15.5 Anaerobic & Spiral-Shaped Bacteria

Anaerobic Bacteria

Anaerobes are organisms that cannot grow in the presence of oxygen (strict anaerobes) or tolerate only low concentrations (aerotolerant). They constitute the vast majority of the colonic and oropharyngeal flora and become pathogenic when they escape their normal niche into sterile sites. Anaerobic infections classically produce foul-smelling pus (short-chain fatty acids), gas in tissues, and abscesses.

Anaerobe Classification by Niche

Bacteroides fragilis Group

Bacteroides fragilis is the most important anaerobe in intra-abdominal infection. Its polysaccharide capsule is a potent virulence factor that promotes abscess formation even in the absence of live bacteria. It also produces an enterotoxin in some strains (BFT) that contributes to diarrheal illness. Resistance to penicillin via beta-lactamase is typical, so metronidazole, piperacillin-tazobactam, or carbapenems are used; abscess drainage is essential.

Clostridium — Spore-Forming Anaerobes

Clostridium species are large Gram-positive, spore-forming strict anaerobes found in soil and the gut. Each major species is defined by a specific exotoxin.

Gas Gangrene (Clostridial Myonecrosis)

Following traumatic inoculation (contaminated wounds, crush injuries), C. perfringens spores germinate in necrotic, avascular tissue. Alpha-toxin destroys tissue membranes, and additional toxins (theta, mu, kappa) lyse RBCs, collagen, and hyaluronate. Clinically: severe pain, edema, gas crepitus on palpation, bronze discoloration, and bullae with serosanguineous, foul-smelling discharge. Surgical debridement plus penicillin G + clindamycin (clindamycin suppresses toxin synthesis) are cornerstones; hyperbaric oxygen is adjunctive.

Clostridioides difficile Colitis

Antibiotic disruption of colonic flora (classically clindamycin, also fluoroquinolones, cephalosporins) allows C. difficile overgrowth. Toxins A and B cause colitis with pseudomembranes (yellow-gray plaques on colonoscopy). Diagnosis combines stool GDH, toxin immunoassay, and NAAT (PCR for toxin genes). Treatment: discontinue the offending antibiotic; oral vancomycin or fidaxomicin for initial disease, bezlotoxumab (monoclonal antibody against toxin B) for recurrence reduction, and fecal microbiota transplantation for refractory/recurrent disease.

Spiral-Shaped Bacteria: Spirochetes

Spirochetes are thin, flexible, Gram-negative–like bacteria with axial filaments (endoflagella) that allow corkscrew motility. They are too thin to see on Gram stain; use dark-field microscopy or fluorescent antibody. Three genera matter for the PA-CAT: Treponema, Borrelia, and Leptospira.

Treponema pallidum — Syphilis

Treponema pallidum subspecies pallidum is transmitted sexually and transplacentally (congenital syphilis). It cannot be cultured in vitro, so diagnosis is serologic.

Stages of Syphilis

Syphilis Serology — Two-Tier Strategy

A classic paradox: neurosyphilis can occur at any stage, and CSF VDRL is specific but insensitive. Treatment of early syphilis is a single dose of benzathine penicillin G; tertiary and neurosyphilis require IV penicillin G for 10–14 days. The Jarisch-Herxheimer reaction — fever, chills, rash, hypotension within hours of treatment — is due to lysis of spirochetes releasing endotoxin-like products; it is self-limited and managed with antipyretics.

Congenital Syphilis

Transplacental transmission causes stillbirth, hydrops, or Hutchinson triad (notched incisors, interstitial keratitis, eighth-nerve deafness), plus saber shins and snuffles. All pregnant women are screened with RPR/VDRL early in pregnancy.

Borrelia — Relapsing Fever and Lyme Disease

Borrelia are larger than other spirochetes and can be Giemsa-stained in blood smears.

• Borrelia burgdorferi causes Lyme disease, the most common vector-borne disease in the United States. Transmitted by Ixodes scapularis (northeast/midwest) and I. pacificus (west) ticks; the white-footed mouse and white-tailed deer are reservoirs. The hallmark early sign is erythema migrans (EM) — an expanding annular 'bullseye' rash ≥5 cm at the tick-bite site, sometimes with central clearing.

  • Stage 1 (early localized): EM, flu-like symptoms.
  • Stage 2 (early disseminated): multiple EM, Bell palsy (often bilateral), atrioventricular block, arthritis, meningitis.
  • Stage 3 (late): chronic arthritis, acrodermatitis chronica atrophicans, encephalopathy.
  • Diagnosis clinically for EM; two-tier serology (ELISA then Western blot) for later disease.
  • Treatment: doxycycline (adults/children ≥8) or amoxicillin (young children, pregnant) early; ceftriaxone for carditis/neuroborreliosis.

• Borrelia recurrentis (louse-borne) and B. hermsii (tick-borne) cause relapsing fever with antigenic variation of variable major protein (Vmp) on the surface; episodes of fever clear when antibody selects variants, then relapse when a new variant expands.

Leptospira — Leptospirosis

Leptospira interrogans is a hooked-end spirochete transmitted by contact with water contaminated by animal urine (rats, dogs, livestock). It is a zoonosis with a biphasic illness: an initial leptospiremic phase (fever, headache, myalgia, conjunctival suffusion) followed by an immune phase. Severe disease (Weil disease) includes jaundice, renal failure, and hemorrhage. Diagnosis by serology (MAT) or PCR. Treatment: penicillin G or doxycycline; doxycycline prophylaxis is used for high-risk exposure.

Spirochete Quick Comparison