Animation of Streptococcus pyogenes bacteria. S. pyogenes, or group-A Streptococcus, is a gram-positive spherical (coccus) bacterium. Humans may carry it harmlessly on the skin or in the throat. Pathogenic strains can cause mild skin infections such as impetigo, uterine infection after childbirth, and blood poisoning (septicaemia). However, they can occasionally cause severe and even life-threatening disease and Streptococcus group A1 can be a deadly form of flesh-eating bacteria.