[4][5] Peptostreptococcus species are commensal organisms in humans, living predominantly in the mouth, skin, gastrointestinal, vagina and urinary tracts, and are members of the gut microbiota.

Peptostreptococcus can cause brain, liver, breast, and lung abscesses, as well as generalized necrotizing soft tissue infections.

Based on DNA homology and whole-cell polypeptide-pattern study findings supported by phenotypic characteristics, the DNA homology group of microaerobic streptococci that was formerly known as Streptococcus anginosus or Streptococcus milleri is now composed of three distinct species: S. anginosus, S. constellatus, and S. intermedius.

[10] Peptostreptococcus infections occur in/on all body sites, including the CNS, head, neck, chest, abdomen, pelvis, skin, bone, joint, and soft tissues.

[9] The most common Peptostreptococcus species found in infections are P. magnus (18% of all anaerobic gram-positive cocci and microaerophilic streptococci), P. asaccharolyticus (17%), P. anaerobius (16%), P. prevotii (13%), P. micros (4%), P. saccharolyticus (3%), and P. intermedius (2%).

Most patients from whom microaerophilic streptococci were recovered in pure culture had abscesses (e.g., dental, intracranial, pulmonary), bacteremia, meningitis, or conjunctivitis.

Bacterial synergy, the presence of which is determined by mutual induction of sepsis enhancement, increased mortality, increased abscess inducement, and enhancement of the growth of the bacterial components in mixed infections, is found between anaerobic gram-positive cocci and their aerobic and anaerobic counterparts.

The ability of anaerobic gram-positive cocci and microaerophilic streptococci to produce capsular material is an important virulence mechanism, but other factors may also influence the interaction of these organisms in mixed infections.

In general, Peptostreptococcus species, particularly P. magnus, have been recovered more often from subcutaneous and soft tissue abscesses and diabetes-related foot ulcers than from intra-abdominal infections.

[9] It is difficult to determine the exact frequency of Peptostreptococcus infections because of inappropriate collection methods, transportation, and specimen cultivation.

Patients who have predisposing conditions are shown to have 5% higher recovery rate of the bacteria in blood cultures.

Anaerobic gram-positive cocci and microaerophilic streptococci are part of the normal skin microbiota, so it is hard to avoid contamination by these bacteria when obtaining specimens.

[citation needed] Microaerophilic streptococci typically account for 5-10% of cases of endocarditis; however, Peptostreptococci have only rarely been isolated.

[8][9] Anaerobic gram-positive cocci and microaerophilic streptococci are most frequently found in aspiration pneumonia, empyema, lung abscesses, and mediastinitis.

Like in upper respiratory tract and dental infections, anaerobic gram-positive cocci are recovered mixed with other bacteria.

In this case they are mixed with organisms of intestinal origin such as E coli, bacteroides fragilis group, and clostridium species.

Management of these infections requires prolonged courses of antimicrobials and is enhanced by removal of the foreign material.