Bacteria; Actinobacteria; Actinobacteria; Propionibacteriales; Propionibacteriaceae; Pseudopropionibacterium (1). Pseudopropionibacterium propionicum (1). The taxa Pseudopropionibacterium propionicum was originally known as Actinomyces propionicus, then renamed Arachnia propionica (1). Until 2018, Pseudopropionibacterium propinicum was also recognized as Propionibacterium propionicus (2).
2. Description and significance
Pseudopropionibacterium propionicum is a Gram-positive species of bacteria found in human or animal hosts, most commonly in the human mouth (3). It grows as filaments as well as branching and diphtheroid rods (3). P. propionicum is an aerotolerant anaerobe that metabolizes carbohydrates through the process of fermentation, producing multiple end products such as propionic acid (3, 4). The nomenclature of this microbe has changed multiple times, resulting in confusion in its identification and treatment in clinical settings (2). Sequencing of the 16S rRNA of P. propionicum and comparison with other taxa to determine sequence homology has aided in the reclassification of this microbe (5). P. propionicum is of interest to the broader public due to its presence in the human oral microbiome and its pathogenic capability, often listed as a leading cause of actinomycosis. P. propionicum can access oral tissues with improper blood supplies and as a result, cause various infections within humans, especially those who are immunocompromised (6).
3. Genome structure
In 2016, the genome sequence of P. propionicum was published online but current literature on the genome still lacks. From what is known, the genome consists of 3,449,360 base pairs on a circular chromosome (7). Within the genome of P. propionicum, there are 2,975 protein-encoding genes, 48 tRNA-encoding genes, 6 rRNA-encoding genes, 3 ncRNA-encoding genes, and 60 pseudogenes (7). The genome’s %GC content is relatively high but the exact percentage is unknown (7). Sequencing the 16S rRNA of P. propionicum allowed for re-classification of Arachnia propionica to P. propionicus in 1988 (5). P. propionicum has been found to have 92.5% and 93.2% sequence homology in 16S rRNA with Propionibacterium freudenreichii and Propionibacterium acnes, respectively (5). In contrast, P. propionicum has less than 90% sequence homology with the 16S rRNA of other taxa, such as Actinomyces bovis and Actinomyces viscosus, that it more closely resembles morphologically or pathologically (5).
4. Cell structure
P. propionicum is a Gram-positive bacteria that grows as filaments, branching, and diphtheroid rods (3). It is spore-forming (3). The rods are nonmotile and not acid fast. The peptidoglycan layer of the cell wall of P. propionicum consists of muramic acid, N-acetylglucosamine, glutamic acid, glycine, and L-diaminopimelic acid (3).
5. Metabolic processes
P. propionicum is an aerotolerant anaerobic bacteria; it can live in the presence of oxygen but does not require oxygen for its survival (4). P. propionicum metabolizes carbohydrates through the process of fermentation (4). As end products of fermentation, P. propionicum produces the following carboxylic acids: propionic acid, acetic acid, formic acid, and succinic acid (in trace amounts) (3, 4). The presence of P. propionicum can be determined in human samples by using gas-liquid chromatography and other tests that identify the characteristic end products of its fermentation, such as glucose fermentation tests to reveal if propionic acid is produced (4). P. propionicum is also capable of reducing nitrates (4). P. propionicum is not known to produce any exotoxins as a result of its metabolic processes (3). If provided a nutrient-rich culture medium, P. propionicum will grow readily in a laboratory setting (3).
P. propionicum lives within human or other animal hosts and has not been found living independently in other ecosystems, such as soil or bodies of water (3). P. propionicum is normally a commensal of humans but can be pathogenic as well (4). P. propionicum has been most often studied as a member of the human oral microbiome, where it can be found in the saliva, on the tongue, and in the tonsils (3). Although the exact composition of the human oral microbiome varies between individuals, P. propionicum is generally found in the presence of bacteria that belong to the following genera: Actinomyces, Rothia, Propionibacteria, Lactobacilli, Bifidobacteria, and Corynebacteria (4). In cases of infection, P. propinionicum can also live within the subgingival cavities (8) and dental pulp (9, 10) of the human mouth and, more rarely, in the lungs (11), lacrimal canaliculi (6), and female pelvis (12, 13).
