Originally posted 26 August 2012
Mycoplasma, the smallest prokaryotic cell capable of cell replication, is a frequent annoyance to scientists and laboratory workers from contamination of painstakingly produced cell cultures. Present as natural flora on the mucosal surfaces of human and animals this organism is ubiquitous in nature.
Anne Fischer and her colleagues, in her 2012 PLOS one paper, using a ‘Mycoplasma mycoides cluster’ composed of 118 strains it was possible to estimate the origin of this cluster revealing that its formation coincided with the domestication of livestock, an estimated 10,000 years ago. Domestication resulted in large populations of cattle, previously having roamed wild, being kept together at a high stocking density with the mixing of ruminant species. The close contact and mixing that occurs with this domestication facilitated the spread of this organism by aerosol and contact. Previous studies have shown genetic exchange is possible between different Mycoplasma species within the genus Mycoplasma in the same host. This study shows that it is possible between different species in different primary hosts.
Mycoplasma mycoides subsp. mycoides, the organism responsible for contagious bovine pleuropneumonia (CBPP), and Mycoplasma capricolum subsp. capripneumoniae, responsible for contagious caprine pleuropneumonia (CCPP), are the two most important organisms in the ‘Mycoplasma mycoides cluster’. The cluster also includes M. capricolum subsp. capricolum, M. leachii and M. mycoides subsp. capri. CBPP is a severe disease of cattle that has wrecked havoc upon the livestock industry over the millennia. Formerly it had a worldwide distribution after introduction from Europe to Africa and Asia by colonists. It is now endemic in Africa, the Middle East and Asia. It remains a constant risk to disease free countries with sporadic occurrences in Europe. However, those occurrences in Europe did not arise from African strains. CCPP first occurred in South Africa in 1881 in goats imported from Turkey and is now present in parts of northern and eastern Africa and Turkey. Nomadic herds often transport the organism to disease free regions. This disease can devastate herds with mortality rates reaching 80%.
This bacteria, first thought to be a fungus by Albert Bernhard Frank, was isolated by Nocard and Roux in 1898. This Mycoplasma was the causative agent of CBPP. CBPP and CCPP result in a high fever, anorexia, pneumonia and increased respiratory rate with a characteristic head down and elbows abducted stance. The disease is usually fatal three weeks after the onset of clinical signs. Less severe strains are characterised by arthritis, mastitis, pneumonia, septicaemia and agalactia.
Previous studies show either evolutionary relationships of individual members or relationships present between certain members of the ‘M. mycoides cluster’. This study creates a comprehensive overview of the entire history of the cluster.
Four distinct populations were found in this study; three of which belonged to the M. mycoides cluster. M. mycoides subsp. mycoides, M. mycoides subsp. capri, M. capricolum/M. leachii and a fourth distinct unassigned population which consisted of Mycoplasma, closely related to others from the cluster, was isolated from caprinae.
M. leachii is a hybrid of M. mycoides subsp. mycoides and M. capricolum with all 11 isolates showing at least 30% ancestry from M. mycoides subsp. mycoides. It is more likely to cause arthritis or mastitis than pneumonia. It has not formed from a distinct genetic population but due to hybridization within a host facilitated by the mixing of species; it formed from different Mycoplasma species in different primary hosts. The strain B144P, formerly Mycoplasma sp. serogroup L, an infectious agent of cattle, shows more evidence for hybridization. It possesses 60% ancestry from M. capricolum, 30% from M. mycoides subsp. mycoides, and 10% from M. mycoides subsp. capricolum and is a member of the M. leachii hybrid subpopulation. Mycoplasma from the ‘Mycoplasma mycoides cluster’ cannot survive in the environment; it requires a host. They can survive in non-primary hosts allowing horizontal gene transfer between species to occur.
Hybridization and recombination are thought to be important mechanisms for evolution of the cluster. M. mycoides subsp. capricolum has the highest genetic diversity while the others in this cluster do not.
Mycoplasma mycoides may have arisen around the same time as domestication of livestock occurred but the two most pathogenic strains did not. They are much younger; only forming during the last few millenia. The mixing of animals and co-infection facilitated Mycoplasma mycoides, but increased movement and intensive high scale production of ruminants in more recent times may have created these more destructive organisms.
A, Fischer et al. (2012) The Origin of the ‘Mycoplasma mycoides Cluster’ Coincides with Domestication of Ruminants. PLoS ONE, 7(4): e36150.doi:10.1371/journal.pone.0036150