The impact of taxonomic family and supplemental zinc on minimum inhibitory concentration against carbapenem antibiotics among bacteria expressing IMP metallo-beta-lactamase

2. Abstract Antibiotic-resistant infections cause an estimated 2.8 million illnesses and 35,900 deaths annually in the US. Carbapenems are a class of antibiotics that are generally reserved to treat life-threatening invasive infections including sepsis. Accurate diagnosis of carbapenem-resistant infections is critical for early and appropriate treatment. blaIMP encodes bacterial production of the IMP metallo-beta-lactamase (MBL) which can confer resistance to all the beta-lactams including carbapenems. Zinc is an essential co-factor in the IMP MBL enzymatic hydrolysis of carbapenems. Tests for the presence of IMP carbapenemase, such as the Carba NP, include zinc-sulfate (ZnSO4) although broth dilution methods for determining minimum inhibitory concentration (MIC) for carbapenems do not. We hypothesized that ZnSO4 availability would improve the accuracy of carbapenem MIC determination for Enterobacterales expressing blaIMP. Thus, the objective of this study was to determine if supplemental ZnSO4 affects the carbapenem MICs of Enterobacterales and other bacteria expressing blaIMP. Isolates utilized for this study were originally recovered from environmental samples collected at farms, wastewater treatment plants, and from surface water. They were selected based on phenotypic non-susceptibility to carbapenems and genetic confirmation of bacterial carriage of blaIMP. Cation-adjusted Mueller-Hinton broth suspensions of each isolate standardized to a 0.5 MacFarland standard were tested with and without ZnSO4 at 0.1 mmol/L concentration to determine MICs using standard extended-spectrum beta-lactamase micro-broth dilution MIC panels. Although we observed that Morganellaceae imipenem MICs were higher (P < 0.001) than those from other Enterobacterales harboring blaIMP, the inclusion of supplemental ZnSO4 did not influence carbapenem MIC. Additional research will be required to identify important factors that may influence the expression of carbapenemase including IMP, and the accurate determination of clinical MICs which is critical to appropriate therapeutic decision-making.