P. propionicum, an otherwise normal bacteria present in the human flora, can become pathogenic if exposed to bodily tissues with inadequate blood supplies. P. propionicum does not produce toxins and for this reason, the mechanisms by which it causes disease are unclear (3). P. propionicum is an important contributor to the formation of extraradicular biofilm on external root surfaces of teeth, as well as to persistent periapical infections (10). In addition, P. propionicum can be found in a significant number of both primary endodontic infections and chronic periradicular lesions in humans, suggesting that P. propionicum may be an endodontic pathogen (9). P. propionicum is a leading cause of actinomycosis, a chronic condition characterized by swelling and formation of abscesses in the sinus cavities of either the neck and face, chest, abdomen, or sometimes the pelvis (3). P. propionicum is most often found in the neck and face of patients with actinomycosis, but recent studies suggest that it can infect the chest and pelvis as well (2, 12, 13). These abscesses are frequently pus-filled and are composed of Gram-positive fibrous walls and sulfur granules (3). In order to diagnose actinomycosis, bacterial cells need to be extracted from the site of infection, isolated, and confirmed. Many who develop actinomycosis usually report a recent surgery, an implantation of a foreign body, or being immunodeficient prior to feeling symptomatic (12). When a patient presents with an infection or disease amplified by P. propionicum, antibiotic drugs are almost always considered in their treatment plans. Penicillin is typically the antibiotic drug of choice when treating P. propionicum-based infections (6). Despite this, it is important to note that different strains of P. propionicum have different susceptibilities to antibiotics. For example, P. propionicum is less susceptible to amikacin and ceftazidime, two other antibiotics commonly used to treat bacterial infections (6).
8. Current Research
The classification of Propionibacterium is constantly changing due to new and more thorough studies of microbes. One of the recent classification changes is the subdivision of the genus Propionibacterium into Propionibacterium, Acidipropionibacterium, Cutibacterium, and Pseudopropionibacterium. The genus Pseudopropionibacterium contains the species Pseudopropionibacterium propionicum, which is the nomenclatural type (14). Both the nomenclatural types Propionibacterium propionicus and Pseudopropionibacterium propionicum were used to refer to this microbe in 2016 (14). In 2018, Pseudopropionibacterium propionicum was recognized as the replaced name for Propionibacterium propionicum after a taxonomy change (2). Due to the many nomenclature changes of P. propionicum, physicians may be unable to recognize the cause of certain infections, and therefore fail to remove P. propionicum from the site of infection (2). This failure to remove the bacteria can cause longer lasting infections and actinomycosis in infected areas (2). While it is widely known that P. propionicum causes primarily endodontic infections and chronic periradicular lesions in humans (8), more recent research has revealed P. propionicum as the primary cause of infections in other body regions, such as the chest, pelvis, and hip (2, 12, 13). In one such case, pelvic actinomycosis presented as a tubo-ovarian abscess in a thirty eight year old woman after removal of an IUD (12). This infection was not initially identified as P. propionicum, and antibiotic treatments were initially unsuccessful (12). Surgery was completed to remove the bacteria along with the abscess, and the patient’s condition was stabilized (12). Consistent with occurrences of P. propionicum infections elsewhere in the body, P. propionicum appears to have become pathogenic in the pelvis of this patient patient after immunosuppression or some sort of medical procedure or trauma, in this case IUD insertion (12). Opportunistic pathogenicity of P. propionicum outside of the head and neck region has also been observed recently in a seventy-one year old male with a recurrent right chest wall abscess. He had surgery on his right chest wall one year prior to the infection, which had been healed for the entire year leading up to this occurrence (2). His chest was drained of pus multiple times, and eventually the bacteria causing this abscess was identified as P. propionicum (2). After drainage of all bacteria and being treated with the proper antibiotics, the infection did not recur in this patient (2). New research is being conducted to aid in early identification of P. propionicum, but few studies have been published on this topic as of 2020. Earlier identification and correct treatment for actinomycotic infection would decrease the frequency of infection relapse, subsequently decreasing some of the painful symptoms associated with infection caused by this bacteria (2). P. propionicum is capable of causing actinomycotic infections in more areas than just the oral cavity, and research to aid in its identification in other areas of the body continues (2).
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Edited by [Matthew Ball, Nina Castano, Nina Hardy, Taylor Epstein, Amy Huang], student of Jennifer Bhatnagar for [http://www.bu.edu/academics/cas/courses/cas-bi-311/ BI 311 General Microbiology], 2020, Boston University.