Cutibacterium acnes is a bacterium that lives on almost everyone’s skin, yet it can transform from a harmless resident into a troublesome pathogen under certain conditions. While most commonly associated with acne, this microorganism has gained recognition as a cause of serious infections following surgery and medical device implantation.

Epidemiology

Cutibacterium acnes exists on the skin of virtually every human being. This bacterium can be found on nearly every person, making it one of the most universal inhabitants of the human body^[1]. The bacterium primarily lives deep within hair follicles and pores, especially in areas rich in sebaceous glands (oil-producing glands in the skin), such as the face, back, chest, and upper thorax^[2].

The presence of C. acnes changes throughout a person’s life. It appears briefly on the skin of newborns but true colonization begins during the one to three years before sexual maturity. During this developmental period, the number of C. acnes bacteria increases dramatically, rising from fewer than 10 bacteria per square centimeter to approximately one million per square centimeter^[4].

When it comes to acne vulgaris, the most common skin condition linked to this bacterium, the numbers are staggering. Acne affects more than 45 million individuals in the United States alone, with approximately 85 percent of the population experiencing some form of acne between the ages of 12 and 24^[1][7]. Nearly 20 percent of all visits to dermatologists are related to acne treatment^[1]. On a global scale, acne vulgaris affects approximately 9.38 percent of the world’s population^[11]. In mainland China, research showed overall pooled prevalence rates of acne at 39.2 percent^[11].

While acne is the most recognized condition associated with C. acnes, post-surgical infections represent another important aspect of its epidemiology. The bacterium has become increasingly recognized as a cause of prosthetic joint infection, particularly following shoulder surgery. Studies examining shoulder arthroplasty have found C. acnes infection rates ranging from 16 to 70 percent at the time of revision surgery, with most common estimates around 50 to 60 percent^[5]. The shoulder is colonized by C. acnes at much higher rates compared to the knee and hip^[5].

Men appear to be at particularly high risk for developing post-surgical infections with C. acnes. This increased vulnerability is related to the fact that men have a greater number of sebaceous follicles than women, which provides more habitat for the bacterium to thrive^[5]. A 2009 study showed that C. acnes is more likely to infect the shoulder than the hip or knee after surgery, and that men have a higher natural bacterial load on their skin than women^[7].

Causes

Cutibacterium acnes is a Gram-positive bacterium (a type of bacteria that retains a particular stain used in laboratory identification) that prefers anaerobic (without oxygen) growth conditions^[1]. However, despite its preference for oxygen-free environments, C. acnes possesses all the necessary proteins for oxidative phosphorylation, which allows it to survive in oxygen-rich environments as well^[10]. This adaptability makes it a versatile organism capable of thriving in various conditions throughout the human body.

Originally identified as Bacillus acnes in the 1900s by Thomas Casper Gilchrist, the bacterium was later named Propionibacterium acnes due to its ability to generate propionic acid as a product of anaerobic metabolism^[2][7]. In 2016, the bacterium underwent taxonomic reclassification based on biochemical and genomic studies. Researchers found that in terms of both evolutionary tree structure and DNA composition, the skin-dwelling species was distinguishable from other species previously categorized under the same name. As part of this restructuring, a novel genus called Cutibacterium was created specifically for skin-associated species^[2].

The bacterium is lipophilic, meaning it thrives in fatty or oily environments. C. acnes lives primarily on fatty acids found in sebum (the oily substance secreted by sebaceous glands) within hair follicles^[2]. The cell wall and envelope of C. acnes contain various lipids, and the cell wall is made up of peptidoglycan. The bacterium also contains lipoglycans with substantial concentrations of mannose, glucose, and galactose^[10].

C. acnes exists in different subspecies and strains that have distinct characteristics. Scientists have identified three main groupings: C. acnes subspecies acnes, C. acnes subspecies defendens, and C. acnes subspecies elongatum^[17]. These groupings correspond to evolutionary groupings that have high associations with whether the strain is disease-causing or protective in nature.

How Cutibacterium Acnes is Transmitted

Cutibacterium acnes exists on the skin of nearly every person, which means transmission occurs primarily through skin contact^[1]. Because it is a normal inhabitant of human skin, the bacterium is ubiquitous in the environment. These organisms are particularly problematic in settings where shedding of skin cells occurs, such as during surgical procedures or in cleanroom environments where microbial contamination must be minimized^[7].

The bacterium can also be found throughout the gastrointestinal tract, on the conjunctiva of the eye, in the oral mucosa, and in the external auditory canal^[10]. It may also be present in other body locations beyond just the skin surface^[2].

Risk Factors

Several factors increase the risk of developing conditions associated with C. acnes, whether acne vulgaris or post-surgical infections. For acne development, elevated production of sebum by hyperactive sebaceous glands or blockage of hair follicles can cause C. acnes bacteria to grow and multiply^[2]. Adolescents and young adults are particularly vulnerable due to hormonal changes that occur during puberty, which stimulate increased sebum production.

