Effect of dissolved versus total zinc levels on bacterial zinc-dependent meropenem co-resistance in River Tyne sediments.

Metal pollution can increase in situ antibiotic resistance (AR), leading to elevated AR exposures and the potential evolution of new resistance. Metal-driven AR selection has been observed downstream of historic zinc (Zn) mining areas, such as the River Tyne catchment in Northeast England. The South Tyne was heavily mined until the early 20th century, leaving its waters and sediments with high Zn levels. Conversely, the North Tyne basin was unmined, providing a stark contrast in environmental Zn levels. Replicate sediment and water samples were collected six times from two North Tyne (Reaverhill, RH; Warks Burn, WB) and three South Tyne (Featherstone, FS; River West Allen, WA; River Nent, RN) sites, and phenotypic Zn, meropenem, and combined meropenem-Zn resistance were quantified relative to total and dissolved Zn levels and water quality conditions. In parallel, a 309-day batch experiment was performed on sediments from WB or WA, amended with inorganic Zn, and meropenem and Zn resistance were quantified over time. Sediment plating showed Zn-resistant and combined-resistant bacteria were significantly more abundant than meropenem-resistant bacteria in WA and RN (p < 0.05), suggesting Zn-dependent meropenem resistance prevailed at elevated Zn levels. Parallel batch studies, however, showed that only dissolved Zn levels correlated with phenotypic meropenem co-resistance, suggesting resistance was probably driven by dissolved rather than total Zn. Subsequent qPCR analysis on sediment samples showed Zn efflux and porin genes, especially oprD, associated with phenotypic Zn and combined Zn-meropenem resistance. Overall, results suggest meropenem co-resistance exists in the Tyne, but it is Zn-dependent, possibly linked to oprD-mediated uptake repression. However, dissolved Zn levels, not total Zn, drive meropenem resistance co-selection in the Tyne. In the future, dissolved and total metal levels should always be quantified in AR risk assessments on metal-polluted sites.