Microbiology Cheat Sheet
Anaerobes & Necrotising Infection
Anaerobic and necrotising infections for NEET-PG: gas gangrene, Nagler reaction, tetanus vs botulinum toxin, Bacteroides fragilis, and necrotising fasciitis.
MedNext Academy | 3 min read
Anaerobes & Necrotising Infection
Anaerobic and necrotising infections for NEET-PG: gas gangrene, Nagler reaction, tetanus vs botulinum toxin, Bacteroides fragilis, and necrotising fasciitis.
This topic covers the classification of anaerobic bacteria, the conditions that permit anaerobic infection, clostridial toxin-mediated disease, and rapidly progressive necrotising soft tissue infection.
High-yield lines
- Anaerobic infections are almost always polymicrobial, because facultative aerobes consume oxygen and lower the redox potential to allow anaerobic growth.
- Clostridium perfringens causes about 90 percent of gas gangrene and produces alpha toxin, a lecithinase (phospholipase C) that destroys cell membranes and causes myonecrosis.
- The Nagler reaction detects C. perfringens alpha toxin by inhibition of lecithinase activity with specific antitoxin on egg yolk agar.
- C. perfringens on Gram stain shows large rectangular boxcar-shaped Gram-positive rods with conspicuously few neutrophils, because the toxins destroy leucocytes.
- Tetanospasmin from Clostridium tetani cleaves synaptobrevin, blocking release of inhibitory glycine and GABA, producing spastic paralysis.
- Botulinum toxin cleaves SNARE proteins and blocks acetylcholine release, producing descending flaccid paralysis, the opposite of tetanus.
- Clostridium tetani shows a terminal round spore giving a drumstick or tennis-racket appearance.
- Bacteroides fragilis is the most common anaerobe recovered from intra-abdominal abscess despite being a minor component of colonic flora, due to its abscess-promoting zwitterionic capsule.
- Fusobacterium necrophorum causes Lemierre syndrome, an internal jugular vein septic thrombophlebitis following pharyngitis with pulmonary septic emboli.
- Necrotising fasciitis Type I is polymicrobial (mixed aerobes and anaerobes), while Type II is monomicrobial due to Streptococcus pyogenes.
- Pain disproportionate to visible skin changes is the single most important early clinical clue to necrotising fasciitis.
- Foul-smelling pus with mixed organisms on Gram stain but no growth on aerobic culture indicates anaerobic infection.
- Gas gangrene and necrotising fasciitis are surgical emergencies where aggressive debridement, not antibiotics alone, is the definitive treatment.
- Actinomyces israelii is an anaerobic branching Gram-positive filamentous bacterium that produces sulphur granules and is not acid-fast, unlike aerobic Nocardia.
Mapped competency codes
- MI4.1
- MI4.3
Continue into the full chapter
This summary maps to MI4-musculoskeletal-skin-and-soft-tissue-infections.
Frequently Asked Questions
What is the most common cause of gas gangrene and its key toxin?
Clostridium perfringens, which produces alpha toxin, a lecithinase (phospholipase C) causing myonecrosis, haemolysis, and shock.
How does tetanus toxin differ from botulinum toxin in effect?
Tetanospasmin blocks inhibitory glycine and GABA causing spastic paralysis, while botulinum toxin blocks acetylcholine causing flaccid paralysis.
What is the earliest clinical clue to necrotising fasciitis?
Pain disproportionate to the visible skin findings, appearing before bullae, crepitus, and skin necrosis become evident.
Which anaerobe most often causes intra-abdominal abscess?
Bacteroides fragilis, whose zwitterionic capsular polysaccharide directly stimulates abscess formation despite being a minor part of colonic flora.
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