In many countries high doses (up to 3000 mg/kg feed) of dietary zinc oxide are used in pig nutrition to suc- cessfully combat post weaning diarrhea in pigs, which is often induced by pathogenic Escherichia coli strains [1]. It has been shown that the intestinal microbiota of freshly weaned pigs is strongly perturbed by such high dietary zinc doses [2]. However, a direct inhibition of total enterobacteria or coliforms was not observed in sev- eral studies on this topic [3–5] and there are even indica- tions that high dietary zinc may increase the diversity of
*Correspondence: Wilfried.Vahjen@fu-berlin.de
Institute of Animal Nutrition, Freie Universität Berlin, Berlin, Germany
Enterobacteriaceae [6]. Possibly connected to the hypoth- esis that dietary zinc enhances enterobacterial diversity is a recent report on the increased occurrence of multi- resistant E. coli strains in pigs fed high zinc doses [7]. Although intrinsic resistances against certain antibiotics are known for Gram-negative bacteria, the lack of anti- biotic use in the mentioned studies makes it much more likely that indeed 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 hori- zontal gene transfer within the Enterobacteriaceae could lead to the emergence of strains with high virulence.