Streptococcus pyogenes bacteria. Coloured transmission electron micrograph (TEM) of a section through Streptococcus pyogenes, or group-A Streptococcus, bacteria. S. pyogenes is a gram-positive spherical (coccus) bacteria. Many humans may carry it harmlessly in their throat or on the skin. 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 - Streptococcus group A1 can be a deadly form of flesh eating bacteria.