70,000X magnification electron micrograph of an ultra-thin section of Streptococcus pyogenes. Image source: Dr. Vincent A. Fischetti, Rockefeller University.
Streptococcus is a genus of spherical or oval bacteria that grow in pairs or bead-like chains. Although some species of streptococci are harmless, others are responsible for many common infections, including strep throat, tonsillitis, scarlet fever, impetigo, cellulitis, erysipelas, and puerperal fevers. A related organism is a common cause of pneumonia and one type may cause endocarditis in damaged heart valves.