The University of Georgia College of Veterinary Medicine

Georgia Veterinary Scholars Program

GVSP Summer 2011 Scholars

Georgia Veterinary Scholar	Faculty Mentor

Camillya Fanning Tuskegee University College of Veterinary Medicine Class of 2013	Dr. Mary Hondalus Department of Infectious Diseases UGA College of Veterinary Medicine

Analysis of Intramacrophage Growth Phenotypes in Distinct Rhodococcus equi Isolates

Fanning CD, Willingham JM, Coulson GB, Hondalus MK

Rhodococcus equi is an intracellular facultative bacterium that replicates within macrophages and causes pyogranulomatous lung lesions in foals and immunocompromised people. In swine, however, R. equi disease presents differently wherein cervical lymphadenitis is the typical clinical manifestation. R. equi virulence is dependent on the possession of a plasmid that confers the capacity for intramacrophage replication. The R. equi virulence plasmid houses a pathogenicity island (PAI) in which a family of genes, called virulence associated proteins or vaps, are located. R. equi strains isolated from foals have a plasmid PAI region that contains vapA while the PAI region of swine plasmids contains vapB. vapA has been shown to be a virulence determinant; essential for bacterial replication in murine macrophages and for chronic disease development in a SCID mouse infection model system. Interestingly, vapA is exclusive to equine isolates and vapB is only present in swine isolates of R. equi and never vice versa. The discovery that distinct plasmid types are restricted to equine isolates and swine isolates specifically, raises the hypothesis that differences in vap gene possession are responsible for host species specificity, perhaps at the level of intramacrophage replication. Testing of this hypothesis is underway where the intracellular growth capacity of vapA and vapB plasmid containing R.equi strains are being analyzed in murine, equine, and swine macrophages cultured in vitro. The intracellular growth kinetics of the R. equi strains are being determined by lysis of infected macrophages at 1, 24, and 48 hours post infection followed by dilution plating of the lysate to determine the macrophage-associated bacterial burden over time. Should results show that only vapA-containing plasmid strains replicate in equine macrophages and the vapB- containing plasmid isolates replicate in swine macrophages, then the hypothesis that specific plasmid types are responsible for host species specificity will be supported