Male gender represents a significant risk factor for post-surgical C. acnes infections. Men have a greater number of sebaceous follicles than women, leading to higher natural bacterial loads on the skin^[5][7]. This increased bacterial presence makes contamination during surgery more likely.

Patients undergoing certain types of surgery face elevated risks. Shoulder surgery carries a particularly high risk of C. acnes infection compared to hip or knee procedures^[5][7]. The shoulder area naturally harbors higher concentrations of C. acnes due to the density of sebaceous glands in this region.

The presence of medical devices and implants creates another major risk factor. C. acnes has the ability to persist on body implants and surgical devices, causing a wide range of post-operative infectious conditions^[1]. The bacterium can colonize various types of prosthetic devices and form biofilms that make infections difficult to treat. It has been recognized as an important pathogen in prosthetic shoulder joint infections, in cases involving breast implants, and in infections caused by cardiovascular devices^[7][10].

C. acnes acts as an opportunistic pathogen, meaning it takes advantage of weakened defenses or disrupted barriers. When normal skin barriers are breached during surgery, or when immune function is compromised, the bacterium can shift from a harmless commensal organism to a disease-causing pathogen^[3].

Symptoms

The symptoms of C. acnes infection vary dramatically depending on whether the infection manifests as acne vulgaris or as a post-surgical device infection. In acne vulgaris, C. acnes bacteria predominantly live deep within follicles and pores. When these bacteria multiply rapidly, they can trigger inflammation that leads to the symptoms associated with common skin disorders^[2].

Acne vulgaris typically presents with both non-inflammatory and inflammatory skin lesions. The condition can manifest as comedones (blackheads and whiteheads), which are non-inflammatory lesions. When inflammation occurs, patients develop papules (small raised bumps), pustules (pus-filled bumps), and nodulocystic lesions (deeper, painful lumps under the skin)^[11]. These inflammatory lesions result from the cellular damage, metabolic byproducts, and bacterial debris produced during rapid bacterial growth within follicles^[2].

The face is most commonly affected by acne lesions, though the back and chest can also develop significant involvement. Acne is common in adolescents and can be identified by facial lesions that can significantly impact quality of life and self-esteem^[11].

Post-surgical C. acnes infections present quite differently. These infections often have delayed presentation following implant surgery, sometimes occurring months or even years after the initial procedure^[12]. The clinical presentation is frequently non-specific, making diagnosis challenging^[5].

In shoulder arthroplasty infections, patients may experience pain, decreased range of motion, and subtle signs of inflammation. Unlike infections caused by more aggressive bacteria, C. acnes infections often do not produce the dramatic symptoms of acute infection such as high fever, severe pain, or obvious wound drainage.

Beyond skin and surgical site infections, C. acnes has been implicated in several other chronic conditions. The bacterium can cause chronic blepharitis (inflammation of the eyelids) and endophthalmitis (inflammation inside the eye), particularly following intraocular surgery^[2]. It has also been associated with endocarditis (infection of the heart valves), infections affecting the spine, prostatitis, sarcoidosis, synovitis, and septic arthritis^[1][10].

Prevention

Preventing C. acnes-related conditions requires different strategies depending on whether the goal is preventing acne or preventing post-surgical infections. Since the bacterium is a normal inhabitant of virtually everyone’s skin, complete prevention of colonization is neither possible nor desirable. Instead, prevention focuses on managing bacterial populations and preventing conditions that allow harmful overgrowth or contamination.

For acne prevention, maintaining skin cleanliness and managing oil production are fundamental approaches. Using products containing active ingredients can help control bacterial populations. Benzoyl peroxide is particularly effective at killing C. acnes and is a staple in acne prevention^[15]. Products with benzoyl peroxide concentrations above 2.5 percent effectively reduce C. acnes populations without requiring stronger formulations that might irritate skin^[15].

C. acnes is susceptible to a wide range of chemicals, making chemical preservation and antimicrobial treatments relevant strategies. Benzoyl peroxide helps remove excess oils from the skin and eliminates dead skin cells that can clog pores^[1]. Combining benzoyl peroxide with other ingredients like salicylic acid or glycolic acid can enhance effectiveness by helping unclog pores and treating existing blemishes^[15].

C. acnes can be killed with ultraviolet light, which represents another potential prevention strategy^[7]. The bacterium grows best in temperatures between 30 and 37 degrees Celsius and thrives in the lipid-rich environment of hair follicles^[7].

For preventing post-surgical infections, rigorous protocols during surgery are essential. The Food and Drug Administration has advised manufacturers to establish suitable methods to prevent contamination, including maintaining adequate quality of incoming materials, ensuring sanitary design, properly cleaning equipment, limiting production and storage times, and monitoring environmental conditions^[1].

