After a doubtful zone of inhibition around a penicillin disk for Streptococcus pneumoniae, which test should be performed to confirm the organism?

After a doubtful zone of inhibition around a penicillin disk for Streptococcus pneumoniae, which test should be performed to confirm the organism?

• A. Subculture onto another BAP
• B. Bile solubility test
• C. Incubate longer
• D. Pyridoxal-enhanced medium

Answer: (B)

The essential idea is using a test that exploits a unique property of Streptococcus pneumoniae to confirm its identity after a questionable penicillin susceptibility result. Pneumococcus is bile soluble: the bile salts activate its autolytic enzymes, causing the bacterial cells to lyse and the colonies to disappear or clear in a bile-containing environment. This sharp contrast with other alpha-hemolytic strep, like viridans, which remain intact in bile, makes the bile solubility test a reliable confirmatory method when the penicillin zone is doubtful. Subculturing onto blood agar would show colony appearance but won’t definitively identify the organism; incubating longer won’t reliably resolve identity, and pyridoxal-enhanced medium isn’t a standard confirmatory test for pneumococcus.

The essential idea is using a test that exploits a unique property of Streptococcus pneumoniae to confirm its identity after a questionable penicillin susceptibility result. Pneumococcus is bile soluble: the bile salts activate its autolytic enzymes, causing the bacterial cells to lyse and the colonies to disappear or clear in a bile-containing environment. This sharp contrast with other alpha-hemolytic strep, like viridans, which remain intact in bile, makes the bile solubility test a reliable confirmatory method when the penicillin zone is doubtful. Subculturing onto blood agar would show colony appearance but won’t definitively identify the organism; incubating longer won’t reliably resolve identity, and pyridoxal-enhanced medium isn’t a standard confirmatory test for pneumococcus.