7.21 anaerobic bacteria

Clostridium perfringens

Gram (+), rod;
Non-motile;
Spore-forming;
Capsule+;
Obligate anaerobe;

Alpha toxin (lecithinase),
Enterotoxin (heat-stable),
Theta toxin (pore-forming);

NMF: Human intestine, soil, decaying vegetation, contaminated food;

Causes: Gas gangrene—rapidly spreading necrosis with gas formation; Food poisoning—diarrhea and abdominal cramps due to enterotoxin.

Clostridium tetani

Gram (+), rod;
non-capsulated
motile;
Spore-forming;
Obligate anaerobe;

Tetanospasmin toxin-(blocks inhibitory neurotransmitters, causing muscle spasms)

NMF: soil, intestinal tracts of animals;

Causes: Tetanus—muscle stiffness and spasms.

Clostridium botulinum

Gram (+), rod;
non-capsulated
motile (peritrichous flagella);
Spore-forming;
Obligate anaerobe;

Botulinum toxin;

NMF: soil, improperly canned foods;

Causes: Botulism—flaccid paralysis due to toxin.

Clostridium difficile

Gram (+), rod;
Motile (peritrichous flagella);
Spore-forming;
Capsule(-);
Obligate anaerobe;

Antibiotic-resistant, colonizes gut after microbiome disruption;

Toxin A (enterotoxin) & Toxin B (cytotoxin) disrupt intestinal epithelium;

NMF: Human intestines (opportunistic pathogen), hospital settings;

Pseudomembranous colitis—severe diarrhea, colonic inflammation after antibiotic use.

Bacteroides fragilis

Gram (-), rod;
Non-motile;
non-spore forming;
Capsule+;
Obligate anaerobe;

Most abundant gut anaerobe, opportunistic pathogen, antibiotic-resistant strains;

Capsular polysaccharide (immune evasion), Beta-lactamase (antibiotic resistance), Endotoxin (weaker than other Gram-negatives);

NMF: Human intestines, mucosal surfaces;

Intra-abdominal abscesses, sepsis, peritonitis.

Prevotella melaninogenica

Gram (-), rod;
Non-motile;
non-spore forming;
Capsule+;
Obligate anaerobe;

Produces black-pigmented colonies, involved in oral and respiratory infections;

Proteases (degrade host tissues),
Hemolysin;

NMF: Oral cavity, upper respiratory tract, genital tract;

Periodontal disease, aspiration pneumonia, brain abscesses.

Porphyromonas gingivalis

Gram (-), rod;
Non-motile;
non-spore forming;
Capsule+;
Obligate anaerobe;

Black-pigmented, associated with chronic periodontitis and systemic inflammation;

Fimbriae (adhesion),
Gingipains (proteolytic enzymes degrading host proteins);

NMF: Oral cavity, subgingival plaque;

Periodontal disease—gum inflammation, tissue destruction, tooth loss.

Fusobacterium nucleatum

Gram (-), rod;
Non-motile;
non-spore forming;
Capsule-;
Obligate anaerobe;

Long, spindle-shaped, key in biofilm formation, opportunistic pathogen;

Adhesins (FadA protein allows tissue invasion),
Endotoxin (triggers inflammation);

NMF: Oral cavity, gastrointestinal tract;

Periodontal disease, Lemierre’s syndrome (throat infection leading to septic thrombophlebitis).

Bifidobacterium bifidum

Gram (+), rod;
Non-motile;
non-spore forming;
Capsule-;
Obligate anaerobe;

Beneficial probiotic, fermentative metabolism, protects gut microbiome;

Exopolysaccharides (gut colonization),
Acid resistance (survives stomach passage);

NMF: Human intestines, fermented dairy products;

Supports digestion, prevents infections, modulates immune response.