The sputum specimen of a 70-year-old male was cultured on a 5% sheep blood agar. The culture showed a-haemolytic colonies next day. The fuher processing of this organism is most likely to yield

The organism is likely to be streptococcus pneumoniae which is gram-positive, diplococci in flame shape or lanceolate shape. Catalase test: Streptococcus pneumoniae does not produce catalase thus gives a negative catalase test. Bile solubility test: Streptococcus pneumoniae are bile soluble. If a few drops of 10% sodium deoxycholate solution are added to 1 ml of an overnight broth culture, the culture clears due to the Lysis of cocci. Alternatively, if a loopful of 10 % deoxycholate solution is placed on a Streptococcus pneumoniae colony on blood agar, the colony lyses within a few minutes. Bile solubility is a constant propey of Streptococcus pneumoniae and is of diagnostic impoance from other streptococci. Principal: based on the presence of an autolytic amidase in Streptococcus pneumoniae that cleaves the bond between an alanine and muramic acid in the peptidoglycan. The amidase is activated by surface active agents such as bile or bile salts. Reference: Textbook of Microbiology; Anathanarayan and panicker's; 10th edition; Page no: 224, 225