In a patient of orbital cellulitis, microorganism on culture shows greenish colonies and optochin sensitivity. The most likely organism is: (A) Streptococcus viridans, (B) Staphylococcus, (C) Pseudomonas, (D) Pneumococcus
Answer is D. Pneumococci or Streptococcus pneumoniae on blood agar, after incubation for 18 hours, colonies are small, dome-shaped, glistening with an area of green discoloration ( alpha hemolysis due to paial hemolysis). Fuher incubation gives flat colonies with raised edges and central combination ( draughtsman or carrom coin appearance). Optochin sensitivity: differentiating features of pneumococci from viridans streptococci. When a disc impregnated with optochin is applied on a plate of blood agar inoculated with pneumococci, a wide zone of inhibition appears on incubation. Reference: Textbook of Microbiology; Anathanarayan and panicker's; 10th edition; Page no: 225