Streptococcus pneumoniae is the most commonly identified causative organism for bacterial pneumonia. Up to two-thirds of bacteremic community-acquired pneumonias are due to this organism. 5-25% of healthy individuals carry S. pneumoniae in their pharynx. The classic description of S. pneumoniae on Gram's stain is that given in the question stem. S. pneumoniae can be verified by the Quellung reaction, or counter immunoelectrophoresis to determine serotypes of isolated strains or for case detection using sputum specimens (there are more than 80 distinct serotypes based on studies of capsular antigens). Chlamydia pneumoniae is not identified on the basis of Gram's stain, but rather, can be seen with Giemsa or immunofluorescence. Haemophilus influenzae is a gram-negative rod. Staphylococcus aureus is also a gram-positive coccus, but the classic description of this organism typically includes a reference to "grape-like clusters." Also know: In 2000, a pneumococcal conjugate vaccine (PCV) was introduced in which polysaccharide was conjugated with protein. This 7-valent vaccine stimulates T-dependent TH2 responses and is effective beginning at 2 months of age and is thus the standard for childhood immunization. Ref: Ray C.G., Ryan K.J. (2010). Chapter 25. Streptococci and Enterococci. In C.G. Ray, K.J. Ryan (Eds), Sherris Medical Microbiology, 5e.