Cutibacterium acnes, formerly known as Propionibacterium acnes, is an anaerobic, Gram-positive rod typically associated with acne. It usually resides harmlessly deep within the sebaceous glands and hair follicles of human skin. While a natural part of the skin’s microbiome, C. acnes can transform into an opportunistic pathogen under specific circumstances, leading to serious, deep-seated infections, particularly involving bone and joint tissues.
From Skin Commensal to Invasive Pathogen
The transition of C. acnes from a benign skin resident to an invasive pathogen is almost always linked to orthopedic surgical procedures. The organism is highly concentrated on the skin of the upper body, especially the shoulder, chest, and back. This explains why shoulder arthroplasty and spinal surgeries are the most common sites of infection. During a surgical incision, the bacteria, which reside deep in the dermal layer, can be inadvertently transferred into the sterile surgical field, allowing them to colonize internal tissues and implanted devices.
Once inside the body, the primary mechanism allowing C. acnes to establish a persistent infection is the formation of a biofilm. This biofilm is a complex, protective matrix that the bacteria secrete, allowing them to adhere securely to foreign materials like titanium or stainless-steel prosthetics, screws, and plates. The formation of this dense layer shields the bacteria from both the patient’s immune system and many common antibiotics.
The bacteria can also be found deep within the bone tissue itself, invading microscopic channels known as osteocyte lacuno-canalicular networks. This ability to hide within the bone’s submicron architecture creates a protected reservoir that contributes to the difficulty of treating the infection effectively. Males are at higher risk because they tend to have a greater overall colonization burden of C. acnes due to a higher density of sebaceous glands. The infection is considered an emerging pathogen in orthopedics, with one study reporting that C. acnes accounts for up to 76% of all shoulder periprosthetic joint infections.
Recognizing Symptoms and Diagnostic Hurdles
Infections caused by C. acnes are difficult to diagnose because the clinical presentation is often vague, indolent, and chronic. Unlike acute infections, which present rapidly with high fever and severe inflammation, C. acnes infections typically manifest with subtle, low-grade symptoms. Patients may experience mild, persistent joint pain, discomfort, or progressive joint stiffness, often mistaken for routine post-operative recovery issues or mechanical failure. These symptoms frequently do not appear until months or even years after the initial surgery, with delays of three to 24 months being common.
The low virulence of the organism means it does not trigger a strong systemic inflammatory response. Standard laboratory markers of infection, such as white blood cell count or C-reactive protein levels, may be only slightly elevated or even normal. This lack of clear clinical or laboratory evidence is a major hurdle that often leads to a delayed or missed diagnosis. To confirm the presence of C. acnes osteomyelitis, specialized diagnostic steps must be taken to overcome the organism’s unique growth characteristics.
C. acnes is a fastidious, slow-growing anaerobe, meaning it thrives in environments with little or no oxygen. Standard hospital culture protocols, which typically incubate samples for only a few days, are often insufficient to allow the organism to grow to detectable levels. To accurately isolate the pathogen, laboratory technicians must utilize specialized, prolonged culture times, often requiring incubation for 10 to 14 days or longer. Without this extended incubation, the organism may be mistakenly dismissed as a transient contaminant from the skin.
Accurate diagnosis also depends on the quality of the sample collected during surgical revision or biopsy. Physicians must obtain multiple tissue samples from the suspected site of infection to increase the likelihood of capturing the bacteria. Furthermore, if an implant is removed, a technique called sonication, which uses ultrasonic waves to dislodge bacteria from the surface of the device, is often necessary to recover the organisms living within the protective biofilm.
Managing the Infection
The treatment strategy for C. acnes bone and joint infections is inherently complex and involves both aggressive surgical management and a prolonged course of antibiotics. Due to the organism’s ability to form a tenacious biofilm on implanted materials, surgical intervention is almost universally required to achieve a cure. This typically involves surgical debridement to remove infected tissue, and often the complete removal or exchange of the contaminated prosthetic device.
The subsequent antibiotic regimen must be planned carefully because of the slow-growing nature of the bacteria and the need to penetrate the biofilm matrix. Treatment is always long-term, lasting an average of three to six months. The initial phase often involves two to six weeks of intravenous antibiotics to achieve high concentrations, followed by an extended course of oral medication.
C. acnes remains susceptible to several classes of antibiotics, notably beta-lactams (such as penicillin, amoxicillin, or cephalexin) and clindamycin. Rifampin is often included in combination therapy because of its specific activity against organisms residing in a biofilm. Due to rising rates of antibiotic resistance, especially to drugs like clindamycin, susceptibility testing of the isolated bacterial strain is mandatory to tailor the most effective treatment plan. Adherence to the full, multi-month treatment plan is paramount, as early cessation increases the risk of recurrence due to residual bacteria hidden within the bone or residual biofilm.