Comparative evaluation of plazomicin MICs obtained using agar dilution versus broth microdilution methods and impact of inoculum size against ESBL-producing Escherichia coli, carbapenemase-producing Klebsiella pneumoniae and methicillin-resistant Staphylococcus aureus clinical isolates.

MICs of plazomicin were determined by agar dilution and broth microdilution in 187 ESBL-producing Escherichia coli (n = 73), carbapenemase-producing Klebsiella pneumoniae (n = 55) methicillin-resistant Staphylococcus aureus (n = 59) clinical isolates. Inoculum effect was determined by broth microdilution assay using two different inocula; 1-5 × 105 (standard inoculum) and 1-5 × 108 CFU/mL. For all isolates tested >98% categorical agreement and ≥95% of essential agreement (±1elog2) was found. At high inocula, MICs of plazomicin increased ≥ eight-fold for 25% of E. coli, 24% of K. pneumoniae and 7% of S. aureus isolates tested. The results indicate that agar dilution and broth microdilution methods were equally suitable for determining plazomicin MICs. Inoculum effect was observed for plazomicin in Escherichia coli and Klebsiella pneumoniae isolates. Further studies that establish the genetic background of the isolates are required to better understand the reasons behind the inoculum effect.