Anaerobic Cocci and Anaerobic Gram-Positive Nonsporulating Bacilli

Members of the Commensal Microbiota

Among gram-negative anaerobic cocci, Veillonella species are considered mainly harmless, or even beneficial, colonizers of the mouth from the early years of life onward. Acidaminococcus and Megasphaera species reside the human intestine at young age; however, geographic or ethnic differences, or both, in their abundance are significant.

Among GPAC organisms, R. gnavus plays an important role as an early colonizer of the human gut. Although members of the genera Anaerococcus, Finegoldia, Parvimonas, Peptoniphilus, and Peptostreptococcus are commonly present as part of the human microbiota, residing the mouth, intestine, and both the female and male genital tract as well as the surfaces of the eye, nasal cavity, and skin, only limited knowledge exists about their role as commensal organisms.

Anaerobic gram-positive nonsporulating bacilli belong to the commensal microbiota of the digestive tract, and some members are part of the microbiota of the urogenital tract and skin. The latter environment is widely colonized by cutaneous propionibacteria, C. (formerly P. ) acnes, in particular. Bifidobacterium and Collinsella species are important in the development of the gut microbiota. In elderly persons, a decrease in the levels of intestinal Bifidobacterium populations is seen with aging. Lactobacilli, such as Lactobacillus rhamnosus, L. paracasei, L. plantarum, and L. salivarius, form an important component on mucosal surfaces of the gastrointestinal tract. The most prevalent lactobacilli in health-associated vaginal biofilms are L. crispatus, L. gasseri, and L. jensenii, while L. iners can also be found in intermediate and dysbiotic states of the vaginal microbiota. Hydrogen peroxide production by these species, except for L. iners, may help to control the overgrowth of bacterial vaginosis–associated organisms and to protect from obstetric infections during pregnancy. With variable results, beneficial Lactobacillus and Bifidobacterium strains have been used as probiotics, aiming to protect the homeostasis or to alter the dysbiotic composition of the microbiota.

Clinical Significance

In infectious lesions, the organisms covered in this chapter often appear together with other anaerobic or facultative bacteria in polymicrobial consortia typical at or close to their natural anatomic site. Hematogenous spread of anaerobes from the commensal microbiota and local infections along the local veins and through the general circulation expose the host to systemic infections, such as brain abscesses and endocarditis. The clinical significance of anaerobic bacteremia varies markedly by pathogen; if patients have an underlying condition and are not treated with drugs active against anaerobes and drainage when appropriate, the mortality rate can be high. According to data from the Mayo Clinic (Rochester, MN), the increased prevalence of anaerobic bacteremia was especially shown for “peptostreptococci”, and their increasing resistance to metronidazole was of concern. However, in a population-based monitoring of anaerobic bloodstream infections in the Province of Alberta, Canada, no increase in resistance rates of anaerobes was observed from 2000 to 2008. Moreover, a decrease in the incidence of anaerobic bacteremia was reported from a Belgian university hospital over a 10-year period between the years 2004 and 2013. Besides bacteremia, typical infectious sites where anaerobic cocci or gram-positive anaerobic nonsporulating rods can be involved include the central nervous system, eye, oral cavity, respiratory tract, abdomen and intestine, genital tract, and wounds.

Anaerococcus

Typical infectious sites for the genus Anaerococcus include the female genital tract; A. hydrogenalis, A. lactolyticus, A. prevotii, A. tetradius, and A. vaginalis have been isolated from vaginal discharges and ovarian abscesses, while A. octavius recoveries come from the skin, nasal cavity, and vagina. A. prevotii, A. vaginalis, and A. lactolyticus are also among the prevalent findings in chronic wound samples. In addition, the involvement of A. prevotii in mixed surgical infections and skin infections and endodontic infections with abscesses has been reported. A. murdochii has been recovered from polymicrobial infections, mainly in the lower extremities but also in the neck and a sternal wound. Recently, two novel Anaerococcus species with clinical relevance have been suggested: A. degenerii (four strains isolated from toe tissue, from drainage fluid after rectal carcinoma surgery, from a pelvic abscess, and from an intraabdominal abscess after rectal carcinoma surgery) and A. nagyae (three strains isolated from an infectious lesion of the groin, from a nephrourecterectomy wound infection, and from blood).

