Peptococcus bacteria, computer illustration. Gram-positive bacteria. As in the case of other species of Streptococcus, these bacteria are characterized by their tendency to form chains. Anaerobic streptococci are normal commensals on the mucous membranes such as the vagina and large intestines. They become pathogenic in association with other anaerobic bacteria, causing gangrene around surgical wounds, puerperal sepsis (infection of the lining of the womb or vagina following childbirth) and septicaemia (blood poisoning).