antibiotic usage to limit resistance development?

We wish to comment on the article by Amyes et al.,1 in which we are referenced2 as stating that ‘the concept that ertapenem is unlikely to increase resistance in non-fermenters is flawed’. We in fact did not state this and, on the contrary, we agree with the conclusions of Livermore et al.3 that selection (of carbapenem-resistant Pseudomonas aeruginosa in particular) is unlikely under physiologically relevant ertapenem concentrations. In this regard, the following points are critical. First, the BSAC/EUCAST ertapenem breakpoint for susceptibility is 0.5 mg/L and for resistance is.2 mg/L. The former value is far below the MICs for most P. aeruginosa strains, which range between 2 and 32 mg/L with MIC50 and MIC90 values of 4–8 and 16 mg/L, respectively.4 Secondly, the free ertapenem plasma concentration is less than the MIC50 by 4 h post-dose.