Successful therapy of treatment-emergent, non-clonal daptomycin-non-susceptible Enterococcus faecium infections

with reduced susceptibility (,23 mm inhibition zone diameter or MIC .1 mg/L) to any of the carbapenems (ertapenem, imipenem or meropenem) will be tested for the carbapenemase phenotype by the combined disc method (with EDTA or boronic acid). All strains with a positive phenotype will be sent to a reference laboratory for molecular testing. Hence, implementation of the new CLSI criteria would not simplify the laboratory testing and reporting procedure in our locality. Although susceptibility of Proteus spp., Providencia spp. and M. morganii to imipenem is recognized to be lowered and could be caused by mechanisms other than carbapenemase production, the extent of the effect that applying the new interpretive criteria would have on clinical isolates has not been previously described. Here, we showed that susceptibility categorization for this group of organisms could change by up to 61.4%. Laboratories that choose to implement the new criteria should clearly inform infection control personnel so that there will not be any misunderstanding of outbreak occurrence. In conclusion, this study showed that applying the new CLSI criteria to interpret carbapenem susceptibility in Enterobacteriaceae would drastically change the susceptibility rates for some isolate groups. Furthermore, the FDA-approved breakpoints have not been changed. This explains why many clinical laboratories continue to use the old breakpoints and perform phenotypic testing for carbapenemase production. Our findings emphasize the need to obtain more clinical outcome data.