Efficacy of multiple metals against copper-resistant bacterial strains

Aims: The antibacterial efficacy of zeolites containing copper (Cu) or silver (Ag) ions or a combination was assessed against several reported copper-resistant (Cu^R) bacterial strains. Methods and Results: Comparison strains were obtained from the American Type Culture Collection that had no documented metal resistance. Reductions in bacterial populations were determined after exposure time intervals of 3, 6 and 24h. All three Cu^R strains of Salmonella enterica exhibited resistance to Cu, Ag and Cu/Ag after three and 6h of exposure. Both the Cu^R and comparison strain of Enterococcus faecium were resistant to both metals and the metal combination. Cu^RPseudomonas putida was significantly reduced by all zeolites within 3h. The Cu^REscherichia coli strain was more sensitive to Cu, but more resistant to Ag than the comparison strain; however, significant reductions were achieved within 3h with both Cu and Cu/Ag, and within 24h with Ag. Conclusions: Some strains with reported resistance to Cu were also resistant to Ag, suggestive of a shared resistance mechanism such as an indiscriminate Cu efflux pump. Ent. faecium appears to have innate resistance to both metals. In general, Ent. faecium was the most resistant species to the individual metals and the combination of metals, Ps. putida the least resistant, and the Salmonella strains were more resistant than E. coli. Significance and Impact of the Study: Several of the comparison strains with no reported copper resistance were resistant to one or both metals. This may call into question the methods for determining bacterial metal resistance, which typically use nutrient-rich media containing metals to assess the ability of the bacteria to grow in comparison with a wild-type strain. Nevertheless, all the Cu^R strains evaluated in this study, with the exception of Ent. faecium, were reduced using the Cu and Ag zeolite combination.