A male patient presents with urethral discharge; Gram stain shows intracellular gram-negative diplococci within neutrophils. Culture on modified Thayer-Martin medium at 37°C in 5% CO2 grows oxidase-positive, catalase-positive colonies. To confirm Neisseria gonorrhoeae and rule out N. meningitidis, the definitive test is:

• A Oxidase test alone
• B Capsule detection by Quellung reaction
• C Bacitracin disk sensitivity test
• D Carbohydrate utilization: fermentation of glucose only (not maltose, sucrose, or lactose) ✓

Explanation

N. gonorrhoeae ferments only glucose; N. meningitidis ferments both glucose and maltose; N. lactamica also ferments lactose. Carbohydrate utilization (CTA sugars) is the definitive biochemical test to differentiate pathogenic Neisseria. Both GC and meningococcus are oxidase-positive and catalase-positive, so oxidase alone cannot differentiate them. Quellung reaction is used for pneumococcal capsular serotyping. Bacitracin sensitivity is used to identify S. pyogenes.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.