Infections Due to Mixed Anaerobic Organisms¶

Chapter 182 | Part 5: Infectious Diseases · Part 5 – Infectious Diseases: Bacterial · Chapter 182

Key Clinical Points¶

Anaerobic infections require an anaerobic environment established by tissue ischemia, trauma, surgery, or necrosis.
Bacteroides fragilis group is the most common anaerobe in intraabdominal infections and bacteremia; its capsular polysaccharide A (PSA) induces abscess formation.
Lung abscesses present with foul-smelling sputum and constitutional symptoms lasting 1–3 weeks; ~90% of cultures identify anaerobes if handled correctly.
Empyema is characterized by thick purulent material, pH >1000 IU/L, and WBC count ≈ 55,000 cells/mL.
Noma (cancrum oris) has a 70–90% mortality rate in malnourished children in sub-Saharan Africa.
Lemierre's syndrome is caused by Fusobacterium necrophorum, leading to septic thrombophlebitis of the internal jugular vein.
Gas is common in anaerobic infections but not diagnostic; putrid smell occurs in 30–50% of cases and develops late.
Obligate anaerobes rarely cause meningitis (1 case in 188 isolates).
Aspiration pneumonia must be distinguished from Mendelson syndrome and foreign body obstruction; anaerobes are the most common isolates.
Synergistic infections involve 3–6 organisms; facultative bacteria maintain anaerobic conditions for obligate anaerobes.

1. DEFINITION & OVERVIEW¶

Anaerobic organisms are the predominant class of bacteria in the normal human microbiota, residing on mucous membranes and causing disease after mucosal barrier disruption. Infections are typically polymicrobial, involving both aerobic and anaerobic bacteria. Challenges in specimen handling and cultivation often leave anaerobic etiology unproven. This chapter focuses on infections caused by anaerobes other than Clostridium species.

1.1 Metabolism & Classification¶

• Anaerobic Respiration: Produces less ATP per glucose molecule compared to aerobic respiration. • Classification: ◦ Obligate anaerobes: Killed by ≥ 0.5% O₂. ◦ Aerotolerant: Tolerate but cannot use O₂ (e.g., Bacteroides fragilis). ◦ Facultative: Prefer oxygen when available; in its absence, they use anaerobic respiration or fermentation. • Aerobic Organisms: Use oxygen as the final electron acceptor.

2. EPIDEMIOLOGY¶

Anaerobes constitute 7–8% of inpatient isolates and 13–15% of outpatient isolates. Bacteremia and soft tissue infections are the most common clinical presentations (Fig. 182-2). Anaerobes account for <1% of bacteremia cases. The lower gastrointestinal tract is a major reservoir, while high anaerobe-to-aerobe ratios are found in the oral cavity (10³:1 in gingival crevices) and colon (10³:1).

2.1 Microbiota Distribution (Table 182-1)¶

• Nose: 10³–10⁴ total bacteria; 2:1 anaerobic/aerobic ratio. • Oral cavity (Saliva): 10⁸–10⁹ total; 10:1 ratio. • Tooth surface: 10¹⁰–10¹¹ total; 1:1 ratio. • Gingival crevices: 10¹¹–10¹² total; 10³:1 ratio. • Stomach: 10⁰–10³ total; 1:10 ratio. • Duodenum: 10¹–10⁵ total; 1:1 ratio. • Jejunum: 10³–10⁶ total; 1:1 ratio. • Ileum: 10⁴–10⁹ total; 10:1 ratio. • Cecum and colon: 10¹¹–10¹² total; 10³:1 ratio. • Female genital tract: 10⁷–10⁹ total; 10:1 ratio. • Skin: 10⁴–10⁶ total; 100:1 ratio.

3. ETIOLOGY & PATHOPHYSIOLOGY¶

Anaerobic infections occur when mucosal barriers are disrupted (e.g., trauma, surgery) or organisms are directly inoculated into tissue (e.g., bite wounds). Infections are polymicrobial, involving synergistic facultative and obligate anaerobes.

3.1 Virulence Factors¶

• Bacteroides fragilis: The most common anaerobe in intraabdominal infections and bacteremia. ◦ Capsular Polysaccharide A (PSA): Major virulence factor inducing abscess formation. ◦ Mechanism: PSA stimulates macrophages to release IL-8, IL-17, and TNF-α via Toll-like receptor 2 → promotes neutrophil chemotaxis and ICAM-1 upregulation. • Synergistic Infections: Involve 3–6 organisms. ◦ Facultative bacteria: Create anaerobic conditions for obligate anaerobes. ◦ Short-chain fatty acids: Inhibit phagocyte function. ◦ Vitamin K: Produced by oral diphtheroids, supporting growth of P. melaninogenica.

4. CLINICAL FEATURES¶

Anaerobic infections present with abscess formation, putrid purulence, septic thrombophlebitis, tissue necrosis, and poor response to non-anaerobic antibiotics.

4.1 Specific Manifestations¶

• Orofacial infections: ◦ Odontogenic infections: Progress from dental caries/periodontitis to periapical abscesses. ◦ Ludwig's angina: Submandibular swelling, trismus, and potential airway obstruction. • Acute necrotizing ulcerative gingivitis: Painful bleeding gums, foul breath, pseudomembrane on gingiva; may progress to noma (cancrum oris) in malnourished children with immune dysfunction. • Peripharyngeal space infections: ◦ Peritonsillar abscess (quinsy): Sore throat, dysphagia, uvular deviation, and peritonsillar swelling.

