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 alpha 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:

Ans. is 'c' i.e. Optochin Respiratory tract infection in an infant, together with the presence of alpha hemolytic gram-positive cocci suggests the presence of Streptococcus pneumonia The sensitivity of pneumococci to optochin is useful in differentiating it from other alpha hemolytic streptococci. Streptococcus pneumoniae is susceptible to optochin while other alpha hemolytic streptococci are not sensitive to optochin. If the diagnosis is still not certain, streptococcus pneumococci can be diagnosed by its bile solubility. Also know Classification of Streptococci and Enterococci Hemolytic reaction Lancefield group Species A Streptococcus pyogenes B Streptococcus agalactiae Beta C S. dysgalactia subspecies D Enterococcus species Small colony variants of A, C, F, G or ungroupable Anginosus group D Enterococcus species D Streptococcus bovis Alpha or Gamma Small colony variants of A, C, F, Anginosus group G or ungroupable None Streptococcus pneumoniae None Viridans streptococci