The lack of a cell wall offers them drug resistance to commonly used antibiotics that inhibit cell wall synthesis such as penicillin or other beta-lactam antibiotics. Mycoplasma species have adapted over time to live as independent bacteria or parasitize the host. They have a wide host range that allows them to infect humans, animals, plants and other genera. While many species of Mycoplasma are considered commensals and are able to live in the host without causing disease or can be considered opportunistic in immunocompromised hosts, others are implicated in clinical conditions. Atypical or walking pneumonia is often caused by Mycoplasma pneumoniae or Mycoplasma hominis while non-specific urethritis is very commonly caused by Mycoplasma genitalium which is also thought to be involved in pelvic inflammatory disease. Several species are pathogenic in humans, including M. pneumoniae, which is an important cause of atypical pneumonia and other respiratory disorders, and M. genitalium, which is believed to be involved in pelvic inflammatory diseases.
Mollicutes have been reported to seriously interfere with life sciences research when cell lines or laboratory animals are infected. The bacteria can live inside and outside of the cells, deplete nutrients from the medium and change the biology of cells, disrupt metabolism, modify the immune response, inhibit or prevent viral replication or have unexplained findings in cell culture experiments.
Nocard and Roux reported the cultivation of the causative agent of Contagious Bovine Pleuro Pneumoniae (CBPP) in 1898, a disease that was widespread in cattle herds and was found to be caused by M. mycoides subsp. mycoides SC (small-colony type). In 1956, the first species of Mycoplasma were isolated in culture. The most commonly encountered species are Acholeplasma laidlawii, Mycoplasma arginini, M. orale, M. salivarium, M. fermentans, and M. hyorhinis. Species such as Ureaplasma urealyticum, M. pneumoniae and M. pirum are rarely present in cell cultures. It is anticipated that many of the cell lines created prior to 1990 were infected by Mycoplasma species that originated from contaminated animal serum and further disseminated by contaminated aerosols in the laboratory and laboratory technicians while they conducted their experiments.
A. laidlawii and M. arginini are of bovine origin; M. orale and M. salivarium are of a human origin (oropharynx) and M. hyorhinis originated from swine. It is hypothesized that presence of Mycoplasmas of swine origin in bovine serum is justified by the contamination of this product in mixed slaughterhouses.
It is estimated that as many as 30% of all cell lines generated prior to 1990 are contaminated with Mycoplasma species.