Comparative genomics of Mycoplasma gallisepticum: Implications for pathogenesis

Mycoplasma gallisepticum causes a significant infectious disease of poultry, which accounts for great losses on commercial farms. Typically, M. gallisepticum induces lesions in the respiratory and reproductive tracts of poultry and is spread both horizontally and vertically. Producers often employ vaccination strategies to control infection, yet little is known about the mechanism(s) of attenuation of the live attenuated vaccines used on these farms. The F strain of M. gallisepticum is commonly used to eradicate field isolates from poultry facilities. To better understand its mechanism(s) of attenuation, we sequenced the complete genome of the F strain and compared it with the previously sequenced virulent Rlow strain to identify genes or loci that are different between these isolates. Among the changes observed, hypothetical gene MGA1107 was missing from the F strain genome. A previous study showed that MGA1107 is up-regulated when Rlow is attached to eukaryotic cells. Taken together, these data implicate MGA1107 as a potential virulence factor of M. gallisepticum. In vivo evaluation of a MGA1107 mutant demonstrated that this gene is essential for virulence in the tracheas of infected chickens, as lesions were similar to those observed in medium inoculated birds, and the organism was not recovered post-challenge. Further examination of the differences between Rlow and the F strain was conducted by comparing their transcriptomes using DNA microarrays. Among the differentially expressed features was hypothetical gene MGA0674. MGA0674 was also identified in previous proteomic studies where it was shown to be an immunogenic, detergent-phase protein which is proteolytically processed into two fragments. In silico analyses indicate that MGA0674 is a lipoprotein which shares conserved domains strictly with other members of the genus Mycoplasma thereby leading to the name Mycoplasma-specific lipoprotein A (mslA). In vivo evaluation of three mslA mutants resulted in reduced lesion scores in chickens compared to those induced by Rlow , indicating that mslA is a virulence factor of M. gallisepticum. These results demonstrate that genomic analyses, combined with virulence assessments in host animals, are useful in the study of Mycoplasma virulence.