Unravelling the main genomic features of Mycoplasma equirhinis

Mycoplasma spp. are wall-less bacteria with small genomes (usually 0.5-1.5 Mb). Many Mycoplasma (M.) species are known to colonize the respiratory tract of both humans and livestock animals, where they act as primary pathogens or opportunists. M. equirhinis was described for the first time in 1975 in horses but has been poorly studied since, despite regular reports of around 14% prevalence in equine respiratory disorders. We recently showed that M. equirhinis is not a primary pathogen but could play a role in co-infections of the respiratory tract. This study was a set up to propose the first genomic characterization to better our understanding of the M. equirhinis species.

Four circularized genomes, two of which were generated here, were compared in terms of synteny, gene content, and specific features associated with virulence or genome plasticity. An additional 20 scaffold-level genomes were used to analyse intra-species diversity through a pangenome phylogenetic approach. The M. equirhinis species showed consistent genomic homogeneity, pointing to potential clonality of isolates despite their varied geographical origins (UK, Japan and various places in France). Three different classes of mobile genetic elements have been detected: insertion sequences related to the IS1634 family, a putative prophage related to M. arthritidis and integrative conjugative elements related to M. arginini. The core genome harbours the typical putative virulence-associated genes of mycoplasmas mainly involved in cytoadherence and immune escape.

M. equirhinis is a highly syntenic, homogeneous species with a limited repertoire of mobile genetic elements and putative virulence genes.