Abstract
High levels of zinc oxide are used frequently as feed additive in pigs to improve gut health and growth performance and are still suggested as an alternative to antimicrobial growth promoters. However, we have recently described an increase of multi-resistant E. coli in association to zinc feeding in piglets. This previous study focused on clonal diversity of E. coli, observing the effect on multi-resistant strains by chance. To shed further light into this highly important topic and falsify our previous findings, we performed a zinc pig feeding trial where we specifically focused on in-depth analysis of antimicrobial resistant E. coli. Under controlled experimental conditions, piglets were randomly allocated to a high dietary zinc (zinc group) and a background zinc feeding group (control group). At different ages samples were taken from feces, digesta, and mucosa and absolute E. coli numbers were determined. A total of 2665 E. coli isolates were than phenotypically tested for antimicrobial resistance and results were confirmed by minimum inhibitory concentration testing for random samples. In piglets fed with high dietary zinc, we detected a substantial increase of multi-resistant E. coli in all gut habitats tested, ranging from 28.9–30.2% multi-resistant E. coli compared to 5.8–14.0% in the control group. This increase was independent of the total number of E. coli. Interestingly, the total amount of the E. coli population decreased over time. Thus, the increase of the multi-resistant E. coli populations seems to be linked with persistence of the resistant population, caused by the influence of high dietary zinc feeding. In conclusion, these findings corroborate our previous report linking high dietary zinc feeding of piglets with the occurrence of antimicrobial resistant E. coli and therefore question the feeding of high dietary zinc oxide as alternative to antimicrobial growth promoters.
Highlights
The European ban of antibiotics as feed additives in 2006 (REGULATION (EC) No 1831/ 2003) has enforced the search for possible alternatives to prevent frequently occurring postweaning diarrhea in pigs
We found that 18.6% of the Escherichia (E.) coli clones isolated from a high zinc group (2425 mg zinc/kg diet) were multi-resistant but not a single clone from the control group (57 mg zinc/kg diet) [20]
Regarding E. coli colonies grown on selective agar plates supplemented with antimicrobial substances, again we observed no significant difference in the number of E. coli between the control and zinc group across different time points, habitats, or the nine different antimicrobial supplements
Summary
The European ban of antibiotics as feed additives in 2006 (REGULATION (EC) No 1831/ 2003) has enforced the search for possible alternatives to prevent frequently occurring postweaning diarrhea in pigs. Besides possible alternatives such as probiotics, prebiotics, enzymes, herbal products, organic acids, and other bioactive compounds, cationic trace elements such as zinc from zinc oxide at pharmacological levels have been propagated frequently by science and industry [1,2,3]. This knowledge potentially encourages feeding durations longer than 14 days in less regulated countries or under conditions of recurrent periods of diarrhea and low performance
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