Cutibacterium acnes (C. acnes) is a bacterium that commonly lives on human skin. While often harmless and associated with acne, it can cause serious infections deep within the body, including in bones. These infections are uncommon but present a challenge due to the bacterium’s unique characteristics and the specific circumstances under which they develop.
The Source of Infection
Cutibacterium acnes is a regular inhabitant of the skin, thriving in the oily, low-oxygen environment of sebaceous glands and hair follicles. It is considered a commensal organism, feeding on lipids in sebum without causing harm.
The feature that allows C. acnes to become infectious is its ability to form a biofilm. A biofilm is a community of bacteria enclosed in a self-produced matrix that adheres to a surface, shielding it from the immune system and antibiotics. The slow-growing nature of C. acnes also means infections can develop gradually over months or years without producing immediate symptoms.
Associated Procedures and Risk Factors
The opportunity for C. acnes to infect bone tissue most often arises during orthopedic surgeries, particularly those involving the shoulder and spine. During an operation, the bacterium can be introduced from the patient’s skin into deeper tissues. Surgical implants, such as joint replacements or screws, provide an ideal surface for C. acnes to attach and form a biofilm.
Certain factors increase the risk of a C. acnes bone infection. Men are at a higher risk, likely due to having more sebaceous follicles and a higher bacterial load on their skin. A personal history of acne may also correlate with a higher density of the bacteria, and existing surgical hardware creates a site for colonization.
Signs and Symptoms
The clinical presentation of a C. acnes bone infection is often subtle. Due to the bacterium’s slow growth, symptoms develop gradually and are chronic rather than acute. Patients experience a persistent, dull, aching pain in the affected area, which may be mistaken for normal postoperative discomfort or arthritis.
Unlike infections from more aggressive bacteria, a C. acnes infection rarely causes high fever, sharp pain, or significant redness and swelling. The primary complaints are persistent joint pain, stiffness, or weakness in the limb, which can emerge months or years after the procedure.
Diagnostic Methods
Diagnosing a C. acnes bone infection is challenging due to its subtle symptoms. Imaging tests like X-rays or MRIs may reveal nonspecific signs, such as a loose orthopedic implant. Blood tests for inflammatory markers like ESR and CRP are often performed, but levels may be only slightly elevated or normal.
A definitive diagnosis requires obtaining tissue or fluid samples from the suspected site through a biopsy or joint aspiration for laboratory culture. Because C. acnes is a slow-growing organism, standard culture protocols can miss it. Cultures must be held for up to 14 days or longer under anaerobic (oxygen-free) conditions. Multiple tissue samples are often taken to increase the chance of a positive culture, and molecular techniques like PCR may offer a faster alternative.
Treatment Strategies
Treating a C. acnes bone infection combines surgical intervention with a long course of antibiotics. Surgery is necessary to address the infection’s source, especially when an implant is involved. This may involve a debridement procedure to clean out infected tissue or a revision surgery to remove the infected implant entirely. Removing the biofilm-covered implant is frequently required for antibiotics to work effectively.
Following surgery, a prolonged course of antibiotic therapy is administered, often lasting from three to six months. Treatment may start with intravenous antibiotics before transitioning to oral medications like penicillin, ceftriaxone, and clindamycin, chosen based on susceptibility testing.