Cystine-tellurite blood agar plates are recommended for isolation of which organism?

• A. Group D streptococci
• B. Corynebacterium diphtheriae
• C. Streptococcus pyogenes
• D. Streptococcus agalactiae

Answer: (B)

Cystine-tellurite blood agar is used because it combines selective inhibition with differential visibility for Corynebacterium diphtheriae. The tellurite component suppresses many other bacteria, while C. diphtheriae reduces tellurite to a dark, often black, colony appearance (frequently with a brown halo) on a blood-containing medium. The cystine in the medium supports the growth of Corynebacteria, helping them form distinct colonies suitable for identification. This makes the medium especially useful for isolating C. diphtheriae from clinical specimens like throat swabs where other flora would overwhelm standard media. After isolation, additional testing (for example, to determine toxin production) is used to confirm toxigenicity. Other organisms such as streptococci do not produce the characteristic black colonies on this medium and are not selectively isolated by it.