Question
Nagler reaction.
Answer
Nagler’s reaction is a laboratory test that demonstrates lecithinase (phospholipase C/alpha-toxin) activity, used most classically to identify Clostridium perfringens and confirm its role in gas gangrene.
Principle: alpha-toxin (lecithinase C) hydrolyses lecithin present in egg-yolk agar, producing an opaque zone of turbidity around bacterial growth (due to the resulting insoluble diglyceride/fatty-acid products). This lecithinase activity can be specifically inhibited by pre-treating half of an egg-yolk agar plate with Clostridium perfringens type A antitoxin.
Procedure: an egg-yolk agar plate is divided into two halves — one half is spread with specific C. perfringens antitoxin, the other half left untreated; the test organism is streaked across both halves and the plate incubated anaerobically. On the untreated half, the organism produces an opaque zone of lecithinase-mediated precipitation around its growth; on the antitoxin-treated half, this opacity is inhibited (no zone, or a reduced zone), since the antitoxin neutralizes the toxin before it can act on the lecithin substrate.
Interpretation: a positive Nagler reaction (opacity on the untreated half, inhibited on the antitoxin half) confirms lecithinase (alpha-toxin) production, supporting identification of C. perfringens and confirming toxin production relevant to its pathogenicity in gas gangrene.
Clinical significance: alpha-toxin is the principal virulence factor in C. perfringens gas gangrene, causing membrane damage, tissue necrosis, and haemolysis — Nagler’s reaction thus links laboratory identification directly to the organism’s pathogenic mechanism.