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

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