Relationship between rifampicin resistance and RpoB substitutions of Rhodococcus equi strains isolated in France.

This research article focuses on the study of rifampicin resistance in Rhodococcus equi strains isolated from French horses over a two-decade timespan, to understand the relationship between rifampicin resistance and specific RpoB substitutions.

Research Study Explanation

The research revolves around a comprehensive study of the rifampicin resistance observed in Rhodococcus equi strains, gathered from horses based in France, over a period of 20 years. To shed light on the details:

• The investigation began by testing the susceptibility of these strains towards rifampicin; two methods were employed for this – disk diffusion (DD) and broth macrodilution.
• The study centered on 40 R. equi strains wherein nearly half demonstrated non-susceptibility towards rifampicin.
• The same strains also went through rpoB gene sequencing as well as multilocus sequence typing (MLST).
• The study found a consistent correlation of results between rifampicin susceptibility testing and rpoB sequencing.
• Strains that were non-susceptible to rifampicin by DD exhibited a substitution at one of the specified sites (Asp516, His526, or Ser531) known to confer rifampicin resistance.

Key Findings

The research article presented the following key findings:

• High-level resistance to rifampicin was linked with either His526Asp or Ser531Leu substitutions.
• Low-level resistance, on the other hand, was associated with the Asp516Tyr substitution, a uniquely documented substitution specific to R. equi.
• Strains that were susceptible to rifampicin by DD demonstrated no substitution except for two strains that had His526Asn and Asp516Val substitutions, which have previously been linked to low-level rifampicin resistance. These exceptions were isolated from an animal from which ten other strains, all non-susceptible to rifampicin by DD, were isolated.
• The MLST test revealed 10 sequence types (STs). Interestingly, these STs did not demonstrate any observable connection with rifampicin resistance.
• The study suggests that there appears to be a non-clonal dissemination of rifampicin-resistant strains across France.

Conclusions and Future Research Directions

The research study concluded that certain RpoB substitutions could possibly confer high or low-level rifampicin resistance, along with describing a new substitution found in R. equi that confers rifampicin resistance. However, it also suggests that the standard DD method could possibly miss strains with low-level rifampicin-resistant substitutions. The authors recommend further research in this field to address the absence of R. equi-specific clinical breakpoints.