The impact of zinc supplementation on carbapenem MICs among bacteria expressing 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. bla _IMP 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 bla _IMP. Thus, the objective of this study was to determine if supplemental ZnSO₄ affects the carbapenem MICs of Enterobacterales, Alteromonadales and Moraxellales expressing bla _IMP. 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 bla _IMP. 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 bla _IMP, 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.

Keywords: antimicrobial resistance; antimicrobial susceptibility testing; blaIMP; carbapenemase; metallo-beta-lactamase; zinc.

Fig. 1.. Observed MIC values against imipenem for 8 Morganellaceae and 17 other taxonomic family isolates (10 Enterobacteriaceae, 5 Shewanellaceae and 2 Moraxellaceae) expressing IMP carbapenemase with and without supplemental ZnSO₄. The supplemental ZnSO₄ did not impact imipenem MIC (P>0.05).

Fig. 2.. Observed MIC values against meropenem for 8 Morganellaceae and 17 other taxonomic family isolates (10 Enterobacteriaceae, 5 Shewanellaceae and 2 Moraxellaceae) expressing IMP carbapenemase with and without supplemental ZnSO₄. The supplemental ZnSO₄ did not impact meropenem MIC (P>0.05).

Fig. 3.. Observed MIC values against imipenem for 8 Morganellaceae and 17 other taxonomic family isolates (10 Enterobacteriaceae, 5 Shewanellaceae and 2 Moraxellaceae) expressing IMP carbapenemase without supplemental ZnSO₄. Morganellaceae isolates had higher (P<0.001) MIC values than the other taxonomic family isolates.