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

## TL;DR

This study investigates whether adding zinc affects the accuracy of antibiotic resistance tests for bacteria that produce a specific enzyme linked to antibiotic resistance.

## Contribution

The study reveals that adding zinc does not improve carbapenem MIC accuracy for IMP-producing bacteria.

## Key findings

- Morganellaceae showed higher imipenem MICs compared to other blaIMP-harboring bacteria.
- Supplemental ZnSO4 had no effect on carbapenem MICs for IMP-expressing environmental bacteria.
- Zinc supplementation does not enhance the accuracy of MIC testing for IMP metallo-beta-lactamase.

## Abstract

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. 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 sulphate (ZnSO4) although broth dilution methods for determining MIC for carbapenems may vary. We hypothesized that ZnSO4 availability would improve the accuracy of carbapenem MIC determination for bacteria expressing blaIMP. Thus, the objective of this study was to determine if supplemental ZnSO4 affects the carbapenem MICs of Enterobacterales, Alteromonadales and Moraxellales expressing blaIMP. 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 blaIMP. Cation-adjusted Mueller–Hinton broth suspensions of each isolate standardized to a 0.5 MacFarland standard were tested with and without ZnSO4 added at 0.1 mmol l−1 concentration to determine MICs using standard extended-spectrum beta-lactamase microbroth dilution MIC panels. Although we observed that Morganellaceae imipenem MICs were higher (P<0.001) than those from other bacteria harbouring blaIMP, the inclusion of supplemental ZnSO4 did not influence carbapenem MIC. This suggests that supplemental ZnSO4 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.