Appropriate use of personal protective equipment and proper gowning procedures are vital in settings where microbial contamination poses risks, particularly in cleanrooms and operating theaters. Because C. acnes is shed with skin cells, controlling this shedding through proper barriers becomes important during surgical procedures^[7].

Scientific studies suggest that establishing appropriate product specifications, including tests, methods, and acceptance criteria, helps ensure drug and implant product components meet quality standards^[1]. When appropriate, additional laboratory tests may be needed to determine whether products are suitable for release.

Pathophysiology

The pathophysiology of C. acnes infection involves complex interactions between the bacterium, the skin environment, and the human immune system. C. acnes is largely commensal and part of the normal skin flora present on most healthy adult humans^[2]. Understanding how this normally harmless bacterium causes disease requires examining its virulence factors, its ability to trigger inflammation, and its capacity to form biofilms.

C. acnes produces several proteins and enzymes that enable it to survive, obtain nutrients, and interact with human tissues. The bacterium secretes many proteins, including several digestive enzymes that help break down sebum and acquire other nutrients^[2]. These enzymes can also destabilize the layers of cells forming the walls of hair follicles. The genome of C. acnes has been sequenced, and studies have shown that several genes can generate enzymes for degrading skin and proteins that may activate the immune system^[2].

During the past decade, many studies have identified and characterized several potential virulence factors involved in the pathogenicity of this bacterium. These virulence factors participate in bacterial attachment to target cells, formation of polysaccharide-based biofilms, molecular structures that trigger inflammation, and enzymatic degradation of host tissues^[3].

The bacterium produces several proteins or glycoproteins that could be considered active virulence factors. These enable C. acnes to adapt to the lipophilic environment of the pilosebaceous unit (the hair follicle and its associated sebaceous gland) of the skin, but also to various other organs it may colonize^[3]. The makeup of the C. acnes cell wall, which contains various lipids and peptidoglycan, contributes to the bacterium’s ability to persist in tissues and resist host defenses^[10].

One of the most important pathogenic mechanisms of C. acnes is its ability to form biofilms. Biofilms are structured communities of bacteria encased in a self-produced protective matrix. C. acnes can form biofilms on prosthetic devices and implants, which significantly complicates treatment. The biofilm protects bacteria from antibiotics and immune defenses, allowing persistent infection. This characteristic ability to adhere to and form biofilms, particularly on prosthetics, is typical of the infections that C. acnes causes^[4].

In acne vulgaris, the pathophysiology involves several interconnected processes. When sebaceous glands become hyperactive or when follicles become blocked, C. acnes bacteria grow and multiply within the follicle. As bacterial populations expand, the organisms secrete enzymes and produce metabolic byproducts. These substances, along with bacterial debris and cellular damage, trigger an inflammatory response^[2].

The inflammation process involves the activation of the innate immune system. C. acnes produces molecular structures that trigger inflammatory pathways, leading to the release of inflammatory mediators. Research has shown that C. acnes-derived extracellular vesicles (tiny particles released by bacteria) can trigger activation of inflammation-related factors including interleukin-8 (IL-8), interleukin-6 (IL-6), tumor necrosis factor alpha (TNFα), and granulocyte-macrophage colony-stimulating factor (GM-CSF)^[6].

Interestingly, the inflammatory response and disease manifestation depend not just on the presence of C. acnes, but also on strain differences and the host’s immune response. Different phylotypes of C. acnes have distinct effects. Specific phylotypes have been associated with acne vulgaris development, while other phylotypes have been linked to healthy skin^[6]. The proteomic profile of cargo contained in bacterial extracellular vesicles reflects distinct characteristics of different C. acnes phylotypes in terms of life cycle, survival, and virulence^[6].

Studies have shown that acneic skin has relatively less C. acnes and more presence of other bacterial species, including an increase of Staphylococcus epidermidis, especially in hair follicles where they can produce biofilms that contribute to pore blockage^[17]. This suggests that acne pathophysiology involves complex microbial community interactions, not just C. acnes alone.

In post-surgical infections, the pathophysiology centers on bacterial contamination during surgery, bacterial persistence on implanted materials, and biofilm formation. The delayed presentation of many C. acnes device infections reflects the slow-growing nature of the organism and its ability to establish low-grade chronic infections. The bacterium requires at least six days for growth in culture, highlighting its relatively slow replication compared to more aggressive pathogens^[4].

The bacterium’s adaptation to life on humans has provided novel understanding of infection mechanisms. C. acnes can colonize various ecological niches beyond skin, demonstrating remarkable adaptability^[3]. Interactions between C. acnes and the human host, including interactions with the human skin microbiota, promote selection of strains capable of producing virulence factors that increase inflammatory capability^[3].