CSF culture from a child with meningitis yields alpha-haemolytic, Gram-positive diplococci. The isolate shows a zone of inhibition around an optochin disc and lysis after addition of sodium deoxycholate. These results identify the organism as:

• A Viridans group streptococcus
• B Streptococcus pneumoniae ✓
• C Enterococcus faecium
• D Streptococcus bovis

Explanation

S. pneumoniae is optochin susceptible and bile soluble because deoxycholate activates its autolytic enzyme LytA, which destroys the cell wall. Viridans streptococci share alpha-haemolysis and diplococcal morphology but are optochin resistant and bile insoluble, making that the key discriminating fact. Enterococci are catalase negative but grow in bile and 6.5 percent salt and are neither optochin sensitive nor bile soluble. S. bovis is associated with endocarditis and colonic lesions.

Reference: Jawetz, Melnick and Adelberg's Medical Microbiology, 27th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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