Etiology of Necrotizing Soft-Tissue Infection There are two subtypes of necrotizing soft-tissue infection (NSTI): Type I Type II Type I NSTI, typically involving the torso and perineum, results from a polymicrobial infection usually including group A streptococci (eg, Streptococcus pyogenes) and a mixture of aerobic and anaerobic bacteria (eg, Bacteroides species).