Comment on: Acinetobacter spp. and time-kill studies

Sir, We are in agreement with Dr Lopardo on various issues that affect antibiotic synergy testing, and in particular, on some of the intrinsic limitations of time–kill methodology. We also agree that the MIC is not a static parameter, and that one of the limitations of our study (as mentioned in the Discussion section) was that we were unable to measure antibiotic concentrations within the testing medium. It is worth noting that MIC endpoints are read after 18 h of incubation, while the combination testing was performed over 24 h. It is also a point of interest that at least two other studies using similar time–killing assays against Acinetobacter baumannii have demonstrated bacterial regrowth at concentrations of 1 MIC of colistin. It remains to be determined if this is a true phenomenon or a limitation of the testing methodology. With regard to the points raised regarding bacterial counts, we would like to point out that a turbidity equivalent to that of a 0.5 McFarland standard (barely detectable turbidity) equates to a bacterial count of 1.5 10 cfu/mL, and Acinetobacter species will achieve substantially higher turbidity levels in Mueller–Hinton broth over a 24 h growth period. Finally, in our study, synergy was defined as a 2 log10 decrease in cfu/mL for the antibiotic combination compared with its more active constituent. We were unable to ascribe the presence of synergy to one isolate in our study because the difference between the colony count in the combined antibiotic testing ( 20 cfu/mL) and the most active single antibiotic tested (colistin;1200 cfu/mL) did not fulfil the study definition.