Streptococcus pyogenes bacteria, illustration. S. pyogenes is a Gram-positive spherical (coccus) bacterium. 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, this bacteria can occasionally cause severe and even life-threatening disease.