Antimicrobial Susceptibility of Australian Virulent R hodococcus Equi Isolates Collected Between 1991 and 2014

Abstract

Bronchopneumonia caused by Rhodococcus equi is an important and often debilitating disease of young horses throughout the world. Advances in diagnostic methods for the early detection of lung lesions, such as ultrasonographic imaging, have been associated with an increase in the use of antimicrobial agents for the treatment of this disease. Concurrent with this increased usage, reports of antimicrobial resistance in R. equi have become more common in recent years. The aim of this study was to determine the level of resistance to three commonly prescribed antimicrobial agents in 97 virulent Rhodococcus equi isolates collected from infected foals between 1991 and 2014. The second objective was to determine the utility of a novel assay for determining the minimum inhibitory concentration (MIC) of R. equi isolates with a single agent, or when rifampicin and erythromycin were combined. Three cohorts of virulent Rhodococcus equi isolates were included in the study. Cohort 1 contained 29 isolates collected from clinical cases between 1991 and 1998 and Cohort 2 consisted of 12 recent isolates collected between 2011 and 2014. Fifty six isolates from clinically affected foals undergoing antimicrobial therapy in 2006 - 2007 on an endemically infected farm formed Cohort 3. The MICs for rifampicin, erythromycin and neomycin were determined for all of the isolates using a novel microtitre assay. Rifampicin resistance was detected in 3 of the 12 isolates in Cohort 2. The MIC was 64 mg/mL for two of the isolates and 16 mg/mL for the third, well above the threshold of 8 mg/mL that defines resistance to this agent. All isolates collected prior to 2013 had MICs less than 0.125 mg/mL (limit of detection). None of the isolates were resistant to either neomycin or erythromycin, with MIC values ranging from 0.25 - 2 mg/mL for neomycin and 0.125 - 1 mg/mL for erythromycin. When rifampicin and erythromycin were tested in combination, the MIC value for the isolates resistant to rifampicin were lower, compared to exposure to rifampicin alone. This is the first report of rifampicin resistance in virulent R. equi isolated from Australian foals. Sanger sequencing of a portion of the RNA polymerase b subunit was conducted to further characterise the resistant isolates from Cohort 2 [1], [2]. The current therapeutic success of the macrolide - rifampicin combination regimen is thought to rely on the synergistic action of these two agents in vivo. It is not clear from these results whether the rifampicin resistance we have detected has any significant impact on synergy in vivo