Propionibacterium is a Gram-positive, anaerobic genus named for its characteristic production of large amounts of propionic acid. First described by Orla-Jensen in 1909, the genus is typically divided into two principal groups, the classical or dairy propionibacteria and the cutaneous propionibacteria or acne group strains. Both groups serve well-known, though very different functions. The classical group is often used in production of dairy products, particularly swiss cheese and other natural fermentations, while the acne group strains is a normal inhabitant of human skin and elsewhere on the body involved in causing acne vulgaris and other bacterial infections (Cummins and Johnson, 1986).
All Propionibacterium sp. exist as pleomorphic rods ranging from 0.5-0.8 um in diameter and 1-5 um in length. Cells are irregularly shaped and can be coccoid, bifid, or branched, though they often appear club-shaped, or diphtheroid. In addition, P. acnes strains are especially long and slender. Colonies range from white, gray, pink, red, yellow, or orange in color and cells may occur singly or appear in a variety of arrangements. All taxon members are Gram-positive and non-sporing (Cummins and Johnson, 1986).
The classical, or dairy, propionibacteria generally inhabit raw-milk, cheese, and silage (Faye et al, 2011). In addition, P. microaerophilum, a classical propionibacteria species, has been found in olive mill wastewater (Koussemon et al., 2001). In contrast, the acne group strains typically are isolated from sebaceous glands on the skin (Cogen et al., 2009) though are also found in the gastrointestinal tract, mouth, eyes, and other tissues (Brook, 1994).
As anaerobic chemoorganotrophs, all species of Propionibacterium require organic material, preferentially lactate or other sugars, for growth. In addition, all strains require pantothenate, or vitamin B5, and experience no or very little growth in aerobic environments. The dairy group typically grows best at 30-32°C while the cutaneous group grows best at 36-37°C. All strains grow well on trypticase-yeast extract-glucose medium under anaerobic conditions (Cummins and Johnson, 1986).
Only one of the two principal groups of Propionibacterium exists regularly with humans. Acne group strains, including P. acnes and P. granulosum, are found in large numbers on human skin after the onset of puberty (Leyden et al., 1975). Propionibacterium sp. regularly colonizes other regions of the body including the gastrointestinal tract and parts of the mouth and eyes (Brook, 1994). P. acnes normally exists as a commensal bacteria, lowering skin pH and releasing bacteriocins to prevent infection from other bacteria, however in some cases acts as an opportunistic pathogen causing infections such as acne vulgaris (Cogen et al., 2009).
In contrast, classical propionibacteria have not been shown to exist with humans or other animals, instead colonizing various foods and natural fermentations, such as dairy products, silage, and olives (Cummins and Johnson, 1986). In particular, P. freudenreichii and other dairy propionibacteria are used as starters in the making of Swiss cheese, lending to the flavor and holes in the cheese (Deustch et al., 2012).
In addition, Propionibacterium sp. has been found to associate with some insects. These bacteria were detected in the midgut of three different Saturniidae species of tropical caterpillars, regardless of the species diet. DNA recovered from eggs suggest the bacterial symbionts may be inherited vertically (Pinto-tomás et al., 2011). Though the nature of its role is not yet understood, Propionibacteria have also been shown to associate with members of the Nasonia genus (Brucker and Bordenstein, 2012).
Diseases and Parasites
Research suggests P. acnes plays a direct role in causing acne vulgaris though the exact mechanism is unknown (Dessinioti and Katsambas, 2010). P. acnes has also been linked to numerous other infections including dental, ophthalmic, and medical device, such as prostheses, infections (McDowell et al., 2012). In rare cases, P. acnes can cause more serious diseases such as brain abscesses and other central nervous system (CNS) infections (Chung et al., 2011).
Other species of Propionibacterium have also been indicated as causative disease agents. P. propionicum has been indicated to cause bacterial infections similar to actinomycosis , a chronic bacterial infection that normally infects the skin an is caused by Actinomyces bacteria (Wunderink et al., 2011).
Life History and Behavior
All members of the genus are all nonmotile (Cummins and Johnson, 1986).
Evolution and Systematics
The acne group strains including P. acnes, P. avidum, P. granulosum, and P. lymphophilum, were reclassified and added to the genus Propionibacterium in 1974 after previously belonging to Corynebacterium. Propionibacterium of both principal groups are irregular, nonsporing, Gram-positive rods. In contrast to other closely related taxa, they grow anaerobically, have a high G + C content, and produce large amounts of propionic acid (Cummins and Johnson, 1986).
Within the genus, species are differentiated through genomic as well as metabolic and compositional differences. Besides variation in habitat, species have been separated based on factors such as their ability to utilize different sugars, reduce nitrate, and the composition of their cell walls (Cummins and Johnson, 1986). More recently, 16S rRNA sequencing has lead to the identification of P. humerusii, which was previously labeled as P. acnes (Butler-Wu et al., 2011).
