Assessment of Antimicrobial Susceptibility of Virulent Strains of Rhodococcus equi Isolated From Foals and Soil of Horse Breeding Farms With and Without Endemic Infections.

Rhodococcus equi is an opportunistic, intracellular saprophyte that causes severe pyogranulomatous pneumonia in foals. The bacterium displays in vitro susceptibility to many antibiotics. The highest efficacy against R. equi in vitro and in vivo is achieved by using a combination of rifampicin and macrolide antibiotics. Recent years have seen an upward trend in the minimum inhibitory concentration (MIC) of rifampicin and erythromycin, suggesting increasing resistance of R. equi to these antibiotics. The aim of the study was to determine the antimicrobial activity of 24 selected antibiotics against R. equi strains isolated from dead foals and from the environment of horse breeding farms with and without endemic R. equi infections. Minimum inhibitory concentration gradient strips were used to determine the lowest concentration of the antibiotic that inhibited the growth of R. equi. Based on normal MIC distribution, an epidemiologic cutoff values (ECOFF) were assessed for particular antibiotics and R. equi strains. The results were compared with ECOFFs for S. aureus, according to the European Committee on Antimicrobial Susceptibility Testing data. The data indicate that the lowest MIC values were obtained for clarithromycin, rifampicin, imipenem, and vancomycin. The majority of R. equi strains can be classified as wild type in relation to the majority of antibiotics. A small percentage of strains presented non-WT (NWT) with the exception of SXT, for which 35% of strains were classified as NWT. The lack of interpretative criteria for R. equi creates a real problem in the assessment of antibiotic sensitivity both for clinical and scientific purposes.