Cutibacterium acnes Infection

Cutibacterium acnes is a bacterium that lives on nearly everyone’s skin, but it can transform from a harmless companion into an opportunistic pathogen, causing infections ranging from common acne to serious complications following surgery and medical device implantation.

Table of contents

• What is Cutibacterium acnes?
• Where the bacterium is found
• Diseases caused by C. acnes
• How the infection spreads
• Who is at higher risk
• Detecting the infection
• Treatment approaches

What is Cutibacterium acnes?

Propionibacterium acnes, Bacillus acnes

Cutibacterium acnes (C. acnes) is a gram-positive bacterium, which means it has a specific type of cell wall that shows up as purple under a microscope when stained^[1]. The bacterium prefers to grow in environments without air, a characteristic called anaerobic growth^[1]. However, it possesses all the necessary proteins for survival in oxygen-rich environments as well^[10].

The bacterium was originally called Bacillus acnes when it was first identified by Thomas Casper Gilchrist in the 1900s^[7]. It was later renamed Propionibacterium acnes because of its ability to produce propionic acid as a byproduct of its metabolism^[2]. In 2016, scientists reclassified it again based on genetic studies and renamed it Cutibacterium acnes^[2].

The bacterium is rod-shaped, does not form spores, and grows relatively slowly, typically requiring at least 6 days to grow in laboratory cultures^[4]. Interestingly, while its natural pigmentation is white to grey, C. acnes appears orange under blacklight^[7].

Where the bacterium is found

• Skin
• Sebaceous glands
• Hair follicles
• Gastrointestinal tract
• Conjunctiva (eye)
• Oral mucosa
• External auditory canal
• Genitourinary tract

C. acnes can be found on the skin of virtually every human^[1]. The bacterium is particularly abundant in areas rich in sebaceous glands, which are tiny organs in the skin that produce an oily substance called sebum^[1]. These areas include the face, upper thorax (chest), and back.

The bacterium lives primarily deep within hair follicles and pores, where it feeds on sebum, cellular debris, and metabolic byproducts from surrounding skin tissue^[2]. It grows best in lipid-rich (fatty) environments at temperatures between 30 to 37 degrees Celsius^[7].

While C. acnes is found briefly on the skin of newborns, true colonization begins during the 1 to 3 years before puberty. During this time, the number of C. acnes bacteria increases dramatically from fewer than 10 per square centimeter to about 1 million per square centimeter, especially on the face and upper thorax^[4]. Beyond the skin, C. acnes can also be found throughout the gastrointestinal tract and in other body areas^[2][10].

Diseases caused by C. acnes

C. acnes is generally considered a harmless bacterium that is part of the normal skin flora present on most healthy adults^[2]. However, there is growing evidence that it acts as an opportunistic pathogen, meaning it can cause disease under certain conditions^[1].

Acne vulgaris

Acne vulgaris is the disease most commonly associated with C. acnes infection^[2]. It is one of the most common skin diseases, affecting more than 45 million individuals in the United States, with nearly 20 percent of all visits to dermatologists related to acne treatment^[1].

When sebaceous glands become overactive or follicles become blocked, C. acnes bacteria can grow and multiply rapidly^[2]. The bacteria secrete digestive enzymes that break down sebum and can destabilize the walls of hair follicles. The cellular damage, metabolic byproducts, and bacterial debris produced by this rapid growth can trigger inflammation, leading to the inflamed spots characteristic of acne^[2].

Post-surgical and device-related infections

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 act as an opportunistic pathogen causing invasive and chronic implant infections^[1].

C. acnes is recognized as an important pathogen in prosthetic shoulder joint infections^[7]. It is more likely to infect the shoulder than the hip or knee after surgery^[7]. Recent studies have found that C. acnes can be detected in 16% to 70% of shoulder revision surgeries, with most common estimates around 50% to 60%^[5].

The bacterium has also been associated with infections of breast implants, cardiovascular devices, and other prosthetic materials^[7][10].

