Pathogenesis of Rhodococcus equi infection in mice: Roles of virulence plasmids and granulomagenic activity of bacteria

Abstract

Virulence of Rhocococcus equi ATCC 33701 and its plasmid-cured derivative ATCC 33701P- was compared in BALB/c and C3H/HeJ mice in terms of bacterial growth kinetics and histological changes in the liver, spleen and lungs, and humoral immune responses. Injection with a sublethal dose of 10(6) ATCC 33701 in mice resulted in microabscess formation after rapid multiplication in the liver and spleen by day 4, and then the bacteria were gradually eliminated with the formation of granuloma and the production of specific antibodies against 15- to 17-kDa antigens of the virulent bacteria. By contrast, ATCC 33701P- was avirulent as shown by early elimination of viable bacteria and no evidence of net multiplication in the organs. Histopathological changes consisted of only slight, transient infiltration of neutrophils and macrophages in the liver. Although live ATCC 33701P- did not evoke any humoral or histological responses in the mice, a large inoculum (10(8)) of killed ATCC 33701 and ATCC 33701P- resulted in the formation of granuloma in the liver and accelerated extramedullary hemopoiesis in the spleen. These results suggest that the pathogenesis of R. equi infection involves at least two important virulence determinants, both of which play critical roles in the disease: one is the virulence plasmid, which is required for R. equi to resist and grow within host cells; and the other is the granulomagenic activity that is related to the lipids and nature of the cell wall of the species, which induces the characteristic pathological changes.

Phenotypic Mutants of the Intracellular Actinomycete Rhodococcus equi Created by In Vivo Himar1 Transposon Mutagenesis

“…At some time around day 5 postinfection, clearance of the bacteria becomes apparent and bacterial numbers quickly decline. The infection is generally resolved in approximately 3 weeks, depending on the size of the initial inoculum (27,46). In contrast to the in vivo growth displayed by wild-type R. equi, the riboflavin auxotroph did not replicate.…”

“…Therefore, BALB/cJ mice were infected with the riboflavin mutant and the clearance of the auxotroph was compared to that of wildtype R. equi. Although the mouse does not develop rhodococcal pneumonia, virulent (virulence plasmid-containing) strains of R. equi will replicate in vivo over the short term (46), and this ability to replicate in the mouse has been correlated to virulence in foals (14,50). In mice intravenously challenged with wild-type R. equi, the bacteria increase in number approximately 5-fold in the liver and 10-fold in the spleen during the first 48 h postchallenge, after which time the bacterial burden plateaus (Fig.…”

“…Thus, we investigated whether the R. equi riboflavin mutant would be similarly attenuated in vivo by using a murine challenge model. Although immunocompetent mice are relatively resistant to R. equi infection, virulent strains of the bac-teria will replicate in the early days postinfection, whereas attenuated strains will not (46). Furthermore, the ability to replicate in the tissues of the mouse has been demonstrated to correlate with the ability of R. equi to cause disease in foals (14,50).…”

Innate Immune Responses to Rhodococcus equi

“…the bacterial burdens plateau (40). In normal mice, bacterial clearance is initiated at approximately day 5 postinfection, and then bacterial numbers quickly decline (41,42). In our experiments, mice were infected with ϳ2 ϫ 10 5 CFU of R. equi, seeding the spleen with ϳ10 4 CFU (data not shown).…”

Expression of Virulence‐Associated Antigens of Rhodococcus equi Is Regulated by Temperature and pH

“…The virulence-associated antigens are synthesized in vitro by virulent R. equi at temperatures similar to those of the mammalian body and at pH values similar to those in macrophage phagolysosomes or abscesses. R. equi acts as a facultative intracellular bacterium which survives within macrophages and eventually destroys them (9,12,22). Low pH is probably an important signal in the intracellular environment, and stress proteins produced by intracellular bacteria have been investigated for their protective role against host defense mechanisms (2, 5, 11).…”