Abstract
BackgroundDietary zinc oxide is used in pig nutrition to combat post weaning diarrhoea. Recent data suggests that high doses (2.5 g/kg feed) increase the bacterial antibiotic resistance development in weaned pigs. Therefore, the aim of this study was to investigate the development of enterobacterial antibiotic resistance genes in the intestinal tract of weaned pigs.FindingsWeaned pigs were fed diets for 4 weeks containing 57 (low), 164 (intermediate) or 2425 (high) mg kg−1 analytical grade ZnO. DNA extracts from stomach, mid-jejunum, terminal ileum and colon ascendens were amplified by qPCR assays to quantify copy numbers for the tetracycline (tetA) and sulfonamide (sul1) resistance genes in Gram-negative bacteria. Overall, the combined data (n = 336) showed that copy numbers for tetracycline and sulfonamide resistance genes were significantly increased in the high zinc treatment compared to the low (tetA: p value < 10−6; sul1: p value = 1 × 10−5) or intermediate (tetA: P < 1.6 × 10−4; sul1: P = 3.2 × 10−4) zinc treatment. Regarding the time dependent development, no treatment effects were seen 1 week after weaning, but significant differences between high and low/intermediate zinc treatments evolved 2 weeks after weaning. The increased number of tetA and sul1 copies was not confined to the hind gut, but was already present in stomach contents.ConclusionsThe results of this study suggest that the use of high doses of dietary zinc beyond 2 weeks after weaning should be avoided in pigs due to the possible increase of antibiotic resistance in Gram-negative bacteria.Electronic supplementary materialThe online version of this article (doi:10.1186/s13099-015-0071-3) contains supplementary material, which is available to authorized users.
Highlights
Dietary zinc oxide is used in pig nutrition to combat post weaning diarrhoea
In many countries high doses of dietary zinc oxide are used in pig nutrition to successfully combat post weaning diarrhea in pigs, which is often induced by pathogenic Escherichia coli strains [1]
The combined data (n = 336) showed a significant increase of copy numbers for tetA and sul1 in the high zinc treatment compared to the low or intermediate zinc treatment (Table 1). This result is in line with data for tetracycline and sulfonamide resistant E. coli strains by Bednorz et al [7]
Summary
Dietary zinc oxide is used in pig nutrition to combat post weaning diarrhoea. Recent data suggests that high doses (2.5 g/kg feed) increase the bacterial antibiotic resistance development in weaned pigs. The combined data (n = 336) showed that copy numbers for tetracycline and sulfonamide resistance genes were significantly increased in the high zinc treatment compared to the low (tetA: p value < 10−6; sul: p value = 1 × 10−5) or intermediate (tetA: P < 1.6 × 10−4; sul: P = 3.2 × 10−4) zinc treatment. Intrinsic resistances against certain antibiotics are known for Gram-negative bacteria, the lack of antibiotic use in the mentioned studies makes it much more likely that zinc oxide is the determining factor for the observed increase in antibiotic resistances. These results are disconcerting, as enterobacteria are known to exchange genetic information frequently [8, 9]. An increased spread of antibiotic resistance genes via horizontal gene transfer within the Enterobacteriaceae could lead to the emergence of strains with high virulence
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