In a sputum sample showing gram-positive diplococci, which test should be performed to presumptively identify the organism?

In a sputum sample showing gram-positive diplococci, which test should be performed to presumptively identify the organism?

• A. Elek test
• B. Hippurate hydrolysis
• C. Optochin test
• D. PYR test

Answer: (C)

Distinguishing Streptococcus pneumoniae from other Gram-positive cocci in respiratory samples hinges on optochin susceptibility. S. pneumoniae is inhibited by optochin, so a clear zone of inhibition around an optochin disk on a blood agar plate points to pneumococcus as the presumptive identity. Viridans streptococci, which can look similar in sputum, are generally resistant to optochin, so they do not show that inhibition.

This test is a quick, room-temperature way to separate the likely pneumococcus from other alpha-hemolytic streptococci, helping guide initial identification. Other tests listed serve different purposes: Elek detects diphtheria toxin production, hippurate hydrolysis helps differentiate certain streptococci like S. agalactiae, and the PYR test identifies group A streptococcus and enterococci. But for a Gram-positive diplococcus in sputum, optochin susceptibility provides the best presumptive identification.