Role of the 85-Kilobase Plasmid and Plasmid-Encoded Virulence-Associated Protein A in Intracellular Survival and Virulence of Rhodococcus equi

Abstract

Rhodococcus equi is a facultative intracellular pathogen of macrophages and a cause of pneumonia in young horses (foals) and immunocompromised people. Isolates of R. equi from pneumonic foals typically contain large, 85- or 90-kb plasmids encoding a highly immunogenic virulence-associated protein (VapA). The objective of this study was to determine the role of the 85-kb plasmid and VapA in the intracellular survival and virulence of R. equi. Clinical isolates containing the plasmid and expressing VapA efficiently replicated within mouse macrophages in vitro, while plasmid-cured derivatives of these organisms did not multiply intracellularly. An isolate harboring the large plasmid also replicated in the tissues of experimentally infected mice, whereas its plasmid-cured derivative was rapidly cleared. All foals experimentally infected with a plasmid-containing clinical isolate developed severe bronchopneumonia, whereas the foals infected with its plasmid-cured derivative remained asymptomatic and free of visible lung lesions. By day 14 postinfection, lung bacterial burdens had increased considerably in foals challenged with the plasmid-containing clinical isolate. In contrast, bacteria could no longer be cultured from the lungs of foals challenged with the isogenic plasmid-cured derivative. A recombinant, plasmid-cured derivative expressing wild-type levels of VapA failed to replicate in macrophages and remained avirulent for both mice and foals. These results show that the 85-kb plasmid of R. equi is essential for intracellular replication within macrophages and for development of disease in the native host, the foal. However, expression of VapA alone is not sufficient to restore the virulence phenotype.

In vivo induced antigenic determinants of Actinobacillus actinomycetemcomitans

“…The vap region is associated with virulence by genotypic association in both organisms. Their precise role in virulence has not yet been described [22][23][24]. Interestingly, the vapA homologue has significant homology to a number of DNAbinding domains of E. coli repressors including LacR, Cro434, and C1 (data not shown).…”

Study of serum amyloid A concentrations as a means of achieving early diagnosis of Rhodococcus equi pneumonia

“…Although many organisms can cause pneumonia in foals, infection with virulent R. equi is considered to be the most common cause of severe pneumonia (Anon 1978;Prescott 1991;Takai et al 1995;Giguere and Prescott 1997). Virulence of R. equi has been associated with presence of a plasmid that encodes a pathogenicity island of several virulenceassociated proteins (Vaps); the protein VapA has been strongly associated with virulence (Takai et al 1991(Takai et al , 1995Giguere et al 1999;Byrne et al 2001). The impact of R. equi pneumonia on the equine industry is severe because prevalence and casefatality rates are high, and treatment is generally prolonged, expensive, associated with adverse effects, and not uniformly successful (Giguere and Prescott 1997;Giguere 2001).…”

Differential mRNA stability of thevapAICDoperon of the facultative intracellular pathogenRhodococcus equi

“…In addition, the pathogenicity island contains two pseudo-vap genes (Takai et al, 2000;Russell et al, 2004). Foals infected with R. equi invariably have high antibody titres against VapA: a small cell surface associated lipoprotein that is essential, but not sufficient, for virulence and proliferation in macrophages (Takai et al, 1991;Giguère et al, 1999;Jain et al, 2003).…”