Spiroplasmavirus group 1: Isolation, growth, and properties

Complete genomes of two dipteran-associated spiroplasmas provided insights into the origin, dynamics, and impacts of viral invasion in spiroplasma

Spiroplasma is a genus of wall-less, low-GC, Gram-positive bacteria with helical morphology. As commensals or pathogens of plants, insects, ticks, or crustaceans, they are closely related with mycoplasmas and form a monophyletic group (Spiroplasma–Entomoplasmataceae–Mycoides) with Mycoplasma mycoides and its relatives. In this study, we report the complete genome sequences of Spiroplasma chrysopicola and S. syrphidicola from the Chrysopicola clade. These species form the sister group to the Citri clade, which includes several well-known pathogenic spiroplasmas. Surprisingly, these two newly available genomes from the Chrysopicola clade contain no plectroviral genes, which were found to be highly repetitive in the previously sequenced genomes from the Citri clade. Based on the genome alignment and patterns of GC-skew, these two Chrysopicola genomes appear to be relatively stable, rather than being highly rearranged as those from the Citri clade. Phylogenetic analyses suggest that the susceptibility to plectroviral invasion probably originated in the common ancestor of the Citri clade or one of its subclades. This susceptibility may be attributed to the absence of antiviral systems found in the Chrysopicola clade. Using the virus-free genomes of the Chrysopicola clade as references, we inferred the putative viral integration sites in the Citri genomes. Comparisons of syntenic regions suggest that the extensive viral invasion in the Citri clade promoted genome rearrangements and expansions. More importantly, the viral invasion may have facilitated horizontal gene transfers that contributed to adaptation in the Citri clade.

Mycoplasma viruses

Unlike bacterial viruses that infect cells bounded by a cell wall, mycoplasma viruses have evolved to enter and propagate in mycoplasma cells bounded only by a single lipid-protein cell membrane. In addition, mycoplasmas have the smallest amount of genetic information of any known cells, so their complexity is constrained by a limited genetic coding capacity. As a consequence of these host cell differences, mycoplasma viruses have been found to have a variety of structures and replication strategies which are different from those of the bacterial viruses. This article is a critical review of mycoplasma viruses infecting the genera Acholeplasma, Spiroplasma, and Mycoplasma; included are data on classification, morphology and structure, biological and physical properties, chemical composition, and productive and lysogenic replication cycles.

Some characteristics of mycoplasma virus Hr 1, isolated from and infecting Mycoplasma hyorhinis. Brief report

Mycoplasma virus Hr 1 is a short tailed bacteriophage with a polyhedral head about 34 nm across and a tail about 14 nm long. It produces plaques on some strains of Mycoplasma hyorhinis.

Some properties of mycoplasma virus Br 1

Morphologically mycoplasma virus Br 1 is a typical contractile-tailed bacteriophage with a head 77 nm in diameter and a tail 104 nm long. Its type of nucleic acid was not determined. Br 1 was closely associated with its host cell and assays reflected infectious centres. During growth of Br 1 in mycoplasma cultures at multiplicities of infection (MOI) greater than 0.001, there was a lag period: this was up to 23 hours at an MOI of 35. The mean generation time of a mycoplasma culture infected at MOI up to 235 was 2 hours, compared with 1 hour for an uninfected culture. However in these infected cultures there were viable mycoplasmas all of which appeared to be fully susceptible to Br 1 infection and did not seem to be carrying the virus. Br 1 formed plaques on M. bovirhinis but failed to produce plaques on strains of 8 Mycoplasma, 2 Acholeplasma and 4 bacterial species.