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Genome Biol. 2018 Jul 18;19(1):94. doi: 10.1186/s13059-018-1469-2.

Epistasis analysis uncovers hidden antibiotic resistance-associated fitness costs hampering the evolution of MRSA.

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Milner Centre for Evolution, Department of Biology and Biochemistry, University of Bath, Bath, UK.
School of Cellular and Molecular Medicine, University of Bristol, Bristol, UK.
Division of Medical Protein Chemistry, Department of Translational Medicine, Lund University, S20502, Malmö, Sweden.
University of Bristol Proteomics Facility, University of Bristol, Bristol, UK.
Antimicrobial Research Centre, Faculty of Biological Sciences, University of Leeds, Leeds, LS2 9JT, UK.
Centre for Biomolecular Sciences, School of Life Sciences, University of Nottingham, Nottingham, NG7 2RD, UK.
Chemical Characterisation and Analysis Facility, Faculty of Science, University of Bath, Bath, BA2 7AY, UK.
Centre for Mathematics and the Environment, University of Exeter, Penryn Campus, Penryn, TR10 9FE, UK.
School of Cellular and Molecular Medicine, University of Bristol, Bristol, UK.



Fitness costs imposed on bacteria by antibiotic resistance mechanisms are believed to hamper their dissemination. The scale of these costs is highly variable. Some, including resistance of Staphylococcus aureus to the clinically important antibiotic mupirocin, have been reported as being cost-free, which suggests that there are few barriers preventing their global spread. However, this is not supported by surveillance data in healthy communities, which indicate that this resistance mechanism is relatively unsuccessful.


Epistasis analysis on two collections of MRSA provides an explanation for this discord, where the mupirocin resistance-conferring mutation of the ileS gene appears to affect the levels of toxins produced by S. aureus when combined with specific polymorphisms at other loci. Proteomic analysis demonstrates that the activity of the secretory apparatus of the PSM family of toxins is affected by mupirocin resistance. As an energetically costly activity, this reduction in toxicity masks the fitness costs associated with this resistance mutation, a cost that becomes apparent when toxin production becomes necessary. This hidden fitness cost provides a likely explanation for why this mupirocin-resistance mechanism is not more prevalent, given the widespread use of this antibiotic.


With dwindling pools of antibiotics available for use, information on the fitness consequences of the acquisition of resistance may need to be considered when designing antibiotic prescribing policies. However, this study suggests there are levels of depth that we do not understand, and that holistic, surveillance and functional genomics approaches are required to gain this crucial information.


Epistasis; Fitness costs; GWAS; MRSA; Mupirocin resistance

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