Taxonomy of mollicutes (mycoplasmas)
Fig. 1. The new taxonomy of the mollicutes (Mycoplasmas). Red taxa represent new names. Green names represent taxa where the names have not been changed and which belong to the same higher taxon as before. Yellow names represent taxa where the names have not been changed, but which have been placed within another higher taxon than before. Genera of importance within veterinary medicine are underlined. Note that not all higher taxa within the phylum Tenericutes are included in this figure. - Click on the image to enlarge it.
Fig. 2. Species of mollicutes (mycoplasmas) with importance for veterinary medicine. At the top of each box is the name of the different genera and below the genus name follow the different species names. The same color coding is used in Fig. 2 as in Fig. 1. All species in the figure are included in VetBact, but A. laidlawii does not have much veterinary significance. - Click on the image to enlarge it.
The taxonomy of mollicutes (mycoplasmas) has previously been controversial because it has not reflected their phylogeny, i.e. their natural relations. A Canadian and an Israeli research group have now carried out a comprehensive study based on whole-genome sequencing, where a thorough revision of the taxonomy of the mollicutes has also been proposed (see reference below).
The revised taxonomy
In the revised taxonomy, a new order has been introduced, namely Mycoplasmoidales, which includes the new families Mycoplasmoidaceae and Metamycoplasmataceae. The family Mycoplasmoidaceae contains the new genera Malacoplasma and Mycoplasmoides as well as the genera Eperythrozoon and Ureaplasma. The genus Eperythrozoon was previously tentatively transferred to the genus Mycoplasma when it became clear that these organisms are not rickettsiae. However, the genus name Eperythrozoon has priority and therefore these organisms should still have the original genus name, even though they belong to the class Mollicutes. The family Metamycoplasmataceae contains the new genera Mesomycoplasma, Metamycoplasma and Mycoplasmopsis.
The only previously called mycoplasmas, which still belong to the order Mycoplasmatales, are those that belong to the so-called the mycoides group. This is good, because in that group one finds the most important "mycoplasmas" from a veterinary and global point of view. Now you should only use the term "mycoplasma" as a trivial name if you are referring to organisms, which belong to the revised genus Mycoplasma. Otherwise, the term "mollicutes" should be used. The genera Spiroplasma, Entomoplasma and Mesoplasma formerly belonged to the order Entomoplasmatales. However, to emphasize the close relationship with the mycoplasmas of the mycoides group, they should be placed in the order Mycoplasmatales. The genera Spiroplasma, Entomoplasma and Mesoplasma have hardly any significance in veterinary medicine, but the latter two should perhaps be placed in the two families (Entomoplasmataceae and Mesoplasmataceae, respectively) under the order Mycoplasmatales.
The new taxonomy for the mollicutes will probably create some misunderstandings and cause confusion because the five new family names are so similar to each other. In the long run, however, the new taxonomy will be a support for those who are interested in the connections between the various organisms within this interesting bacterial group.
Gupta RS, Sawnani S, Adeolu M et al. 2018. Phylogenetic framework for the phylum Tenericutes based on genome sequence data: proposal for the creation of a new order Mycoplasmoidales ord. nov., containing two new families Mycoplasmoidaceae fam. nov. and Metamycoplasmataceae fam. nov. harbouring Eperythrozoon, Ureaplasma and five novel genera. Antonie van Leeuwenhoek 111:1583–1630.