JAC Advance Access originally published online on December 19, 2003
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Journal of Antimicrobial Chemotherapy (2004) 53, 203-207
© 2004 The British Society for Antimicrobial Chemotherapy
Rifampicin resistance and its fitness cost in Enterococcus faecium
1 Department of Pathology and Microbiology, University of Bristol, Medical Sciences Building, University Walk, Bristol BS8 1TD; 2 Division of Farm Animal Science, Department of Clinical Veterinary Science, University of Bristol, Langford BS40 5DU, UK
Received 8 August 2003; returned 8 October 2003; revised 24 October 2003; accepted 3 November 2003
Objectives: The genetic basis of rifampicin resistance and the associated fitness cost in Enterococcus faecium were investigated.
Methods: Twelve spontaneous rifampicin-resistant E. faecium mutants were selected from four parent strains recently isolated from porcine faecal material. The DNA sequence of the complete rpoB gene from the parent strains and of nucleotides –189 to +1785 from the mutants was determined from PCR amplicons. The fitness of the mutants was assessed by determining growth rate, by direct growth competition and by the ability of some of the mutants to survive in the pig intestine.
Results: The rpoB genes of the parent strains diverged from each other by 1–10% and each encoded proteins that were 1208 amino acids in length. All mutants had a single amino acid substitution in the region implicated in rifampicin resistance in other organisms. Six mutants carried the substitution H489Y/Q, two mutants carried the substitution R492H, one mutant carried the substitution Q480H, two mutants carried the substitutions S494L and V224I, and one mutant carried the substitutions G485D and V224I. Per generation fitness costs of the mutants ranged from a gain of 2.5% to a cost of 10%. Mutants with the substitution H489Y/Q were the most fit, whereas the double mutants were the least fit. The mutant with the substitution H489Q was able to survive in the pig gut for 12 days. There was some correlation between the rifampicin MIC and fitness cost, with higher MICs being associated with higher fitness costs.
Conclusions: Substitutions in RpoB are associated with rifampicin resistance in E. faecium. The fitness cost of resistance is variable and can sometimes be absent.
Keywords: rpoB, biological cost, in vivo fitness
* Corresponding author. Tel: +44-117-9287522; Fax: +44-117-9287896; E-mail: V.I.Enne{at}bristol.ac.uk
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