Mycoplasma orale is a small bacterium found in the class Mollicutes. It belongs to the genus Mycoplasma, a well-known group of bacterial parasites that inhabit humans. It also is known to be an opportunistic pathogen in immunocompromised humans. As with other Mycoplasma species, M. orale is not readily treated with many antibiotics due to its lack of a peptidoglycan cell wall. Therefore, this species is relevant to the medical field as physicians face the task of treating patients infected with this microbe. It is characterized by a small physical size (0.1 micrometer in diameter on average), a small genome size, and a limited metabolism. It is also known to frequently contaminate laboratory experiments. This bacteria is very similar physiologically and morphologically to its sister species within the genus Mycoplasma; however, its recent discovery leaves many questions still unanswered about this microbe.
Discovery and isolation
Taylor-Robinson et al. identified and named M. orale in 1964 after isolating it from the oral cavity of a child at a Washington hospital and three servicemen stationed in North Carolina. Taylor-Robinson et al. cultured the organism under anaerobic conditions on PPLO agar and broth. The resulting colony growth had a "fried egg" appearance when cultured on solid PPLO agar. PPLO is stands for "Pleuropneumonia-like organisms" and was originally designed for growth of parasitic bacteria found in the respiratory tract of cattle; it is now known that these bacteria were Mycoplasma species. PPLO agar is used for isolation and growth of Mycoplasma and Ureaplasma species; it contains beef heart infusion, yeast extract, phenol red, sodium chloride, the antibiotic Polymyxin B, the antifungal Amphotericin B, the antibiotic Penicillin, deionized water, and horse serum. Taylor-Robinson et al. confirmed M. orale was distinct from previously discovered Mycoplasma species with a serological test using haemagglutination of sheep erythrocytes, tannic acid and sonnicated Mycoplasma extracts.
L.G. Tallgren et al. also isolated M. orale in 1974 from a sample of bone marrow taken from a three-year-old boy with Eosinophilic leukaemia. The sample was obtained via a bone marrow biopsy performed at Aurora Hospital in Helsinki, Finland. L.G. Tallgren et al. noted this organism had a typical Mycoplasma "fried egg" appearance when cultured on solid media.
Taxonomic and phylogenetic classification
As determined by 16S rRNA sequencing, M. orale is a bacterium belonging to the genus Mycoplasma. Its closest relative within the genus is M. salivarius, while its most distant is M. mycoides. M. orale and M. salivarius have both been observed in the human oral cavity, evidencing their close proximity in the genus Mycoplasma.
Of the over 100 documented species in the genus Mycoplasma, 14 are known pathogens to humans. This comes as no surprise, as the class Mollicutes contains common commensals or pathogens of several different organisms. Many species in the genus Mycoplasma are commonly found associated with pelvic or genital region infections including M. fermentans and M. hominis. Other species in this genus are the causative agents of respiratory related infections; these species include the well-known Mycoplasma pneumoniae. The family Mycoplastaceae includes the genera Mycoplasma and Ureaplasma. Bacteria in the genus Ureaplasma are known commensals in humans and possess the enzyme Urease (catalyzes urea to carbon dioxide and ammonia). Both Mycoplasma and Ureaplasma species can be grown on the same PPLO medium due to their similarities in metabolism and growth requirement.