Cutibacterium acnes

Cutibacterium acnes (formerly Propionibacterium acnes) is the relatively slow-growing, typically aerotolerant anaerobic, Gram-positive bacterium (rod) linked to the skin condition of acne; it can also cause chronic blepharitis and endophthalmitis, the latter particularly following intraocular surgery. Its genome has been sequenced and a study has shown several genes can generate enzymes for degrading skin and proteins that may be immunogenic (activating the immune system).
The species is largely commensal and part of the skin flora present on most healthy adult humans' skin. It is usually just barely detectable on the skin of healthy preadolescents. It lives, among other things, primarily on fatty acids in sebum secreted by sebaceous glands in the follicles. It may also be found throughout the gastrointestinal tract.
Originally identified as "Bacillus acnes", it was later named "Propionibacterium acnes" for its ability to generate propionic acid. In 2016, "P. acnes" was taxonomically reclassified as a result of biochemical and genomic studies. In terms of both phylogenetic tree structure and DNA G + C content, the cutaneous species was distinguishable from other species that had been previously categorized as "P. acnes". As part of restructuring, the novel genus "Cutibacterium" was created for the cutaneous species, including those formerly identified as "Propionibacterium acnes", "Propionibacterium avidum", and "Propionibacterium granulosum". Characterization of phylotypes of "C. acnes" is an active field of research.
Role in diseases.
Acne vulgaris.
"C. acnes" bacteria predominantly live deep within follicles and pores, although they are also found on the surface of healthy skin. In these follicles, "C. acnes" bacteria use sebum, cellular debris and metabolic byproducts from the surrounding skin tissue as their primary sources of energy and nutrients. Elevated production of sebum by hyperactive sebaceous glands (sebaceous hyperplasia) or blockage of the follicle can cause "C. acnes" bacteria to grow and multiply.
"C. acnes" bacteria secrete many proteins, including several digestive enzymes. These enzymes are involved in the digestion of sebum and the acquisition of other nutrients. They can also destabilize the layers of cells that form the walls of the follicle. The cellular damage, metabolic byproducts and bacterial debris produced by the rapid growth of "C. acnes" in follicles can trigger inflammation. This inflammation can lead to the symptoms associated with some common skin disorders, such as folliculitis and acne vulgaris. Acne vulgaris is the disease most commonly associated with "C. acnes" infection.
"Staphylococcus" "epidermidis".
The damage caused by "C. acnes" and the associated inflammation make the affected tissue more susceptible to colonization by opportunistic bacteria, such as "Staphylococcus aureus". Preliminary research shows healthy pores are only colonized by "C. acnes", while unhealthy ones universally include the nonpore-resident "Staphylococcus epidermidis", amongst other bacterial contaminants. Whether this is a root causality, just opportunistic and a side effect, or a more complex pathological duality between "C. acnes" and this particular "Staphylococcus" species is not known.
Corneal ulcer.
"C. acnes" has also been found in corneal ulcers, and is a common cause of chronic endophthalmitis following cataract surgery.
Disk herniation.
"C. acnes" has been found in herniated discs. The propionic acid which it secretes creates micro-fractures of the surrounding bone. These micro-fractures are sensitive and it has been found that antibiotics have been helpful in resolving this type of low back pain.
Sarcoidosis.
"C. acnes" can be found in bronchoalveolar lavage of approximately 70% of patients with sarcoidosis and is associated with disease activity, but it can be also found in 23% of controls. The subspecies of "C. acnes" that cause these infections of otherwise sterile tissues (prior to medical procedures), however, are the same subspecies found on the skin of individuals who do not have acne-prone skin, so are likely local contaminants. Moderate to severe acne vulgaris appears to be more often associated with virulent strains.
Opportunistic diseases.
"C. acnes" is often considered an opportunistic pathogen, causing a range of postoperative and device-related infections e.g., surgical infections, post-neurosurgical infections, infected joint prostheses (especially shoulder), neurosurgical shunt infections and endocarditis in patients with prosthetic heart valves (predominantely men). "C. acnes" may play a role in other conditions, including SAPHO (synovitis, acne, pustulosis, hyperostosis, osteitis) syndrome, sarcoidosis and sciatica. It is also suspected a main bacterial source of neuroinflammation in Alzheimer's disease brains. It is a common contaminant in blood and cerebrospinal fluid cultures.
Antimicrobial susceptibility.
"C. acnes" bacteria are susceptible to a wide range of antimicrobial molecules, from both pharmaceutical and natural sources. The antibiotics most frequently used to treat acne vulgaris are erythromycin, clindamycin, doxycycline, and minocycline. Several other families of antibiotics are also active against "C. acnes" bacteria, including quinolones, cephalosporins, pleuromutilins, penicillins, and sulfonamides.
Antibiotic-resistance.
The emergence of antibiotic-resistant "C. acnes" bacteria represents a growing problem worldwide. The problem is especially pronounced in North America and Europe. The antibiotic families that "C. acnes" are most likely to acquire resistance to are the macrolides (e.g., erythromycin and azithromycin), lincosamides (e.g., clindamycin) and tetracyclines (e.g., doxycycline and minocycline).
However, "C. acnes" bacteria are susceptible to many types of antimicrobial chemicals found in over-the-counter antibacterial products, including benzoyl peroxide, triclosan, chloroxylenol, and chlorhexidine gluconate.
Several naturally occurring molecules and compounds are toxic to "C. acnes" bacteria. Some essential oils such as rosemary, tea tree oil, clove oil, and citrus oils contain antibacterial chemicals. Natural honey has also been shown to have some antibacterial properties that may be active against "C. acnes".
The elements silver, sulfur, and copper have also been demonstrated to be toxic towards many bacteria, including "C. acnes".
Photosensitivity.
"C. acnes" glows orange when exposed to blacklight, possibly due to the presence of endogenous porphyrins. It is also killed by ultraviolet light. "C. acnes" is especially sensitive to light in the 405–420 nanometer (near the ultraviolet) range due to an endogenic porphyrin–coporphyrin III. A total irradiance of 320 Joules/cm2 inactivates this species "in vitro". Its photosensitivity can be enhanced by pretreatment with aminolevulinic acid, which boosts production of this chemical, although this causes significant side effects in humans, and in practice was not significantly better than the light treatment alone.
Other habitats.
"C. acnes" has been found to be an endophyte of plants. Notably, grapevine appears to host an endophytic population of "C. acnes" that is closely related to the human-associated strains. The two lines diverged roughly 7,000 years ago, at about the same time when grapevine agriculture may have been established. This "C. acnes" subtype was dubbed "Zappae" in honour of the eccentric composer Frank Zappa, to highlight its unexpected and unconventional habitat.