Tellurite reduction in corynebacteria cultures produces which characteristic appearance on Hoyle's tellurite blood agar, and what does this indicate?
- A Black/grey colonies; ability to reduce potassium tellurite — used for selective isolation of C. diphtheriae ✓
- B Yellow colonies; presence of urease enzyme
- C Green colonies; presence of pyoverdin pigment
- D Blue colonies; oxidase positivity
Correct answer: A. Black/grey colonies; ability to reduce potassium tellurite — used for selective isolation of C. diphtheriae
Explanation
Corynebacterium diphtheriae reduces potassium tellurite to metallic tellurium, producing black or grey colonies on Hoyle's medium, which also contains blood and lithium chloride to inhibit normal flora. This selective property allows C. diphtheriae to be distinguished from most other throat commensals. The colonial appearance also helps identify biotypes (gravis = grey, flat, rough; mitis = black, smooth; intermedius = small, dark). Pyoverdin is a fluorescent pigment of Pseudomonas aeruginosa; urease and oxidase are not properties of C. diphtheriae.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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