5. DIFFERENTIAL DIAGNOSIS¶

Aspiration pneumonia must be distinguished from Mendelson syndrome and foreign body obstruction.

5.1 Aspiration Syndromes¶

• Aspiration pneumonia: Characterized by anaerobic organisms (most common isolates). • Mendelson syndrome: Chemical pneumonitis (gastric acid aspiration without infection). • Foreign body obstruction: Identified by radiopaque objects on imaging.

6. INVESTIGATIONS & DIAGNOSIS¶

Microbiologic handling is critical for anaerobe identification.

6.1 Microbiologic Handling¶

• Transport: Must be transported in anaerobic conditions to prevent aerobic contamination. • Culture: Requires specialized media (e.g., blood agar with hemin) and incubation at 35–37°C for 24–72 hours.

6.2 Empyema Criteria¶

• Diagnosis of Empyema: 1. Identify thick purulent material. 2. Confirm pH >1000 IU/L. 3. Confirm WBC count ≈ 55,000 cells/mL.

7. MANAGEMENT & TREATMENT¶

Treatment requires surgical drainage/debridement combined with antibiotics targeting anaerobes.

7.1 Treatment Protocol¶

Surgical Intervention: ◦ Mandatory for abscesses and necrotizing infections.
Antibiotic Selection: ◦ Metronidazole: 500 mg PO TID (or IV). ◦ Clindamycin: 600–900 mg Q8H. ◦ Mixed Infections: Add β-lactams (e.g., piperacillin-tazobactam, imipenem-cilastatin) for aerobic organisms.
Clinical Caveats (Table 182-2): ◦ Metronidazole: Clinically unreliable against gram-positive non-spore-forming anaerobes (e.g., Actinomyces spp., Propionibacterium spp., Peptostreptococcus spp.). ◦ Clindamycin: Rates of resistance are increasing in Bacteroides spp. ◦ Chloramphenicol: Potential for clinical failure even when isolate is in vitro susceptible.

8. PROGNOSIS & COMPLICATIONS¶

Prognosis depends on infection severity and timely treatment. • Complications: ◦ Septic shock. ◦ Disseminated intravascular coagulation (DIC) (rare in anaerobic bacteremia). ◦ Airway obstruction (in Ludwig's angina).

9. SPECIAL CONSIDERATIONS¶

Pediatric patients are at higher risk for noma and Lemierre's syndrome.

9.1 Pediatric & High-Risk Cases¶

• Noma (cancrum oris): Occurs in 1–7/1000 children in sub-Saharan Africa; mortality is 70–90% without treatment. • Lemierre's syndrome: Caused by Fusobacterium necrophorum, leading to septic thrombophlebitis of the internal jugular vein.

9.2 Immunocompromised Hosts¶

May present with atypical symptoms (e.g., absence of fever) or have prolonged courses requiring extended antibiotic therapy.

10. KEY PEARLS & CLINICAL TRAPS¶

• Gas in Imaging: Common but not diagnostic; putrid odor occurs in only 30–50% of cases and develops late. • Lung Abscess: ~90% of cultures identify anaerobes if handled correctly; symptoms typically last 1–3 weeks. • Meningitis: Obligate anaerobes rarely cause meningitis (1/188 isolates). • Synergy: 3–6 organisms involved; facultative bacteria maintain anaerobic conditions for obligate anaerobes. • Microbiota: High anaerobic ratios in gingival crevices (10³:1) and the colon (10³:1).

Reference Tables¶

TABLE 182-1 The Anaerobic Human Microbiota: An Overview¶

Harrison's 22e, p.1371

ANATOMIC SITE	TOTAL BACTERIAa	ANAEROBIC/ AEROBIC RATIO	POTENTIAL PATHOGEN(S)
Nose	103–104	2:1	Peptostreptococcus spp., Prevotella spp.
108–109	10:1
1010–1011	1:1
1011–1012	103:1
Gastrointestinal tract
Stomach	100–103	1:10	Lactobacillus spp.
Duodenum	101–105	1:1	Lactobacillus spp., Streptococcus spp.
Jejunum	103–106	1:1	Streptococcus spp., Lactobacillus spp., Peptostreptococcus spp.
Ileum	104–109	10:1	Bacteroides spp., Streptococcus spp., Enterococcus spp.
Cecum and colon	1011–1012	103:1	Bacteroides spp. (principally members of the B. fragilis group), Prevotella spp., Clostridium spp.
	107–109	10:1
Skin	104–106	100:1	Cutibacterium acnes

TABLE 182-2 Antimicrobial Therapy That Is Typically Active against Commonly Encountered Anaerobes ANTIBIOTIC(S)…¶

Harrison's 22e, p.1379

ANTIBIOTIC(S)	CAVEATS
Metronidazole	This drug is clinically unreliable against gram-positive non-spore-forming anaerobes (e.g., Actinomyces spp., Propionibacterium spp., Peptostreptococcus spp.).
Clindamycin	Rates of resistance are increasing in Bacteroides spp.
Chloramphenicol	Some clinical failures have been noted, even when the isolate is found to be susceptible by in vitro testing.