Antibiotic susceptibility testing by the disk diffusion method (Kirby-Bauer) uses Mueller-Hinton agar. Which two special supplements must be added to Mueller-Hinton agar when testing Streptococcus pneumoniae and Haemophilus influenzae respectively?

• A S. pneumoniae: 5% sheep blood (blood agar base); H. influenzae: X+V factors (chocolated blood agar)
• B Both organisms: Mueller-Hinton broth alone is sufficient for disk diffusion
• C S. pneumoniae: Chocolate agar with NAD; H. influenzae: 5% sheep blood agar
• D Both organisms: Cation-adjusted Mueller-Hinton with 2–5% lysed horse blood and 20 mg/L NAD ✓

Explanation

EUCAST/CLSI guidelines specify that Mueller-Hinton agar supplemented with 5% defibrinated sheep blood (or lysed horse blood) and 20 mg/L NAD (beta-NAD) is used as Haemophilus test medium (HTM) for Haemophilus influenzae susceptibility testing, and the same cation-adjusted Mueller-Hinton + 5% sheep blood (without NAD) is used for Streptococcus pneumoniae. Both organisms are nutritionally fastidious and do not grow on plain Mueller-Hinton agar. The cation adjustment (Mg2+, Ca2+) is critical for accurate aminoglycoside MIC determination in all gram-negative bacteria.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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