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Transfer of the virulence-associated protein A-bearing plasmid between field strains of virulent and avirulent Rhodococcus equi

W. Stoughton, T. Poole, K. Kuskie, M. Liu, K. Bishop, A. Morrissey, S. Takai, N. Cohen
Journal of veterinary internal medicine 2013 v.27 no.6 pp. 1555-1562
Rhodococcus equi, feces, foals, genetic conjugation, immunoblotting, plasmids, pneumonia, soil sampling, strains, virulence
Virulent and avirulent isolates coexist in equine feces and the environment and serve as a source of infection for foals. The extent to which conjugative plasmid transfer occurs between these strains is unknown and is important for understanding the epidemiology of Rhodococcus equi infections of foals. The objective of this study was to estimate the frequency of conjugative transfer of the virulence plasmid between virulent and avirulent strains of Rhodococcus equi derived from foals and their environment. Five rifampin-susceptible virulent R. equi isolates obtained from foals with pneumonia were each conjugated with 5 rifampin-resistant, avirulent isolates derived from soil samples using solid medium at a ratio of 10 donor cells per 1 recipient cell. Transconjugates were detected by plating on media with rifampin and colony immunoblotting to detect the presence of the virulence-associated protein A. Three transconjugates were detected among 2,037 recipient colonies, indicating an overall estimated transfer frequency of 0.0015 (95% CI, 0.0003 to 0.0043). All three transconjugates were associated with a single donor and two recipient strains. Conjugation of the virulence plasmid occurred with a high frequency. Thus, strategies for control and prevention based on eradication of virulent isolates from the environment will be difficult to achieve and will require persistent application.