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

item STOUGHTON, WILLIAM - Texas A&M University
item Poole, Toni
item KUSKIE, KYLE - Texas A&M University
item LIU, MEI - Texas A&M University
item BISHOP, KIM - Texas A&M University
item MORRISSEY, AUDREY - Texas A&M University
item TAKAI, SHINJI - Texas A&M University
item COHEN, NOAH - Texas A&M University

Submitted to: Journal of Veterinary Internal Medicine
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/12/2013
Publication Date: 10/15/2013
Citation: Stoughton, W., Poole, T.L., Kuskie, K., Liu, M., Bishop, K., Morrissey, A., Takai, S., Cohen, N. 2013. Transfer of the virulence-associated protein A-bearing plasmid between field strains of virulent and avirulent Rhodococcus equi. Journal of Veterinary Internal Medicine. 27:1555-1562.

Interpretive Summary: A bacterium called Rhodococcus equi is the cause of severe pneumonia in newborn horses (foals). This bacterium has worldwide distribution and causes severe economic losses for the equine industry. Factors that contribute to the spread of this infectious disease have remained elusive. It is known that R. equi possesses a virulence gene called VapA that leads to the disease, and VapA is carried on a small circular piece of mobile DNA called a plasmid. Bacteria can share or transfer plasmids by a process called conjugation. Conjugation occurs when a bridge is formed between two bacterial cells and the plasmid migrates across the bridge. After this process is completed, the recipient cell that acquired the plasmid will display the genetic traits of the original bacterial cell if they were present on the plasmid. In this case, the recipient bacterium would be converted from a non-disease-causing causing bacterium to a disease-causing bacterium. Some plasmids transfer more easily than others, and some do not transfer at all. This study was done to determine if the VapA plasmid could be transferred. The results of this study showed that the VapA plasmid did transfer to two different recipient strains of R. equi. This provides insight into one mechanism of how the disease-causing forms of R. equi may spread and indicates that strategies to combat the disease may be more difficult than once thought.

Technical Abstract: 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.