Paper I — 2013 September (Supplementary) (2010 Scheme) — Q4
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Paper I
2013 September (Supplementary) (2010 Scheme) · 40 marks · 120 min
Question
Eleke’s gel precipitation test
Q42 marksShort Notes
Answer
Elek’s gel precipitation test is used to detect toxigenicity of Corynebacterium diphtheriae — i.e., whether an isolated strain actually produces diphtheria exotoxin.
Principle/procedure: a strip of filter paper impregnated with diphtheria antitoxin is embedded across a Petri dish of serum agar. The test strain (along with known toxigenic and non-toxigenic control strains) is streaked at right angles to the strip and the plate incubated at 37°C for 24–48 hours. As the organism grows, secreted toxin diffuses outward through the agar and meets antitoxin diffusing from the paper strip; at the zone of equivalence, a visible line of precipitation forms at an angle to the growth streak.
Interpretation: a precipitin line continuous with that of the known toxigenic control confirms the strain is toxigenic; its absence indicates a non-toxigenic strain. Since only toxigenic strains cause the classical exotoxin-mediated disease (myocarditis, neuropathy, pseudomembrane formation), this test is essential for confirming clinical significance of an isolate, independent of its biotype (gravis/intermedius/mitis).
