Streptococcus pyogenes. Coloured Scanning Electron Micrograph (SEM) of the bacteria Streptococcus pyogenes. These Gram-positive bacteria are rounded or spherical cocci. They typically form chains of cells, as seen here. Healthy humans may carry these bacteria harmlessly in their throat or nose. Pathogenic strains of S.pyogenes, however, cause infections of the skin (including impetigo); the uterus following childbirth (puerperal sepsis); and the bloodstream (septicaemia). S.pyogenes accounts for the high incidence of sore throats, tonsillitis, and middle ear infections (otitis media) in children. Treatment is with penicillin drugs. Magnification: x3250 at 6x6cm size.