The impact of zinc supplementation on carbapenem MICs among bacteria expressing IMP metallo-beta-lactamase.

Antibiotic-resistant infections cause an estimated 2.8 million illnesses and 35,900 deaths annually in the USA. 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. 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 sulphate (ZnSO) although broth dilution methods for determining MIC for carbapenems may vary. We hypothesized that ZnSO availability would improve the accuracy of carbapenem MIC determination for bacteria expressing . Thus, the objective of this study was to determine if supplemental ZnSO affects the carbapenem MICs of , and expressing . Isolates utilized for this study were originally recovered from environmental samples collected at farms, wastewater treatment plants and surface water. They were selected based on phenotypic non-susceptibility to carbapenems and genetic confirmation of bacterial carriage of . Cation-adjusted Mueller-Hinton broth suspensions of each isolate standardized to a 0.5 MacFarland standard were tested with and without ZnSO added at 0.1 mmol l concentration to determine MICs using standard extended-spectrum beta-lactamase microbroth dilution MIC panels. Although we observed that imipenem MICs were higher (<0.001) than those from other bacteria harbouring , the inclusion of supplemental ZnSO did not influence carbapenem MIC. This suggests that supplemental ZnSO will not improve the accuracy of carbapenem MICs in environmental bacteria expressing IMP carbapenemase. 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.