Physiology and Cell Biology
Propionibacterium are anaerobic to aerotolerant bacteria that undergo fermentation as their form of metabolism. The name Propionibacterium is derived from the production of large amounts of propionic, and in lesser amounts acetic acid and carbon dioxide, as the major end products of metatabolism. In addition, fermentation of lactate and other sugars may produce lesser amounts of other organic acids (Cummins and Johnson, 1986). Metabolism is complex, undergoing multiple cycles in which sugars, preferentially lactate in dairy strains, is fermented to produce propionic acid (Piveteau, 1999). Some strains of P. acnes demonstrate resistance to the antibiotics tetracycline and erythromycin due to mutations to rRNA genes (McDowell et al., 2012).
Molecular Biology and Genetics
Five genomes of Propionibacterium species and sequencing data from a P. granulosum plasmid are available via the National Center for Biotechnology Information (NCBI).
- Propionibacterium acnes (Bruggemann et al., 2004)
- Size = 2.49 Mb
- GC content = 60%
- Propionibacterium freudenreichii (Falentin et al., 2010)
- Size = 2.62 Mb
- GC content = 67.3%
- Propionibacterium humerusii (Butler-Wu et al., 2011)
- Size = 2.62 Mb
- GC content = 59.9%
- Propionibacterium propionicum*
- Size = 3.45 Mb
- GC content = 66.1%
- Propionibacterium avidum*
- Size = 2.53 Mb
- GC content = N/A
*Genome sequencing in progress
A plasmid, pPG01, was analyzed after being isolated from P. granulosum. The plasmid is the first to be isolated from cutaneous propionibacteria and is composed of 3539 basepairs (bp) with 57.4% G + C content. Analysis revealed three open reading frames (ORFs), two of which encode proteins with homologs in other bacteria. Two of the ORFs encode proteins homologous to other proteins involved in DNA transfer and plasmid replication found in Streptomyces ghanaensis and Arcanobacterium pyogenes respectively. The third ORF produced no homologous proteins (Farrar et al., 2007).
Through comparisons of multiple P. acnes strains possible virulence factors were identified. Inducing inflammation appears to occur with some P. acnes and not others due to both differences between strain genomes and variation in gene expression. Potential virulence factors including enzymes involved in metabolism as well as cell adhesion molecules were all demonstrated to be upregulated in certain strains of P. acnes that caused skin inflammation suggesting the variation in gene expression between types may be the major cause of disease (Brzuszkiewicz et al., 2011). Distinct virulence factors in the form of surface antigens may contribute to pathogenic instead of commensal strain activity as well (McDowell et al., 2012).
Different members of Propionibacterium produce a variety of unique antimicrobial peptides. Though structurally similar to other bacteriocins, some of the peptides produced by this genus are produced and interact with other species in novel ways. P. jensenii produces a protease-activated antimicrobial peptide (PAMP) in a precursor form as a proprotein that is constitutively produced and released from the cell. PAMP is then only activated after undergoing proteolytic processing outside the cell. However, the necessary proteases are not secreted with PAMP, instead relying on the enzymatic activity of other competing cells (Faye et al., 2002). Two other unique bacteriocins, Propionicin T1, an unmodified general secretory pathway bacteriocin, and Propionicin F, which displays intraspecies specificity to P. freudenreichii are produced by members of this genus. (Faye et al. 2011).
Relevance to Humans and Ecosystems
Classical Propionibacterium sp. have long been used in the production of different dairy products, in particular the ripening of swiss cheese, but may serve a number of uses in the future due to their antimicrobial properties (Piveteau, 1999). Propionibacterium jensenii have been shown while in mixture with certain Lactobacillus sp. to inhibit yeast growth, thereby preventing spoilage in dairy products such as yogurt and cheese (Schwenniger and Miele, 2004). In addition, research has shown classical or dairy Propionibacterium produce unique bacteriocins that may serve biotechnological applications in the future (Faye et al., 2011).
Its members are primarily facultative parasites and commensals of humans and other animals, living in and around the sweat glands, sebaceous glands, and other areas of the skin. They are virtually ubiquitous and do not cause problems for most people, but propionobacteria have been implicated in acne and other skin conditions. One study found that Propionibacterium were the most prevalent human skin-associated microorganism.
- Madigan, Michael T (2012). Brock: Biology of microorganisms (13th ed.). p. Apendix 2 page 12.
- Propionibacterium at National Center for Biotechnology Information (NCBI)
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- Bojar, R., and Holland, K. "Acne and propionibacterium acnes." 2004. Clinics in Dermatology 22(5), pg. 375-379.
- Rust, Susanne (4 February 2012). "UC Berkeley Bacteria Study: Research Shows Humans A Major Source Of Germs". Huffington Post (San Francisco). Retrieved 2012-04-06.
- Kiatpapan P., Murooka Y. Genetic manipulation system in propionibacteria. Journal of Bioscience and Bioengineering. 93 (1) (pp 1-8), 2002
- Making Swiss Cheese - David B. Fankhauser, Ph.D.
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