Characterization of mutations in the rpoB gene associated with rifampin resistance in Rhodococcus equi isolated from foals

Treatment with a combination of erythromycin and rifampin has considerably improved survival rates of foals and immunocompromised patients suffering f rom severe pneumonia caused by Rhodococcus equi. Frequently, because of mon otherapy, emergence of rifampin-resistant strains has been responsible for treatment failure. Using consensus oligonucleotides, we have amplified and sequenced the rifampin resistance (Rif)determining regions of 12 rifampin-r esistant R. equi strains isolated from three foals and of mutants selected in vitro from R. equi ATCC 3701, a rifampin-susceptible strain. The deduced amino acid sequences compared to those of four rifampin-susceptible R. equ i strains showed several types of mutations. In 3 of the 10 strains isolate d from one foal, His526Asn (Escherichia coli numbering) and Asp516Val mutat ions were associated with low-level resistance (rifampin MIC, 2 to 8 mug/ml ), whereas His526Asp conferred high-level resistance (rifampin MIC, 128 mug /ml) in the 7 remaining strains. In strains from the two other foals, His52 6Asp and Ser531Leu mutations were found to be associated with high-level an d low-level resistance, respectively. The in vitro mutants, highly resistan t to rifampin, harbored His526Tyr and His526Arg substitutions. As described in other bacterial genera, His526, Ser531, and Asp516 are critical residue s for rifampin resistance in R. equi, and the resistance levels are depende nt on both the location and the nature of the substitution.