streptococcus , any of a group of gram-positive bacteria, genus Streptococcus, some of which cause disease. Streptococci are spherical and divide by fission, but they remain attached and so grow in beadlike chains. The incidence and severity of streptococcal diseases decreased dramatically after the introduction of antibiotics (penicillin, erythromycin, and selected cephalosporins are all effective against the organisms), but the medical community was shaken by the arrival in the late 1980s of several severe forms of streptococcal infection and by the emergence of several drug-resistant strains (see drug resistance).
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