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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).
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).
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).
All members of the genus are all
nonmotile (Cummins and Johnson, 1986).
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).
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
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
Five genomes of Propionibacterium species and sequencing data from a P. granulosum plasmid are available via
the National Center for Biotechnology Information (NCBI).
*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).
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).
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).
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).
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