Correlation between Mutations inliaFSRof Enterococcus faecium and MIC of Daptomycin: Revisiting Daptomycin Breakpoints

Abstract

ABSTRACTMutations inliaFSR, a three-component regulatory system controlling cell-envelope stress response, were recently linked with the emergence of daptomycin (DAP) resistance in enterococci. Our previous work showed that aliaFmutation increased the DAP MIC of a vancomycin-resistantEnterococcus faecalisstrain from 1 to 3 μg/ml (the DAP breakpoint is 4 μg/ml), suggesting that mutations in theliaFSRsystem could be a pivotal initial event in the development of DAP resistance. With the hypothesis that clinical enterococcal isolates with DAP MICs between 3 and 4 μg/ml might harbor mutations inliaFSR, we studied 38Enterococcus faeciumbloodstream isolates, of which 8 had DAP MICs between 3 and 4 μg/ml by Etest in Mueller-Hinton agar. Interestingly, 6 of these 8 isolates had predicted amino acid changes in the LiaFSR system. Moreover, we previously showed that among 6 DAP-resistantE. faeciumisolates (MICs of >4 μg/ml), 5 had mutations inliaFSR. In contrast, none of 16E. faeciumisolates with a DAP MIC of ≤2 μg/ml harbored mutations in this system (P< 0.0001). All but one isolate withliaFSRchanges exhibited DAP MICs of ≥16 μg/ml by Etest using brain heart infusion agar (BHIA), a medium that better supports enterococcal growth. Our findings provide a strong association between DAP MICs within the upper susceptibility range and mutations in theliaFSRsystem. Concomitant susceptibility testing on BHIA may be useful for identifying theseE. faeciumfirst-step mutants. Our results also suggest that the current DAP breakpoint forE. faeciummay need to be reevaluated.