Mycoplasma - Taxonomy

Taxonomy

The medical and agricultural importance of members of the genus Mycoplasma and related genera has led to the extensive cataloging of many of these organisms by culture, serology, and small subunit rRNA gene and whole genome sequencing. A recent focus in the sub-discipline of molecular phylogenetics has both clarified and confused certain aspects of the organization of the class Mollicutes.

Originally the trivial name "mycoplasmas" has commonly denoted all members of the class Mollicutes. The name "Mollicutes" is derived from the Latin mollis (soft) and cutes (skin), and all of these bacteria do lack a cell wall and the genetic capability to synthesize peptidoglycan. Now Mycoplasma is a genus in Mollicutes. Despite the lack of a cell wall, many taxonomists have classified Mycoplasma and relatives in the phylum Firmicutes, consisting of low G+C Gram-positive bacteria such as Clostridium, Lactobacillus, and Streptococcus based on 16S rRNA gene analysis. The order Mycoplasmatales contains a single family, Mycoplasmataceae, comprising two genera: Mycoplasma and Ureaplasma.

Historically, the description of a bacterium lacking a cell wall was sufficient to classify it to the genus Mycoplasma and as such it is the oldest and largest genus of the class with about half of the class' species (107 validly described), each usually limited to a specific host and with many hosts harboring more than one species, some pathogenic and some commensal. In later studies, many of these species were found to be phylogenetically distributed among at least three separate orders.

A limiting criterion for inclusion within the genus Mycoplasma is that the organism have a vertebrate host. In fact, the type species, M. mycoides, along with other significant mycoplasma species like M. capricolum, is evolutionarily more closely related to the genus Spiroplasma in the order Entomoplasmatales than to the other members of the Mycoplasma genus. This and other discrepancies will likely remain unresolved because of the extreme confusion that change could engender among the medical and agricultural communities.

The remaining species in the genus Mycoplasma are divided into three non-taxonomic groups, hominis, pneumoniae and fermentans, based on 16S rRNA gene sequences.
The hominis group contains the phylogenetic clusters of M. bovis, M. pulmonis, and M. hominis, among others. M. hyopneumoniae is a primary bacterial agent of the porcine respiratory disease complex.

The pneumoniae group contains the clusters of M. muris, M. fastidiosum, U. urealyticum, the currently unculturable haemotrophic mollicutes, informally referred to as haemoplasmas (recently transferred from the genera Haemobartonella and Eperythrozoon), and the M. pneumoniae cluster. This cluster contains the species (and the usual or likely host) M. alvi (bovine), M. amphoriforme (human), M. gallisepticum (avian), M. genitalium (human), M. imitans (avian), M. pirum (uncertain/human), M. testudinis (tortoises), and M. pneumoniae (human). Most if not all of these species share some otherwise unique characteristics including an attachment organelle, homologs of the M. pneumoniae cytadherence-accessory proteins, and specialized modifications of the cell division apparatus.

A study of 143 genes in 15 species of Mycoplasma suggests that the genus can be grouped into four clades: the M. hyopneumoniae group, the M. mycoides group, the M. pneumoniae group and a Bacillus-Phytoplasma group. The M. hyopneumoniae group is more closely related to the M. pneumoniae group than the M. mycoides group.