A 45-year-old man with poor dentition develops subacute infective endocarditis involving the mitral valve. Blood cultures grow alpha-haemolytic, optochin-resistant Gram-positive cocci in chains. Which organism is the most likely cause?

• A Streptococcus pneumoniae
• B Streptococcus pyogenes (Group A)
• C Enterococcus faecalis
• D Viridans group Streptococcus ✓

Explanation

Viridans group Streptococci (e.g. S. mitis, S. sanguis) are commensals of the oral cavity and the most common cause of subacute bacterial endocarditis, particularly in patients with valvular abnormalities or poor dentition. They are alpha-haemolytic and optochin-resistant, distinguishing them from S. pneumoniae which is optochin-sensitive. S. pyogenes is beta-haemolytic. Enterococcus causes endocarditis but is bile-esculin positive and grows in 6.5% NaCl.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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