An infant had a high-grade fever and respiratory distress at the time of presentation to the emergency room. The sample collected for blood culture was subsequently positive showing growth of a haemolytic colonies. On Gram staining, these were gram-positive cocci. In the screening test for identification, the suspected pathogen is likely to be susceptible to the following agent: (A) Bacitracin, (B) Novobiocin, (C) Optochin, (D) Oxacillin
Answer is C. The pathogen is likely to be streptococcus pneumoniae, which is alpha hemolytic ( greenish discoloration due to incomplete hemolysis ) and gram-positive. Streptococcus pneumoniae is optochin sensitive. Optochin sensitivity: the sensitivity of Streptococcus pneumoniae to optochin is useful in differentiating it from other streptococci. When a disc impregnated with optochin is applied on a plate of blood agar inoculated with Streptococcus pneumoniae, a wide zone of inhibition appears on incubation. Reference: Textbook of Microbiology; Anathanarayan and panicker's; 10th edition; Page no: 225