As we move away from the preventive use of antimicrobials for enteric disease in post-wean pigs there has been more dependence on the inclusion of heavy metal ions such as zinc, copper, prebiotics or probiotics in post-wean rations. For several decades the feeding of high levels of zinc (2000 ppm +) has been used to reduce the risk of scours and improve growth rates.
In the European Union there has been concern about the accumulation of zinc in the soil where manure from pigs fed high zinc rations is applied to the land. This does appear to be an issue in Europe because levels of zinc in soil are naturally very high to begin with. In contrast, the zinc levels in soil in Canada are naturally low and almost deficient. Added zinc from high zinc rations should not be a problem in Canada.
Some recent studies have suggested that feeding high dietary zinc levels will increase the proportion of multi-drug resistant E. coli in the gut. The underlying mechanisms of zinc effects on resistant bacteria remains unclear, but co-selection processes for anti-microbial resistance is one suggested explanation of this proposed interaction. These German researchers wanted to determine whether E. coli isolates from intestinal contents of piglets that had been supplemented with high concentrations of zinc acquired a higher tolerance towards zinc, and whether multi-drug resistant isolates seemed to develop higher tolerance for zinc concentrations.
The researchers screened phenotypic zinc/copper tolerance of 210 bacterial isolates that included antimicrobial resistant, multi-drug resistant, and non-resistant E. coli. These isolates were selected from two, independent zinc-feeding animal trials by determining a zinc/copper minimal inhibitory concentration (Merlin, Bornheim-Hersel, Germany). In both trials, groups of piglets were supplemented either with high dietary zinc (> 2000 ppm) or control (50-70 ppm, background) concentrations.
The researchers found that:
- high concentration zinc exposure did not have an effect on either zinc or copper phenotypic tolerance of E. coli isolates from the animals
- no significant association was found between antimicrobial resistance and phenotypic zinc/copper tolerance of the same isolates.
Take Home Messages:
- These findings suggest that there is no evidence for the existence of a co-selection mechanism of antimicrobial drug-resistance and zinc tolerance after feeding dietary zinc supplementation at high levels in weaning piglets.
- Feeding high levels of dietary zinc has already been banned in the European Union but that trend may not make it’s way to North America based on co-selection for antimicrobial resistance arguments alone.
Ref: Ghazisaeedi F, Ciesinski L, Bednorz C, Johanns V, Pieper L, Tedin K, Wieler LH, Günther S. Phenotypic zinc resistance does not correlate with antimicrobial multi-resistance in fecal E. coli isolates of piglets. Gut Pathog. 2020 Jan 21;12:4. doi: 10.1186/s13099-019-0342-5. eCollection 2020.