obligate anaerobe

**Definition & mechanism:** Obligate anaerobes lack an electron transport chain that uses oxygen as the terminal electron acceptor. They also lack or have very little superoxide dismutase (SOD), catalase, and peroxidase. Oxygen exposure generates toxic reactive oxygen species (superoxide, hydrogen peroxide) that damage the cells. They ferment or use other terminal electron acceptors. Aerotolerant anaerobes (e.g., Clostridium tertium, some Lactobacillus) tolerate O2 but do not use it.

**Classic obligate anaerobes (high-yield):** - *Clostridium* species (gram-positive, spore-forming rods): C. perfringens (gas gangrene, food poisoning), C. tetani (tetanus), C. botulinum (botulism), C. difficile (pseudomembranous colitis). Mnemonic: "Anaerobes Can't Breathe Air": Actinomyces, Clostridium, Bacteroides. - *Bacteroides fragilis* (gram-negative rod): most common anaerobe in the GI tract. It causes intra-abdominal and pelvic abscesses. Its polysaccharide capsule promotes abscess formation. - *Actinomyces israelii* (gram-positive, branching filamentous rod): cervicofacial "lumpy jaw" abscesses with draining sinus tracts and sulfur granules. It is not acid-fast, unlike Nocardia. - *Fusobacterium* (gram-negative, spindle-shaped): oral flora, Lemierre syndrome (septic thrombophlebitis of the internal jugular vein), aspiration pneumonia and lung abscess. - *Prevotella*, *Peptostreptococcus*, *Cutibacterium (Propionibacterium) acnes* (acne, prosthetic device infections).

**Clinical clues for anaerobic infection:** - Foul-smelling discharge or pus (short-chain fatty acid and sulfur compound production) - Gas in tissue (crepitus) - Abscess formation - Infection adjacent to mucosal surfaces (oral, GI, vaginal flora) - Necrotic or poorly perfused tissue (low redox potential) - Aspiration pneumonia and lung abscess (typically in dependent lung segments) - Cultures that are negative on routine aerobic media despite Gram stain showing organisms

**Lab/diagnosis:** Culture requires anaerobic transport and incubation (anaerobic chamber, GasPak jar, or thioglycolate broth). In thioglycolate broth, obligate anaerobes grow at the bottom of the tube, where oxygen is lowest. Obligate aerobes grow at the top, and facultative anaerobes grow throughout, with the most growth at the top.

**Pharmacology:** - **Aminoglycosides are ineffective** against anaerobes because uptake across the cell membrane requires oxygen-dependent transport. - Effective drugs: metronidazole (forms free radicals that damage DNA in anaerobic conditions; also covers Giardia, Entamoeba, Trichomonas, Gardnerella), clindamycin (good for above the diaphragm, such as aspiration and lung abscess), beta-lactam/beta-lactamase inhibitor combinations (ampicillin-sulbactam, piperacillin-tazobactam), carbapenems, and cefoxitin/cefotetan. - Treatment of abscesses also requires **source control (drainage/debridement)**.

**Comparison of oxygen requirements:** - Obligate aerobes (Pseudomonas, Mycobacterium tuberculosis, Nocardia, Bacillus): require O2 for ATP generation. M. tuberculosis favors the high-O2 upper lobes. - Facultative anaerobes (E. coli, Staphylococcus, Streptococcus, Enterobacteriaceae): use O2 if present, otherwise ferment or use alternative acceptors. - Microaerophiles (Campylobacter, Helicobacter): need low O2 (about 5%). - Obligate anaerobes: killed or inhibited by O2.