¶
Biofilm formation via exopolysaccharides and pilus formation, phospholipases C/D, outer-membrane porins, and type VI secretion systems. Capsule production enhances resistance to complement-mediated killing.
3.2 Resistance Mechanisms¶
Intrinsic AmpC β -lactamases, acquired carbapenemases (OXA-23-like, OXA-51-like), and efflux pumps. Chromosomal cephalosporinases and plasmid-mediated β -lactamases contribute to multidrug resistance.
4. CLINICAL FEATURES¶
Clinical manifestations include pneumonia, bloodstream infections, wound infections, and meningitis. Community-acquired infections are rare but reported in tropical regions. Disseminated infections are associated with high mortality (up to 70% for carbapenem-resistant strains).
4.1 Common Infections¶
Nosocomial pneumonia (ventilator-associated), bloodstream infections (ICU-acquired), wound infections (trauma/ burns), and meningitis (post-neurosurgical procedures).
4.2 Complications¶
Hepatic abscesses, endocarditis, septic arthritis, and necrotizing fasciitis. Multidrug-resistant strains increase mortality due to treatment challenges.
5. DIFFERENTIAL DIAGNOSIS¶
Differential diagnoses include other gram-negative bacilli (e.g., Pseudomonas, Klebsiella), Staphylococcus aureus, and fungal infections. A. baumannii should be considered in soft tissue infections following trauma or in disaster settings.
6. INVESTIGATIONS & DIAGNOSIS¶
Diagnosis requires culture and antimicrobial susceptibility testing. Molecular methods (PCR, MALDI-TOF-MS) are needed for species identification. Blood cultures and imaging (chest X-ray, CT) are critical for assessing infection extent.
6.1 Laboratory Tests¶
Culture of blood, sputum, urine, or wound exudates. Identification of hydrogen sulfide production and β -lactamase activity. Antimicrobial susceptibility testing for carbapenems, cephalosporins, and tetracyclines.
6.2 Imaging¶
Chest X-ray for pneumonia, CT for abscesses, and ultrasound for soft tissue infections. Radiologic findings are nonspecific but aid in localization.
7. MANAGEMENT & TREATMENT¶
Treatment is challenging due to multidrug resistance. Combination therapy with carbapenems, polymyxins, and β -lactamase inhibitors is recommended. Dosing strategies vary by syndrome and resistance profile.
7.1 Empirical Therapy¶
Carbapenems (meropenem, imipenem) combined with sulbactam-durlobactam or polymyxins. High-dose ampicillin-sulbactam with colistin or tigecycline for carbapenem-resistant isolates.
7.2 Definitive Therapy¶
Guided by antimicrobial susceptibility testing. Cefiderocol is recommended for carbapenem-resistant strains. Bacteriophage therapy and eravacycline are emerging options.
8. PROGNOSIS & COMPLICATIONS¶
Mortality rates are high (up to 70% for carbapenem-resistant infections). Complications include septic shock, multiorgan failure, and prolonged ICU stays. Early intervention and antibiotic stewardship improve outcomes.
8.1 Mortality Factors¶
Severity of underlying illness, drug resistance, and delayed antimicrobial therapy. Crude mortality for bloodstream infections is 40–70%.
8.2 Long-Term Outcomes¶
Chronic colonization, persistent infection, and increased risk of recurrent infections. Antibiotic stewardship is critical to prevent resistance spread.
9. SPECIAL CONSIDERATIONS¶
Prevention strategies include strict hand hygiene, environmental disinfection, and cohorting of infected patients. Special attention is needed in burn units, trauma centers, and disaster response settings.
9.1 Infection Control¶
Daily terminal disinfection, chlorhexidine baths, and contact precautions. Limiting shared equipment and isolating A. baumannii-positive patients reduces transmission.
9.2 Disaster Settings¶
A. baumannii is linked to infections in trauma victims during tsunamis, earthquakes, and terrorist attacks. Soft tissue infections and bloodstream infections are common in these settings.
10. KEY POINTS & CLINICAL PEARLS¶
- A. baumannii is a multidrug-resistant, nosocomial pathogen with high environmental persistence.
- Carbapenem resistance is common due to OXA-type β -lactamases and efflux pumps.
- Combination therapy with carbapenems, polymyxins, and β -lactamase inhibitors is essential.
- Strict infection control measures are critical to prevent outbreaks in healthcare settings.