An Elek test is performed on a throat swab isolate suspected to be Corynebacterium diphtheriae. A precipitin line forms at 45° to the central antitoxin-impregnated filter paper strip. What does this finding indicate, and on which medium is colony morphology first assessed?

• A Non-toxigenic; precipitin lines are formed only by non-toxigenic strains on blood tellurite agar
• B Toxigenic strain; the precipitin line indicates toxin produced in Loeffler's serum slope medium and should be confirmed by Schick test
• C The Elek test result is invalid; precipitin lines at 45° indicate non-specific cross-reaction with C. ulcerans
• D Toxigenic strain; Elek test detects diphtheria toxin by immunodiffusion; colonies are assessed on Tellurite blood agar (McLeod's medium) for black-grey appearance ✓

Explanation

The Elek test (modified Elek-Ouchterlony) is an immunodiffusion test on special agar where the bacterial growth strip and the antitoxin filter paper strip are placed at right angles. A toxigenic strain produces diphtheria toxin (encoded by the tox gene carried by corynephage beta); toxin diffuses and forms a visible precipitin line with antitoxin at the intersection — classically at 45° to the strip. Primary isolation for colonial morphology uses Tellurite blood agar (McLeod's medium), where C. diphtheriae produces grey-black colonies due to tellurite reduction. The Schick test is a skin test for host immunity, not for strain toxigenicity.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.