Streptococcus agalactiae (Group B Streptococcus) can be distinguished from Group A Streptococcus on blood agar by which test?

• A Sensitivity to bacitracin (0.04 unit disc) — GBS is sensitive
• B Optochin (ethylhydrocupreine) sensitivity — GBS is optochin-sensitive
• C Bile solubility — GBS dissolves in bile salts
• D CAMP test positivity — GBS produces a factor that enhances sphingomyelinase-C activity of Staphylococcus aureus ✓

Explanation

The CAMP test (Christie-Atkins-Munch-Petersen) is a rapid test for GBS identification. GBS produces a CAMP factor (a diffusible protein) that acts synergistically with the beta-toxin (sphingomyelinase-C) of S. aureus to produce enhanced haemolysis. In the test, GBS is streaked perpendicular to an S. aureus streak; an arrowhead-shaped zone of complete haemolysis at the intersection is positive. Group A Streptococcus is sensitive to bacitracin (0.04 unit disc) while GBS is resistant. Optochin sensitivity and bile solubility identify Streptococcus pneumoniae.

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

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