Gram-positive anaerobic cocci

Abstract

Gram-positive anaerobic cocci (GPAC) are a heterogeneous group of organisms defined by their morphological appearance and their inability to grow in the presence of oxygen; most clinical isolates are identified to species in the genus Peptostreptococcus. GPAC are part of the normal flora of all mucocutaneous surfaces and are often isolated from infections such as deep organ abscesses, obstetric and gynecological sepsis, and intraoral infections. They have been little studied for several reasons, which include an inadequate classification, difficulties with laboratory identification, and the mixed nature of the infections from which they are usually isolated. Nucleic acid studies indicate that the classification is in need of radical revision at the genus level. Several species of Peptostreptococcus have recently been described, but others still await formal recognition. Identification has been based on carbohydrate fermentation tests, but most GPAC are asaccharolytic and use the products of protein degradation for their metabolism; the introduction of commercially available preformed enzyme kits affords a physiologically more appropriate method of identification, which is simple and relatively rapid and can be used in routine diagnostic laboratories. Recent reports have documented the isolation in pure culture of several species, notably Peptostreptococcus magnus, from serious infections. Studies of P. magnus have elucidated several virulence factors which correlate with the site of infection, and reveal some similarities to Staphylococcus aureus. P. micros is a strongly proteolytic species; it is increasingly recognized as an important pathogen in intraoral infections, particularly periodontitis, and mixed anaerobic deep-organ abscesses. Comparison of antibiotic susceptibility patterns reveals major differences between species. Penicillins are the antibiotics of choice, although some strains of P. anaerobius show broad-spectrum beta-lactam resistance.

FIG. 1

Clustering of 26 reference and 101 clinical strains of GPAC on the basis of whole-cell composition as assessed by PyMS. Numbered but unnamed clusters did not contain type strains of recognized or proposed species. Adapted from reference with permission of the publisher.

FIG. 2

Phylogenetic tree constructed by the neighbor-joining method, showing the position of Peptostreptococcus species within Clostridium rRNA clusters XI, XIII, and XIVa (67). Significant bootstrap values (90% and higher), expressed as a percentage of 500 replications, are indicated at the branching points. Adapted from reference with permission of the publisher.

FIG. 3

Sites of pathology for species of GPAC. Species of major importance are underlined, ? denotes uncertain pathology.