Finegoldia

Among GPAC organisms, F. magna has the highest pathogenic potential and, subsequently, it is found in a variety of human infections. This high pathogenicity is explained by its specific virulence properties. Although GPAC species usually appear in polymicrobial infections, F. magna can be identified as the only infectious finding. Most typically, F. magna is involved in wound, soft tissue, and bone and joint infections. In addition, there are reports on its involvement in endocarditis, breast abscesses, pleural empyema, mediastinitis, chronic balanitis, and bacterial vaginosis. F. magna is one of the principal species in diabetic foot, venous leg, and decubitus ulcers, which are chronic, polymicrobial biofilm-associated skin and skin structure infections. According to a report of 61 bone and joint infections with anaerobic organisms, F. magna was present in 21% of the cases, which all were polymicrobial and were located in the lower extremities, especially in the ankle. There is increasing evidence for F. magna as a causative agent in orthopedic implant–associated infections.

Parvimonas

P. micra is the far most prevalent GPAC species in the mouth, where it is considered a putative pathogen within subgingival biofilms connected to severe periodontitis, infected root canals with or without abscesses, failing dental implants, and odontogenic infections. Strikingly, in a Norwegian study, most brain abscesses proved to be infected by P. micra or other Parvimonas sp., suggesting an oral origin. Some recoveries come from bacteremias and bone and joint infections. Furthermore, P. micra has been shown, for the first time, to be capable of causing bacterial meningitis. This organism is also found among the bacterial vaginosis–related microbiota exposing to chronic genital inflammation.

Peptoniphilus

In clinical specimens with GPAC organisms, P. asaccharolyticus appears as a frequent finding ; however, some doubt has been raised about the correct identification of the species. Instead, a biochemically similar species, P. harei, seems to cover a considerable proportion of clinically relevant GPAC findings. P. asaccharolyticus/P. harei was frequently detected among 61 bone and joint infections. P. harei is one of the primary findings in biofilms of venous leg ulcers, and P. harei, P. ivorii, and P. indolicus in pressure ulcers. P. lacrimalis has been isolated from a lacrimal gland abscess and osteoarticular and vaginal specimens but is also detected as a rather common finding in decubitus ulcer samples. Novel Peptoniphilus species have been isolated from polymicrobial human infections below the waistline, including P. coxii (leg infections, back cyst, flank abscess, endometrial fluid, and tonsil biopsy), P. duerdenii (vaginal abscess), P. gorbachii (lower extremities), P. koenoeneniae (buttock abscess), P. olsenii (lower extremities), P. rhiniditis (chronic rhinosinusitis), and P. tyrrelliae (leg infection, ischial wound, and perirectal abscesses).

Peptostreptococcus

P. anaerobius has long been considered an important human pathogen. A similar species, P. stomatis, was not described until 2006 ; therefore the term P. anaerobius sensu lato is used when citing earlier literature. There are reports on its involvement in various types of infections all over the body, including the head, respiratory tract, gastrointestinal tract, genitourinary tract, skin and soft tissue, bone and joints, and cardiovascular sites. P. anaerobius sensu lato is frequently isolated from abscesses of various anatomic sites, mixed surgical infections, diabetic wounds, and cutaneous wounds caused by dog or cat bites. In the original description of P. stomatis, it was considered an oral species, whereas P. anaerobius was suggested to harbor sites below the waistline. Both P. anaerobius and P. stomatis (oral clone CK035) have been detected in infected root canal specimens and bronchoalveolar lavage samples from patients with ventilator-associated pneumonia. In a study of 64 P. anaerobius sensu lato strains, P. stomatis strains originated not only from oropharyngeal but also gastrointestinal specimens, whereas P. anaerobius strains originated mainly from ulcer and skin specimens of the lower extremities and pus specimens of the genitourinary tract.