The classification of the Mollicutes has always been difficult. The individuals are tiny, and being parasites, they have to be cultivated on special media. Until now, many species could not be isolated at all. In the beginning, whether they were fungi, viruses, or bacteria was not clear. Also, the resemblance to
L-forms was confusing. At first, all members of the class Mollicutes were generally named "mycoplasma" or pleuropneumonia-like organism (PPLO). Mollicutes other than some members of genus
Mycoplasma were still unidentified. The first species of
Mycoplasma/Mollicutes, that could be isolated was
Mycoplasma mycoides. This bacterium was
cultivated by Nocard and Roux in 1898. In 1956, D.G. Edward and E.A. Freundt made a first proposal for classifying and naming PPLOs. They left undecided, however, whether they belong to the bacteria (
prokaryotes, in 1956 called "Schizomycetes") or to the
eukaryotes. As
type species (name-giving species) of the PPLOs/mycoplasmas, Edward and Freundt proposed
Mycoplasma mycoides, being the causative organism of bovine pleuropneumonia and referring to the pleuropneumonia-like organisms. Until then,
Mycoplasma mycoides was known as
Asterococcus mycoides, but later that name was not recognized as valid. In their publication of 1956, they described 15 species of
Mycoplasma. In 1967 the class Mollicutes, containing the order
Mycoplasmatales, was proposed by the Subcommittee on Taxonomy of the Mycoplasmata. Now, the name
Mycoplasma should exclusively be used for members of the
genus Mycoplasma, rather than the use as a trivial name for any Mollicute. As the trivial name has been used in literature for a long time, this is yet not always the case. For classification and nomenclature of Mollicutes, there are special rules, which are maintained by the
International Committee on Systematics of Prokaryotes (ICSP) Subcommittee on the Taxonomy of Mollicutes (formerly the
International Committee on Systematic Bacteriology (ICSB) Subcommittee on taxonomy of Mycoplasmatales). Traditionally, Mollicutes
taxonomy has been based on
serology and
phenotypic characteristics. However, most modern classifications are based on DNA or RNA sequences, especially
16S rRNA sequences) Bacteria into three divisions (= phyla) on the basis of the cell wall types: •
Gram-negative Gracilicutes, with a thin cell wall and little
peptidoglycan; •
Gram-positive Firmicutes, with a thicker cell wall and more peptidoglycan (the name was later changed in "Firm
icutes"), and • the "Mollicutes", without a cell wall.
The phylum for Mollicutes The results of Mollicutes
phylogenetic analyses have been controversial. Some taxonomists place them in Bacillota, others in Mycoplasmatota.
Woese et al. suggested that the Mollicutes might have been derived from different branches of bacteria. They concluded, that the Mollicutes are not a phylogenetically coherent group and therefore do not form a distinct higher level taxon. Instead, they cluster within Gram-positive bacteria of the phylum Bacillota. The results of
molecular phylogenetic analyses have been partly dependent on the chosen molecular marker, like
rRNA,
elongation factor or another protein. Phylogenetic trees based on
phosphoglycerate kinase (Pgk) amino acid sequences' indicated a monophyletic origin for the Mollicutes within the Bacillota. An early edition of ''
Bergey's Manual of Systematic Bacteriology'' placed class Mollicutes within phylum
Bacillota, whereas in the announced 2nd edition, they are moved to a separate phylum Tenericutes (later renamed
Mycoplasmatota). The change is motivated by "their unique phenotypic properties, in particular the lack of rigid cell walls, and the general low support by alternative markers". Up-to-date analysis of the SSU (16S) rRNA puts Mollicutes in Bacillota (see LTP below).
GTDB's 120-protein approach does the same (see below).
The 2018 Mycoplasma split It has been known since the 1990s that
Mycoplasma includes several groups that were not very related to its
type species. Four lineages were defined by 2007. In 2018, Gupta et al. re-circumscribed the genus
Mycoplasma around
M. mycoides. A total of 78 species were removed from
Mycoplasma, creating five new genera and a number of higher taxonomic levels. Under this new scheme, a new family
Mycoplasmoidaceae was created to correspond to the "pneumoniae" group, with
M. pneumoniae and related species transferred to a new genus
Mycoplasmoides. Another new family
Metamycoplasmataceae was created to correspond to the "hominis" group. Both families belong to a new order
Mycoplasmoitales, distinct from the
Mycoplasmatales of
Mycoplasma. The taxonomy was accepted by the ICSB with validation list 184 in 2018 and became the
correct name. Both
List of Prokaryotic names with Standing in Nomenclature (LPSN) and
National Center for Biotechnology Information (NCBI) now use the new nomenclature. Gupta's proposed taxonomy, as expected, moved the medically important "pneumoniae" group out of
Mycoplasma into its own genus. As a result, a number of mycoplasmologists petitioned to the ICSB to reject the name in 2019. They argue that although Gupta's phylogenetic methods were likely solid, the proposed name changes are too sweeping to be practically adopted, citing some principles of the Code such as "name stability". Gupta and Oren wrote a rebuttal in 2020, further detailing the pre-existing taxonomic problems. In 2022, the ICSP's Judicial Opinion 122 ruled in favor of the name changes proposed by Gupta, meaning they remain valid under the
Prokaryotic Code Gupta
et al. 2019 performed some uncontroversial sorting of the order Mycoplasmatales. ==Phylogeny==