Anaerobes, Actinomyces, Nocardia & Gardnerella

Branching filaments: Actinomyces and Nocardia

Shared look, different lives. Both are gram-positive rods that grow as branching filaments, but they differ in oxygen use, acid-fastness, source, and treatment.

Actinomycosis. The cervicofacial form ("lumpy jaw") follows dental extraction, poor dentition, or jaw trauma: a hard, slowly enlarging mass near the angle of the jaw that forms abscesses and sinus tracts crossing tissue planes and draining pus with yellow sulfur granules, which are tangles of filaments. Thoracic disease follows aspiration, abdominal disease follows bowel surgery or perforation, and pelvic abscesses occur with long-term IUD use. Because it infiltrates like a tumor, it is sometimes diagnosed only on a resected mass. Treatment is high-dose penicillin for weeks to months plus drainage or debridement.

Nocardiosis. Inhaled Nocardia causes nodular or cavitary pneumonia in immunocompromised hosts that is easily mistaken for tuberculosis, and it spreads hematogenously to the brain, so brain imaging is warranted in every case of pulmonary disease. In immunocompetent people, soil entering a wound causes cellulitis, lymphocutaneous nodules, or mycetoma. TMP-SMX is given for months, with combination therapy (for example, adding imipenem) for severe or CNS disease.

Anaerobes above and below the diaphragm

Anaerobic infection clues. Obligate anaerobes lack enzymes such as catalase and superoxide dismutase that detoxify oxygen radicals. Their infections are usually polymicrobial, foul-smelling, abscess-forming, and sometimes gas-producing. Aminoglycosides are ineffective against them because drug uptake needs oxygen-dependent transport.

Below the diaphragm. Bacteroides fragilis is a bile-tolerant anaerobic gram-negative rod of colonic flora and the key anaerobe in intra-abdominal and pelvic abscesses and in peritonitis after perforation. Its polysaccharide capsule promotes abscess formation, and its β-lactamase inactivates penicillin. Metronidazole is the classic drug; piperacillin-tazobactam or a carbapenem also cover it.

Above the diaphragm. Oral anaerobes (Prevotella, Fusobacterium, Peptostreptococcus) are aspirated by patients with alcohol use, seizures, stroke-related dysphagia, or poor dentition. Pneumonia develops in dependent segments and can cavitate into a lung abscess with putrid sputum and an air-fluid level. Clindamycin or ampicillin-sulbactam is used; metronidazole alone fails because aerobic and microaerophilic streptococci are also present.

Lemierre syndrome. In adolescents and young adults, Fusobacterium necrophorum spreads from pharyngitis or a peritonsillar abscess into the parapharyngeal space and causes septic thrombophlebitis of the internal jugular vein: persistent fever, lateral neck pain and swelling, and septic pulmonary emboli seen as multiple cavitating nodules. Contrast CT of the neck shows the thrombus, and prolonged IV antibiotics with anaerobic coverage are required.

Cutibacterium acnes. This aerotolerant anaerobic gram-positive rod of sebaceous follicles contributes to acne and causes indolent infections of CSF shunts and prosthetic joints, especially the shoulder. It is also a frequent blood culture contaminant. It grows slowly, so cultures from suspected device infections are held for about 2 weeks.

Gardnerella and bacterial vaginosis

Pathogenesis. Bacterial vaginosis is a shift in vaginal flora rather than an infection with one organism: lactobacilli, which keep the pH low with lactic acid and H2O2, decline, and Gardnerella vaginalis, a gram-variable rod, overgrows along with other anaerobes that release volatile amines. It is linked to sexual activity but is not a classic STI, and it causes no inflammation.

Diagnosis (Amsel criteria, 3 of 4). Thin, homogeneous gray-white discharge; vaginal pH above 4.5; a positive whiff test (fishy odor when KOH is added); and clue cells, which are epithelial cells whose borders are obscured by adherent coccobacilli. Few WBCs are seen, and there is no vulvar itching or erythema. The odor worsens after intercourse or menses, when alkaline fluid releases the amines.

Treatment. Symptomatic women, including pregnant women, receive oral or vaginal metronidazole or vaginal clindamycin; male partners are not routinely treated. BV raises the risk of preterm birth, post-procedure PID, and acquisition of STIs including HIV.

Weakly acid-fast plus aerobic means Nocardia and sulfonamides; not acid-fast plus anaerobic means Actinomyces and penicillin.