In a patient of orbital cellulitis, microorganism on culture shows greenish colonies and optochin sensitivity. The most likely organism is

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