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This study was conducted to characterize the role of chemical interventions, biocides and heavy metal micronutrients, in particular, in swine production systems on the emergence of heavy metal and biocide tolerant Salmonella and its association with antimicrobial resistance. A total of 353 Salmonella isolates with different antimicrobial resistance profiles identified from 36 barns exposed to three different classes of biocides were analyzed. The sources of isolates included feed (n=30), fecal (n=226), and environment (n=97) samples that were systematically selected. The minimum inhibitory concentrations (MIC) of each isolate against heavy metals copper (Cu) and zinc (Zn) was determined on Mueller-Hinton-II (MH) agar plates containing serial dilutions of copper sulfate (1-32mM) and zinc chloride (0.25-16mM). A non-parametric Wilcoxon Rank Sum test for trends across ordered groups (Stata 10, College Station, TX) was used to determine association between concentration of metal in feed and MIC. The most common MDR patterns among the more heavy metal tolerant isolates were AmClStSuTe (n=81) and AmStTeKm (n=58), which are common multi-drug resistance patterns found in swine production systems. There was a significant association between the concentration of copper in feed and the MIC of isolates recovered from fecal samples for copper (p<0.001). Heavy metal tolerance was also significantly associated with distinct multi-drug resistance types. The odds of finding high Zinc MIC were 15 times higher for the AmClStSuTe R-type than AmStTeKm (Chi-square= 47.2; p<0.05). On the other hand, the odds ratio for association between copper tolerance and R-type AmStTeKm was 4.6 (Chi-square=17.9; P<0.05). No association between biocide use and heavy metal tolerance was detected in this study. Unique genes that encode for tolerance to copper and zinc and physical linkage to antibiotic resistance determinants are being investigated. The findings in this study suggest that the use of copper in swine feed results in higher tolerance of Salmonella strains to copper which in turn co-selects for antimicrobial resistance.

Use of heavy metals in swine feed and its association with the co-selection of metal tolerant and multi-drug resistant Salmonella