Zinc oxide can inhibit E. coli, and is an alternative solution to antibiotics for preventing post-weaning diarrhea. Some bacteria, like Staphylococcus aureus (MRSA), have ZnO resistance and are also carrying the genes for multidrug resistance. Therefore, feeding high levels of ZnO in starter diets could actually be putting pressure on selection for antibiotic resistance in bacteria. A herd colonized by MRSA was used, and at weaning pigs were fed a diet with either 100 or 3,000 ppm of ZnO. Neither group experienced diarrhea during the trial, but the 3,000 ppm group had a significantly higher level of MRSA. This suggest selection for drug resistance in MRSA could be caused by high levels of ZnO.