Copper Resistant Bacteria Better Tolerate Commercially Available Antimicrobial Treatments Based in Silver and Silver-Copper Ions

Abstract

In the current study, the antibacterial efficacy of zeolites containing silver or copper ions or a combination of these metals was assessed against several diverse copper resistant (CuR) and copper sensitive (CuS) strains of clinically relevant bacterial species. CuR Pseudomonas putida was significantly reduced in comparison to the unamended zeolite control. Unexpectedly, a CuS P. putida strain with no reported metal resistance appeared to be more resistant to the zeolite containing either Ag or Ag/Cu than the CuR strain. Contrary to expectations, after three and six hours of exposure, the CuS Escherichia coli displayed significantly more resistance to the Ag/Cu and Cu treatments than the reportedly CuR E. coli. All three reportedly CuR strains of Salmonella enterica exhibited resistance to Cu and Ag, as well as to the combination of the two metals after three and six hours of exposure. The reductions observed after 24 hours for all three CuR strains with Cu alone were still statistically significant compared to that of the CuS S. enterica strain. In addition, two of the CuR strains were more resistant to silver after 24 hours of exposure, suggesting a shared resistance mechanism such a copper efflux pump that also removes silver ions from the cell. Both the CuR and CuS strains of E. faecium were highly resistant to all of the treatments. In general, after comparison of all the resistances with all the treatments, E. faecium was the most resistant species, P. putida was the least resistant species, and the Salmonella strains were more resistant than E. coli in most cases.