The bacterial species Propionibacterium acnes belongs to the propionic acid bacteria of the order Actinomycetales and the division Actinobacteria. As commensals, anaerobes naturally colonize human skin, but they can also become a pathogen in the form of individual strains. In this context, they play a role for Acnes vulgaris, for example.
What is Propionibacterium acnes?
Propionibacteria or propionic acid bacteria have gram-positive staining behavior and belong to the order Actinomycetales of the division Actinobacteria. They produce propionic acid as a metabolic end product by the methylmalonyl-CoA pathway. One bacterial species of the genus Propionibacteria is Propionibacterium acnes, of which about 100 different strains exist. The bacterial species are anaerobic bacteria formerly known as Bacillus acnes or Corynebacterium acnes. Their shape corresponds to a short rod shape. Ellipsoidal cell shapes also occur. Single cells have a width of up to 0.5 micrometers and a length of up to 0.9 micrometers. The cells are usually arranged in angled pairs. As cell divisions progress, they thus form V- and Y-shaped cell chains. The bacterial species does not possess flagella for active locomotion. The species does not form terminal spores, nor other forms of survival. Among the 100 strains of the species, most are commensals that neither benefit nor harm humans. Some strains are under suspicion of pathogenicity. Research has not been completed for all strains.
Occurrence, distribution, and characteristics
Propionibacterium acnes forms roundish colonies in solid culture media with an anaerobic environment after about four days, which appear raised when viewed from the side and also appear smooth or shiny. The diameter is up to four millimeters. As anaerobic bacteria, they are oxygen tolerant, but do not necessarily need oxygen for their metabolism, but are rather inhibited in growth by 02. The bacteria possess the enzyme catalase and form cytochromes. 37 degrees Celsius corresponds to the optimal cultivation temperature, so that colonization is favorable for their growth. The ideal pH value in the culture medium is neutral. At the same time, growth is rather slow even under ideal conditions. Propionibacteria acnes engage in chemoorganotrophic heterotrophic metabolism. In this metabolic pathway, organic compounds become energy sources and serve to build up cellular substances. A major product of its fermentation is propionic acid, so the fermentation activity is sometimes referred to as propionic acid fermentation. Acetic acid and carbon dioxide are formed as by-products. Carbohydrates such as glucose, fructose or mannose and galactose usually serve as substrates. In addition to catalase, the bacteria possess proteolytic enzymes for the degradation of gelatin and, via nitrate reductase, for nitrate liquefaction. Some strains engage in hemolysis in the blood. The bacteria of the species preferentially colonize the sebum of the hair follicle and are sometimes found in the digestive tract of humans.
Significance and function
Up to 100,000 Propionibacteria acnes are present on one square centimeter of human skin without causing disease. In this context, Propionibacterium acnes is predominantly considered a commensal and thus naturally colonizes the skin flora of humans without benefiting or harming them. Commensals do not harm their host for their own interests and thus create a longer-term habitat for themselves. Of the at least 100 different strains of the species Propionibacterium acnes, most strains are considered commensals, but there appears to be some pathogenicity for individual strains. Pathogenic bacteria harm their host and cause disease. The Biosubstances Ordinance and TRBA assign Propionibacterium acnes to risk group 2 because of its, albeit small, number of pathogenic strains. According to the ordinance, microorganisms in this group are “biological substances that can cause a disease in humans”. On the human skin, the bacterium can therefore correspond to a harmless inhabitant, but also to a pathogen. The spread of the bacteria into the blood can have all the more pathogenic consequences, so especially for immunodeficient patients. Under which conditions the individual strains are pathogenic on the skin and under which conditions they correspond to commensals on the skin has not yet been conclusively clarified.
Diseases and ailments
As a causative agent, the bacterial species is primarily associated with acne vulgaris and is thought to be a secondary causative factor. Accordingly, strains of the bacterial species may be detectable on healthy human skin without causing acne vulgaris. On the other hand, however, increased bacterial colonization with Propionibacterium acnes is frequently found on the skin of patients with acne vulgaris. The pathogenic strains of this species are thought to stimulate cytokine and chemokine production in the cells of the sebaceous glands. These substances promote inflammatory processes in acne vulgaris. Acne is the most common skin disease worldwide. Acne-related inflammation can be accompanied by redness, swelling and itching. Frequently, scars remain. Propionibacterium acnes multiplies in the comedones under largely anaerobic conditions and uses the enzyme lipase to break down components of the skin sebum for energy production. This leads to the formation of pro-inflammatory substances which, by means of chemotaxis, attract leukocytes into the tissue which, after dying off, cause pus-filled pustules to form. In addition to acne, diseases such as keratitis and sarcoidosis are now also associated with Propionibacterium acnes. Cases of spondylodiscitis, central nervous system infections, endocardial infections, osteomyelitis, and endophthalmitis or joint infection have already been documented in association with Propionibacterium acnes. In addition, circulating immune complexes against Propionibacterium acnes antigens are associated with SAPHO syndrome from the rheumatic form circle. The attachment of the immune complexes to bones and joints is thought to possibly induce immune responses in the context of SAPHO syndrome, which could explain the rheumatic symptoms.
