A 65 years old man presenting with complains of chest pain, fever and cough with sputum. On examination of sputum, pus cells with the gram-positive organisms seen. How will you differentiate this from other gram-positive cocci?

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