A simple disk pre-diffusion test to predict in vitro aztreonam/avibactam activity against NDM-producing Klebsiella pneumoniae complex.

Objectives: The aim of this study was to describe a simple test to predict in vitro efficacy of aztreonam/avibactam (ATM-AVI) combination based on a pre-diffusion assay involving routinely available ceftazidime/avibactam (CAZ-AVI) and aztreonam (ATM) disks.

Methods: A total of 113 non-repetitive NDM-producing Klebsiella had the species identified by multiplex PCR. Minimum inhibitory concentrations (MICs) for ATM and ATM-AVI were determined by broth microdilution. For the combined disk pre-diffusion method, a disk containing ceftazidime 10 μg and avibactam 4 μg (CAZ-AVI) was applied on the surface of an uninoculated Mueller-Hinton agar plate. Following incubation for 2 h at 36°C, the disk was removed, the bacterial suspension was applied and a 30 μg ATM disk was placed precisely in the same position as the removed CAZ-AVI disk. Following incubation for 16-20 h, inhibition zone diameters were measured and correlated with ATM-AVI MICs.

Results: The distribution of species among the 113 isolates was 85 Klebsiella pneumoniae (75.2%), 19 Klebsiella quasipneumoniae (16.8%) and 9 Klebsiella variicola (8.0%). A total of 99 isolates had only bla and 14 had both bla and bla genes. Regarding the isolates positive for bla only, 38.4% were susceptible to ATM and 7.1% were susceptible, increased exposure. All isolates had ATM-AVI MICs of ≤1 mg/L, and the smallest inhibition zone diameter observed was 23 mm.

Conclusion: Modified disk pre-diffusion can be used as a simple test to screen for ATM-AVI in vitro activity against Klebsiella, since ATM-AVI disks, gradient strips or microdilution panels are not commercially available.