Anaerobic infections

BASIC INFORMATION
An anaerobic infection is caused by one of a group of bacteria that require a reduced oxygen tension for growth. Recent studies have emphasized the role of these organisms in many infections commonly encountered in clinical practice. Features shared by these infections are: 1. Acquisition from the flora of the host's mucocutaneous surfaces and 2. Polymicrobial with multiple bacteria recovered from the infected site, often including aerobes as well as anaerobes.
PHYSICAL FINDINGS & CLINICAL PRESENTATION
• May occur at any site, but most are anatomically related to mucosal surfaces
• Should be suspected when there is foul-smelling tissue, soft-tissue gas, necrotic tissue, or abscesses
• Head and neck
1. Odontogenic infections from dental or soft tissue possibly progressing to periapical abscesses, at times extending to bone

2. Both anaerobic and aerobic pathogens in chronic sinusitis, chronic mastoiditis, and chronic otitis media
3. Peritonsillar abscess possible
4. Complications: deep neck space infections, brain abscesses, mediastinitis
• Pleuropulmonary
1. May involve anaerobes present in the oropharynx
2. Aspiration more common in persons with altered mental status or seizures
3. Anaerobic bacteria more likely in those with gingivitis or periodontitis
4. Manifestations: necrotizing pneumonia, empyema, lung abscess

5. Osteomyelitis especially when associated with decubitus ulcers or vascular insufficiency
6. Facial bone osteomyelitis from adjacent infections of the teeth or sinuses
ETIOLOGY
• Most commonly endogenous, arising from bacteria that normally line mucosal surfaces
• Disruption of mucosal barriers resulting from various conditions (trauma, ischemia, surgery, perforation), with infection occurring when organisms gain access to normally sterile sites, causing tissue destruction and abscess formation
• Synergy between different anaerobes or between anaerobes and aerobes important
• Most commonly involved: gram-negative anaerobic bacilli
WORKUP
• Specimens submitted for culture processed within 30 min
• Large volume of material more likely to have significant growth; swabs less efficient for transporting infected material
• Blood cultures-preferably before antibiotic administration
LABORATORY TESTS
• Elevated WBC count, with extremely high WBC counts sometimes seen with pseudomembranous colitis
• Positive stool C. difficile toxin assay
• Increased lactate levels in ischemia or perforation
• Possible positive blood or wound cultures, but failure to grow anaerobes in culture may be common, attributed to inadequate culturing techniques and/or fastidious organisms
IMAGING STUDIES
• Plain film of an affected area to show gas in tissues, free air resulting from a perforated viscus, or an air/fluid level inside an abscess
• Ultrasound, CT scan, or MRI to reveal abscesses or tissue destruction
TREATMENT
NONPHARMACOLOGIC THERAPY
• Removal of necrotic tissue
• Drainage of abscesses (accomplished by CT scan-guided percutaneous drainage)
ACUTE GENERAL Rx
Oral antibiotics with anaerobic activity: clindamycin, metronidazole, and chloramphenicol
• Broader spectrum of activity with amoxicillin/clavulanate
• Penicillin VK in odontogenic infections
• Oral metronidazole for C. difficile-associated diarrhea, with oral vancomycin reserved for recurrent or recalcitrant infections
Parenteral antibiotics for more serious illness
• IV clindamycin, metronidazole, and chloramphenicol
• Cephalosporins (anaerobic or mixed infections): cefoxitin and cefotetan
• Extended-spectrum penicillins (i.e., piperacillin) and combination b-lactamase plus b-lactamase inhibitor drugs
1. Significant anaerobic activity, plus various degrees of broad-spectrum coverage
2. Include ampicillin/sulbactam, ticarcillin/clavulanate, and piperacillin/tazobactam
• Imipenem: a broad-spectrum agent with extensive anaerobic activity
• Actinomycosis treated with penicillin for 6 to 12 mo
• SMX/TMP and fluoroquinolones: ineffective

• Intraabdominal
1. Disruption of intestinal integrity leading to infection involving anaerobic bacteria
2. Bacteria from colonic neoplasm, perforated appendicitis, diverticulitis, or bowel surgery, causing bacteremia, peritonitis, at times intraabdominal abscesses
3. Resulting infections usually mixed, containing both anaerobes and aerobes

• Female genital tract
1. Anaerobes in bacterial vaginosis, salpingitis, endometritis, pelvic abscesses, septic abortion; infections tend to be mixed
2. Possible pelvic thrombophlebitis when resolving pelvic infection is accompanied by new or persistent fever
• Other anaerobic infections
1. Skin and soft-tissue infection at any site
2. More commonly associated infections: synergistic gangrene, bite wound infections, infected decubitus ulcers
3. Clinical significance of anaerobes in diabetic foot infections unclear
4. Anaerobic bacteremia uncommon with source usually intraabdominal, followed by female genital tract, pleuropulmonary, and head and neck infections