The human body hosts trillions of microorganisms, but certain species, like Peptostreptococcus anaerobius, possess a dual nature. This common resident of internal surfaces can transform into an aggressive pathogen under specific conditions. Recent investigations highlight its role in acute infections and the development of diseases like colorectal cancer. Understanding this microbe requires examining its characteristics, its commensal life, and its pathogenic mechanisms.
Defining Peptostreptococcus Anaerobius
P. anaerobius is a member of the diverse group of Gram-positive anaerobic cocci (GPAC). It is a strict anaerobe, meaning it cannot survive in the presence of oxygen. Microscopic examination reveals small, spherical cells, typically appearing in pairs or short chains.
The bacterium is classified as Gram-positive because its cell wall retains the crystal violet stain. It is non-spore-forming, distinguishing it from other anaerobic genera like Clostridium. P. anaerobius is a normal part of the human microflora, colonizing mucocutaneous surfaces, including the mouth, gastrointestinal tract, skin, and female genitourinary tract.
The organism is often overlooked in laboratory settings because it is slow-growing and requires specialized, oxygen-free conditions for isolation. Identifying the species can be challenging, which historically limited understanding of its frequency in clinical infections. P. anaerobius is a hyper-ammonia-producing bacterium, utilizing amino acids as its primary source of carbon, nitrogen, and energy.
Commensal Presence and Opportunistic Infections
P. anaerobius lives as a commensal, contributing to the balance of the gut and mucosal microbiomes. Infection occurs when natural protective barriers, such as the skin or mucosal lining, are breached by trauma, surgery, or underlying disease. This breach allows the organism to access deeper tissues, where the low-oxygen environment supports its growth.
Infections involving P. anaerobius are often polymicrobial, found alongside other anaerobic and aerobic species. It rarely acts alone, frequently participating in synergistic infections involving multiple microbes. The organism is commonly isolated from deep-seated infections, particularly in the abdomen and female urogenital tract.
Infections include soft tissue abscesses, bone and joint infections, and head and neck infections, such as dental abscesses. It also contributes to anaerobic pleuropulmonary infections, like aspiration pneumonia and lung abscesses. Treatment is complicated by the organism’s slow growth and increasing antimicrobial resistance.
Emerging Link to Colorectal Cancer
The most significant research involves the association between P. anaerobius and colorectal cancer (CRC). Studies comparing the gut microbiota of CRC patients to healthy individuals show the bacterium is selectively enriched in tumor tissue and surrounding mucosa. This increased abundance suggests the organism actively contributes to the disease process, rather than being a bystander.
Scientists identified a surface protein on P. anaerobius called putative cell wall binding repeat 2 (PCWBR2). This protein enables the bacterium to adhere specifically to CRC cells by interacting with the host cell receptor integrin \(\alpha2/\beta1\). Since this integrin is often overexpressed on CRC tumor cells, it facilitates colonization of the tumor site.
The binding of PCWBR2 to integrin \(\alpha2/\beta1\) initiates an internal signaling cascade, activating the PI3K-Akt pathway. This activation promotes cell proliferation, stimulating cancer cells to grow and divide rapidly. Simultaneously, this cascade activates nuclear factor kappa-light-chain-enhancer of activated B cells (NF-\(\kappa\)B), triggering a pro-inflammatory response.
This chronic inflammation creates a tumor-favoring microenvironment, contributing to tumor progression. P. anaerobius has also been linked to chemoresistance, hindering the effectiveness of agents like oxaliplatin in animal models. This resistance is mediated by the recruitment of myeloid-derived suppressor cells (MDSCs) into the tumor microenvironment, which secrete high levels of interleukin-23 (IL-23).
Clinical Diagnosis and Management
Diagnosing an infection requires specialized laboratory techniques due to the organism’s strict anaerobic requirement and slow growth. Specimens must be transported and cultured in an oxygen-free environment. Identification is achieved through Gram staining, which shows characteristic Gram-positive cocci in pairs and chains, or through rapid molecular methods like mass spectrometry.
Management follows the general protocol for anaerobic infections, requiring surgical intervention and antimicrobial therapy. Surgical drainage of associated abscesses or pus is necessary to control the infection and remove necrotic tissue. Without proper drainage, the infection may persist despite antibiotic treatment.
Antimicrobial treatment involves agents effective against anaerobic bacteria. Metronidazole is the preferred empirical therapy for anaerobic infections and shows excellent activity against most strains of P. anaerobius. Penicillin G is also an effective option. While clindamycin is an alternative, increasing resistance rates have been noted among isolates.