Daptomycin, a cyclic lipopeptide antibiotic, is one of the last-line agents for the treatment of certain severe multidrug-resistant Staphylococcus aureus infections, including those caused by methicillin-resistant S. aureus (MRSA). However, the exact mechanisms giving rise to daptomycin-non-susceptibility in S. aureus are not fully revealed but appear to involve diverse genetic events and several genetic loci, including mprF, yycG (walK), vraSR, tagA, and dltABCD. Drs. Feng-Jui Chen and Tsai-Ling Lauderdale from the National Institute of Infectious Diseases and Vaccinology and collaborators’ previous study of eight sequential clinical MRSA isolates from a patient with persistent bacteremia revealed an L431F amino acid substitution in the MprF protein of daptomycin-nonsusceptible isolates. The team then initiated the study to determine the effect of this mutation on the cellular response to daptomycin by replacing the mprF gene on the chromosome of a daptomycin-susceptible progenitor strain, CGK5, to obtain CGK5mut having the L431F MprF mutation.
Comparing to CGK5, the team found that daptomycin and vancomycin MICs of CGK5mut increased from 0.5 to 3 μg/ml and from 1.5 to 3 μg/ml, respectively; however, its oxacillin MIC decreased from 128 to 1 μg/ml in medium without added 2% NaCl. The expression levels of vraSR and several other cell-wall synthesis-related genes were significantly increased in CGK5mut, and the mutant also had significantly reduced negative cell membrane charge, thicker cell wall, and longer doubling time. These features were abolished in the reverse mutant carrying F431L MprF, confirming the pleiotropic effects of the L431F MprF mutation.
This finding is the first work that shows a single MprF missense mutation can lead to not only changes in the cell membrane but also increased expression of vraSR and subsequently increased resistance to daptomycin and vancomycin while simultaneously conferring increased susceptibility to oxacillin in an isogenic MRSA strain.
Citation: Chen, FJ;Lauderdale, TL;Lee, CH;Hsu, YC;Huang, IW;Hsu, PC;Yang, CS. Effect of a point mutation in mprF on susceptibility to daptomycin, vancomycin, and oxacillin in an MRSA clinical strain. Frontiers in Microbiology. 2018 May 25;9:Article number 1086