Laboratory Identification of Streptococcus pneumoniae

Bile solubility is a rapid, confirmatory test often used to differentiate S. pneumoniae from viridans streptococci, in which a drop of 10% deoxycholate is placed on an area of growth. The bacterial cells of S. pneumoniae are bile soluble, as shown by the disappearance (lysing) of the colonies in the area where a drop of 10% deoxycholate had been added to the surface of the agar (indicated by yellow arrows in the top image). The colonies of viridans streptococci would remain intact and visible.

Optochin or "P" disk susceptibility is a presumptive test using a 6mm filter paper disk impregnated with 5 µg of ethylhydrocupreine hydrochloride (optochin). In 2004, the discovery of the organism Streptococcus pseudopneumoniae made it essential to perform optochin testing under increased CO₂ conditions to differentiate it from S. pneumoniae. When testing is performed in an ambient atmosphere (35°C for 18–24 hours), both S. pneumoniae and S. pseudopneumoniae are susceptible to optochin (zone >14 mm). If testing is performed in 5% CO₂, S. pseudopneumoniae is resistant to optochin (zone ≤14 mm). Therefore, testing should be confirmed with bile solubility tests or serological identification. The table summarizes this testing.

Gram stain: Gram-positive cocci (GPC) in pairs, often demonstrating a lancet shape. The organisms can be inside or outside the polymorphonuclear (PMN) cells.