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Non-Sporing Anaerobes - Complete Notes

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Overview and General Characteristics

• Fail to grow in the presence of 10% O2; some killed by concentrations as low as 0.5%
• Lack cytochromes needed to use oxygen as terminal electron acceptor - generate energy solely by fermentation
• Most lack catalase and superoxide dismutase (enzymes that neutralize H2O2 and superoxide) - hence killed by O2
• Virulent pathogens (e.g., B. fragilis) do produce catalase/SOD as a virulence advantage
• Many have fastidious nutritional requirements and grow slowly on culture media

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Classification of Non-Sporing Anaerobes

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A. Gram-Negative Rods

1. Bacteroides fragilis Group

• Anaerobic, pleomorphic gram-negative rod (resembles mixed population on Gram stain)
• Growth is stimulated by bile (characteristic of the Bacteroides genus)
• Has typical gram-negative cell wall but LPS lacks endotoxin activity - lipid A lacks phosphate groups on glucosamine residues and has fewer fatty acids
• Surrounded by polysaccharide capsule - major virulence factor
• Normal stools contain 10¹¹ B. fragilis per gram (vs 10⁸/g for facultative anaerobes)
• Other B. fragilis group members: B. thetaiotaomicron, B. ovatus, B. vulgatus, B. distasonis

• Colonizes the gastrointestinal tract as a minor microbiome member
• Rare or absent from oropharynx or genital tract of healthy individuals
• Produces endogenous infections when gut mucosa is breached

• Polysaccharide capsule - antiphagocytic; stimulates leukocyte migration and abscess formation
• Fimbriae - adhere to epithelial cells, fibrinogen, fibronectin, lactoferrin
• Short-chain fatty acids (succinic acid) - inhibit phagocytosis
• B. fragilis toxin (fragilysin) - heat-labile zinc metalloprotease; causes diarrheal disease by F-actin rearrangement in intestinal epithelium, stimulates chloride secretion, fluid loss, and IL-8 secretion

• Intraabdominal abscesses (most common - perforated appendix, diverticulitis, post-surgical)
• Pleuropulmonary infections
• Soft-tissue infections, skin infections
• Bacteremia
• Brain abscess (less common)
• Diarrheal disease (enterotoxigenic strains)

• Direct Gram stain from clinical specimens (faintly staining pleomorphic GNRs)
• Grows rapidly in anaerobic culture (unlike other anaerobic GNRs)
• Identification by biochemical tests, gene sequencing, or MALDI-TOF mass spectrometry

• Metronidazole - uniformly susceptible
• Carbapenems (imipenem, meropenem) - active against most strains
• Piperacillin-tazobactam, β-lactam + β-lactamase inhibitors
• Resistant to penicillin (β-lactamase producer)
• ~25% isolates resistant to clindamycin
• Resistant to most cephalosporins (exception: cefotaxime)
• Primary treatment: surgical drainage + antibiotics

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2. Prevotella

• Small gram-negative rods; previously classified as Bacteroides
• Normal inhabitants of the oral cavity, GI tract, and female genitourinary tract
• Pigmented species: Prevotella melaninogenica, P. intermedia - produce black colonies on blood agar (due to protoporphyrin)
• Other species: P. bivia, P. disiens (gynecologic infections)

• Fimbriae for adhesion
• Proteases that degrade immunoglobulins
• Short-chain fatty acids inhibit phagocytosis

• Oral/head and neck infections (P. melaninogenica, P. intermedia)
• Gynecologic infections - pelvic abscesses, endometritis (P. bivia, P. disiens)
• Aspiration pneumonia, lung abscess
• Brain abscess (polymicrobial)
• Associated with periodontal disease (P. intermedia)

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3. Porphyromonas

• Very small gram-negative rods; previously classified as Bacteroides
• Colonize oral cavity and genitourinary tract
• P. gingivalis - key periodontal pathogen; produces gingipains (cysteine proteases)
• P. asaccharolytica - involved in head/neck and gynecologic infections

• Fimbriae of P. gingivalis induce proinflammatory cytokines (TNF-α, IL-1β)
• Proteases degrade immunoglobulins and complement
• Black-pigmented colonies

• Periodontal disease (P. gingivalis is major pathogen)
• Head and neck abscesses
• Gynecologic infections

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4. Fusobacterium

• Long, thin gram-negative rods with tapered ("fusiform") ends - spindly morphology
• ~13 species; most human infections caused by F. nucleatum and F. necrophorum
• Normal inhabitants of the oral cavity

• Found in dental plaque; recurrent tonsillitis, brain abscess, bacteremia
• Female genital tract infections

