A 62 year old patient presents with a persistent cough and rust-coloured sputum, fever, and shortness of breath. Sputum culture shows Gram-positive cocci in chains; the organism forms grey colonies with alpha hemolysis and a green halo on blood agar; catalase is negative; optochin shows growth; bile solubility is positive. Which organism is suspected, and what would you use for penicillin susceptibility testing?

Prepare for the CAMLPR Microbiology Test. Utilize flashcards and engaging multiple-choice questions, each equipped with descriptions and explanations. Ready yourself for an excellent performance!

Penicillin susceptibility testing for Streptococcus pneumoniae is typically started with a screening using an oxacillin disk on Mueller-Hinton agar with 5% blood. If the disk yields a clear zone, penicillin is likely active and further MIC testing isn’t needed; if the zone is reduced or absent, exact penicillin MIC should be determined, usually by an E-test or broth microdilution.

The organism in this scenario is Streptococcus pneumoniae, as suggested by the respiratory infection pattern and lab findings: Gram-positive cocci in chains, alpha-hemolysis with a greenish hue on blood agar, catalase negativity, and bile solubility positivity. These features—especially bile solubility—help distinguish pneumococcus from viridans streptococci, while the oxacillin screen provides a practical way to assess penicillin susceptibility. The other options describe organisms or testing approaches that don’t fit this combination of traits and testing strategy.