Two new variants of the Rhodococcus equi virulence plasmid, 90 kb type III and type IV, recovered from a foal in Japan.

The research study describes the identification of two new and closely-related plasmid types, which contribute to the virulence or disease-causing ability of the bacterium Rhodococcus equi, in a sick foal in Japan.

Details of the Study

• The foal, from which the bacterium was isolated, was suffering from a severe case of Rhodococcus equi pneumonia and ulcerative enteritis.
• The focus of the study was on identifying the presence of virulence-associated antigens (VapA) which play a key role in the pathogenicity of R. equi. Identifying different variations of the VapA gene can help understand the diversity and adaptability of this bacterium to host organisms.

Methods Employed

• The investigators used colony blotting and a special kind of stain (monoclonal antibody 10G5) to identify the presence of the VapA antigens in the bacterium.
• The detection of the VapA coding gene was facilitated by Polymerase Chain Reaction (PCR), a molecular biology technique used to amplify and thus detect targeted DNA sequences.
• Further exploration of the virulence-associated plasmids was carried out by using certain enzymes (restriction endonucleases) to break down the DNA at specific points and study the resultant digestion patterns.

Findings and Conclusions

• The digestion patterns identified were compared with those of eight other known plasmid types, previously reported.
• This comparison revealed that the plasmids extracted from the foal did not match any of the known types, therefore indicating the discovery of two new types. These were tentatively named as 90 kb type III and type IV.
• The results of this study add to our understanding of R. equi by highlighting the existence of at least 10 distinct, yet closely-related, plasmids in this bacterium within horse populations worldwide.