Rhodococcus equi: the many facets of a pathogenic actinomycete.

This article explores Rhodococcus equi, a bacterium found in soil that causes infections in a range of animals and humans, with a particular emphasis on its impact on young foals. The research investigates the role and structure of the bacterium and its virulence plasmid, exploring how these aspects influence infections and potential treatments.

Key Characteristics of Rhodococcus equi

• Rhodococcus equi is a bacterium that is usually found in the soil and is responsible for causing infections, primarily in young foals but also in other animals and humans.
• The bacterium causes pulmonary and extrapulmonary pyogranulomatous infections, with young foals being particularly vulnerable. This disease is endemic at many horse-breeding farms across the globe.
• R. equi is a facultative intracellular parasite of macrophages, meaning that it can survive and multiply within these white blood cells. The bacterium changes the structure of the macrophages’ phagocytic vacuoles to facilitate its survival.

The Role of the Virulence Plasmid

• The virulence plasmid is a vital component of R. equi’s pathogenicity. It allows the bacterium to survive within host cells by preventing the fusion of phagosomes and lysosomes, a process that would typically destroy the bacterium.
• The type of virulence plasmid present in the bacterium also appears to determine its tropism, or predilection for certain hosts. Different strains of the bacterium, carrying different plasmid types, show a preference for horses, pigs, and cattle.

R. equi Infection in Humans

• Research suggests that R. equi infection in humans may be zoonotic, meaning that it is transmitted from animals to humans. This is supported by molecular epidemiological studies of the virulence plasmids.
• Further understanding of the bacterium’s genome has identified other virulence determinants located on the bacterial chromosome.

Current Understanding and Further Research

• The review provides a comprehensive summary of available knowledge regarding the clinical aspects, biology, pathogenesis, and immune response to R. equi.
• Delving deeper into the genomic structure of R. equi and its virulence plasmids could be key to developing a better understanding of how the bacterium causes disease and to creating effective treatments.