lancet

On Gram stain of sputum or cerebrospinal fluid, S. pneumoniae appears as pairs of elongated cocci with tapered ends resembling the point of a lance, surrounded by a clear capsular halo. Its polysaccharide capsule is the principal virulence factor, resisting phagocytosis; it is the basis of the quellung reaction, in which type-specific antiserum causes apparent capsular swelling, and of both vaccines — the conjugate vaccine, which produces T-cell-dependent immunity and immunological memory and works in infants, and the older polysaccharide vaccine, which is T-cell independent and ineffective under two years of age. S. pneumoniae is alpha-haemolytic, optochin-sensitive, and bile-soluble, which distinguishes it from viridans streptococci, which are optochin-resistant and bile-insoluble. It is the leading cause of community-acquired pneumonia, otitis media in children, bacterial meningitis in adults, and sinusitis, summarized by the mnemonic MOPS. Asplenic and sickle cell patients are at particular risk of overwhelming pneumococcal sepsis, hence the vaccination requirement.