Fitness Cost of Daptomycin-Resistant Obtained from Daptomycin Selection Pressure.

Daptomycin-resistant (DAP-R) strains are well documented, but have not been reported in China. To elucidate the evolution adaptability and fitness cost of DAP-R , three DAP susceptible strains, Pre3 (MRSA, ST239-t037), Pre5 (MRSA, ST239-t037), and Pre14b (MSSA, ST188-t189), were isolated from patients with bloodstream infections, and serially passaged in Mueller-Hinton broth with a gradient of DAP concentration to select for resistance. Highly DAP-R mutants were obtained after screening for 34 passages. The DAP minimum inhibitory concentrations increased from 0.5 μg/ml in the parent strains to 16 μg/ml in the mutants, which remained tolerant to 4 μg/ml of DAP for more than 160 generations. The growth of the three mutant strains was slower than that of the parent strains, with relative fitness cost of 34.8%, 19.2%, and 15.0%, respectively. The serum tolerance of the mutants was decreased, and the lethality and pathogenicity in mice were weakened ( < 0.01). Transmission electron microscopy found that the cell walls of the mutants were significantly thicker (from 38.6% to 75.4%) than those of the parent cells. Mutation L826F of was found in Post14b, G299V, and L473I of and Y225N of were found in Post3, while T345A of , S52N of , and F473I of were found in Post5. Thus, stable DAP-R mutants could be obtained from a middle-short term of DAP selection, and according to their fitness cost, some prevention and control work may be done to cope with DAP-R that may appear in China in the future.