Other chronic conditions

C. acnes is involved in several types of chronic conditions, including:

• Endocarditis (infection of the heart’s inner lining)^[1]
• Blepharitis and endophthalmitis (eye infections), particularly following intraocular surgery^[2]
• Spinal conditions and disk herniation^[1]
• Sarcoidosis^[10]
• Prostatitis, synovitis, osteomyelitis, and septic arthritis^[10]

How the infection spreads

Since C. acnes exists on the skin of nearly every person, it is easily transmitted through skin contact^[1]. The bacterium is ubiquitous in the environment, and contamination is particularly problematic in settings where skin cells can be shed^[7].

C. acnes can contaminate surgical sites during operations when skin bacteria enter the surgical field. The bacterium’s ability to form biofilms (protective layers of bacteria that stick to surfaces) allows it to colonize medical devices and implants^[3][5].

Who is at higher risk

Male patients are at particularly high risk for C. acnes infections because they have a greater number of sebaceous follicles than females^[5]. Men have a higher natural bacterial bioburden than women^[7].

People undergoing shoulder surgery face higher risk compared to those having hip or knee procedures^[5]. The shoulder area has more sebaceous glands and higher colonization rates of C. acnes compared to other joint areas^[5].

Individuals receiving any type of prosthetic implant or medical device are at risk for C. acnes infection. Those with overactive sebaceous glands producing excess sebum are more prone to acne-related infections^[2].

Detecting the infection

Detecting C. acnes is challenging and requires validated testing methods that consider the bacterium’s unique characteristics^[1]. The presentation of C. acnes infection is often non-specific and can occur long after surgery, leading to delays in diagnosis^[5].

C. acnes is difficult to culture, typically taking 14 to 17 days for a positive culture result^[5]. The bacterium often does not show abnormal results on standard laboratory workup for infection, including tests for ESR (erythrocyte sedimentation rate), CRP (C-reactive protein), and synovial white blood cell count^[5].

For successful microbiological diagnosis of implant infections, multiple conventional tissue cultures, sonication (using sound energy to dislodge bacteria) of the removed implant or its mobile parts, and synovial fluid aspiration (removing fluids from joints) are recommended^[1].

Media such as Tryptone Soya Agar Blood Agar can be used to examine for the presence of C. acnes. Colonies appear convex, semi-opaque, and glistening and can be pigmented in colors ranging from white to red^[7]. Identification is then confirmed using techniques such as MALDI-ToF (Matrix Assisted Laser Desorption Ionization-time of Flight)^[7].

Treatment approaches

Treatment for acne vulgaris

Most cases of acne vulgaris are controlled with combinations of oral or topical vitamin A derivatives (retinoids), topical benzoyl peroxide, oral or topical antibiotics, and topical beta-hydroxy acids such as salicylic acid^[12].

Benzoyl peroxide kills C. acnes and helps remove excess oils from the skin as well as dead skin cells that clog pores^[1]. Products with benzoyl peroxide strength levels above 2.5% are highly effective^[15].

Oral antibiotics are useful for inflammatory acne, with improvement usually seen after several weeks of use^[9]. However, antibiotic resistance is beginning to emerge, making treatment more challenging^[9][11].

Treatment for implant-associated infections

C. acnes is generally highly susceptible to a wide range of antibiotics, including penicillins, carbapenems, and clindamycin. Vancomycin and teicoplanin have also been used^[9].

Implant-associated infections typically require surgical removal of the infected implant and debridement (cleaning) of any infected tissue^[12]. Extensive and aggressive debridement of all infected tissue with removal of the implant is recommended, as C. acnes infection typically has a delayed presentation following implant surgery^[12].

Surgical therapy needs to be accompanied by prolonged antibiotic treatment to ensure eradication of remaining bacteria^[12]. Recent studies have shown that single-stage exchange (immediate replacement of the device) with oral antibiotics such as rifampin and linezolid can be successful^[8].

Prevention strategies

C. acnes is susceptible to a wide range of chemicals^[1]. The bacterium can be killed with ultraviolet light^[7].

Drug manufacturers and healthcare facilities are advised to maintain adequate quality of incoming materials, ensure sanitary design and cleaning of equipment, establish production and storage time limitations, and monitor environmental conditions^[1]. Appropriate personal protective equipment and gowning procedures are vital in settings where microbial contamination may be an issue^[7].