• Causes Lemierre syndrome (postanginal sepsis):
  • Pharyngitis/tonsillitis → septic thrombophlebitis of internal jugular vein → metastatic septic emboli (lungs, joints, liver)
  • Classically in young adults
  • Unlike other anaerobic infections, Fusobacterium is often the sole organism isolated in Lemierre syndrome
• Also causes liver abscess, soft-tissue infections

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B. Gram-Negative Cocci

Veillonella

• Small gram-negative cocci that may resemble Neisseria on Gram stain
• Normal flora of the oral cavity, intestine
• Rarely pathogenic; may be isolated from mixed infections
• Inhabit oropharynx, GI tract; occasionally cause endocarditis, bacteremia, meningitis

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C. Gram-Positive Cocci

Peptostreptococcus / Anaerobic Gram-Positive Cocci (AGPC)

• Long chains of tiny cocci on Gram stain
• Reclassified from a single genus (Peptostreptococcus) into multiple genera based on gene sequencing
• Normal flora of oral cavity, GI tract, genitourinary tract, and skin
• Specific identification of different genera generally unnecessary clinically

• Oral/upper airway origin: sinusitis, pleuropulmonary infections
• Intestinal origin: intraabdominal infections
• Genitourinary origin: endometritis, pelvic abscess, salpingitis
• Skin origin: cellulitis, soft-tissue infections
• Hematogenous spread: osteomyelitis, solid organ abscesses, endocarditis

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D. Gram-Positive Rods

1. Actinomyces

• Thin, branching, filamentous gram-positive rods
• NOT a true anaerobe - microaerophilic (some are aerotolerant)
• Normal flora of oral cavity, GI tract, and vagina
• Infections are characteristically chronic (weeks to months to develop)

• No classic toxins; pathogenicity due to invasion of damaged mucosal barriers

• Cervicofacial actinomycosis (most common form - 50-60%) - "lumpy jaw"; associated with poor dental hygiene, dental trauma
• Thoracic actinomycosis - aspiration; mimics lung cancer
• Abdominal/pelvic actinomycosis - associated with IUD use (intrauterine device); follows bowel surgery or trauma
• Infection characterized by: chronic granulomatous lesions → suppurative abscesses → sinus tracts
• "Sulfur granules" (yellowish granules in pus) = tangled mass of gram-positive filamentous rods embedded in amorphous mineral deposits (hallmark of actinomycosis)

• Difficult - specimens must avoid oral contamination
• Sulfur granules should be crushed and examined microscopically - shows gram-positive branching rods
• Culture requires prolonged incubation (1 week or more) under anaerobic/microaerophilic conditions
• Many diagnoses not confirmed by culture; often clinical/histological

• Prolonged high-dose penicillin (weeks to months) - drug of choice
• Alternative: amoxicillin, tetracycline, clindamycin
• Responds slowly to antibiotics

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2. Cutibacterium acnes (formerly Propionibacterium acnes)

• Gram-positive rod; aerotolerant anaerobe
• Normal flora of skin (hair follicles, sebaceous glands)
• Produces propionic acid from glucose fermentation

• Acne vulgaris (role in pathogenesis via inflammatory mediators)
• Opportunistic infections in patients with prosthetic devices or intravascular lines (e.g., prosthetic joint infections, endocarditis, post-neurosurgical infections)

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3. Bifidobacterium & Eubacterium

• Normal flora of the GI tract
• Rarely pathogenic; may be found in mixed infections
• Bifidobacterium found in infant intestines (first colonizers)

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E. Laboratory Diagnosis of Non-Sporing Anaerobes

• High-quality specimen: pus or fluid directly from infected site
• Must be transported in oxygen-free containers (anaerobic transport tubes) or syringe with all air expelled
• Cannot use swabs (contaminate with surface flora; expose to O2)
• Generous collection of pus = best transport medium

• Direct smear showing pleomorphic gram-negative and/or gram-positive bacteria is highly suggestive of anaerobic infection
• Most useful test for clinical decision-making (culture results are delayed)

• Anaerobic incubation atmosphere required (anaerobic jar with reducing agents)
• Enriched media (cysteine, thioglycollate, growth factors)
• Selective media with aminoglycosides (to which ALL anaerobes are resistant) to inhibit overgrowth by Enterobacteriaceae
• Incubation for 5-7 days (B. fragilis grows faster; others may take 3+ days)
• Identification: morphology, biochemical tests, MALDI-TOF mass spectrometry, gas chromatography of metabolic end-products

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F. Treatment Principles

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G. Important Clinical Associations (High-Yield Summary)

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