Sputum from a 60-year-old man with lobar pneumonia shows lancet-shaped Gram-positive diplococci. The isolate is susceptible to optochin and is lysed within minutes on addition of bile salts. The biochemical basis of bile solubility in this organism is:
- A Inhibition of peptidoglycan cross-linking by bile salts
- B Dissolution of the polysaccharide capsule by bile hydrolases
- C Activation of an autolytic amidase (LytA) that destroys the cell wall ✓
- D Disruption of teichoic acid anchoring in the cell membrane
Correct answer: C. Activation of an autolytic amidase (LytA) that destroys the cell wall
Explanation
The organism is Streptococcus pneumoniae, identified by optochin susceptibility and bile solubility. Bile salts activate the autolytic enzyme LytA, an amidase that cleaves the bond between muramic acid and L-alanine in peptidoglycan, causing the cell to lyse itself. This property separates pneumococcus from optochin-resistant viridans streptococci. Bile does not dissolve the capsule, and the capsule is demonstrated separately by the Quellung reaction.
Reference: Jawetz, Melnick and Adelberg's Medical Microbiology, 28th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.