The fastidious nature of the mollicutes (mycoplasmas), their lack of a classic bacterial cell wall, and their very small genome, make phylogenetic placements of new species in this enlarging group of prokaryotes an important and valuable aid in their classification. In this report we have determined the phylogeny of the Mycoplasma hominis cluster of the hominis group. The 16S rDNA sequences from several previously described Mycoplasma species were determined and ten species were found to belong to the M. hominis cluster. With almost complete sequences available, the phylogenetic analysis revealed that the M. hominis cluster currently comprises 19 species, forming a distinct clade as judged from branch lengths, bootstrap percentage values, nucleotide signature analysis, and structural elements in the 16S rRNA molecule. The 16S rRNA gene sequences of species in the M. hominis cluster were found to be > or = 94% similar and the range within which similarities can be used in the classification of new species is discussed. Members of the M. hominis cluster all share a major biochemical property of M. hominis, in that they hydrolyse arginine and are incapable of fermenting glucose. This consistency in phenotypic pattern has not been found in any of the other phylogenetic clusters of the hominis group. Two species, the non-cultivable agent of Grey Lung disease in rodents (tentatively named 'Candidatus Mycoplasma ravipulmonis') and the avian species Mycoplasma gypis strain B1/T1T, were regarded as close relatives to the M. hominis cluster, but are clearly separated from the species of this cluster. Both species formed early branches of the M. hominis cluster and should be regarded as individual lines containing one species.


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