Zinc resistance of Staphylococcus aureus of animal origin is strongly associated with methicillin resistance.

This study was conducted to determine the occurrence of zinc and copper resistances in methicillin-resistant Staphylococcus aureus (MRSA) from swine and veal calves in a global strain collection. The test population consisted of 476 porcine MRSA isolates from ten European countries, 18 porcine MRSA isolates from Canada and seven MRSA from China, 92 MRSA and 60 methicillin-susceptible S. aureus (MSSA) isolates from veal calves in the Netherlands and 88 porcine MSSA isolates from four European countries. Most porcine MRSA (n=454) and all bovine MRSA belonged to clonal complex (CC) 398 whereas 37 of the pig MRSA from Europe and the seven Chinese isolates belonged to other CCs and 3 isolates were not classified into a CC. All isolates were tested for susceptibility to zinc chloride and copper sulphate using agar dilution and tested by PCR for the czrC gene encoding zinc resistance. Phenotypic zinc resistance (MIC>2mM) was observed in 74% (n=324) and 42% (n=39) of European MRSA CC398 from pigs and veal calves, respectively, and in 44% of the Canadian isolates (n=8), but not among the Chinese isolates. Almost all (99%) zinc-resistant MRSA carried czrC. Of the 37 European non-CC398 MRSA, 62% were resistant to zinc, but only 46% of them carried czrC. The MICs of the MSSA isolates to zinc chloride ranged from 1 to 4mM and none carried czrC. The MICs of copper sulphate were associated neither with methicillin resistance nor with the detection of czrC. This study showed that zinc resistance and the czrC gene are widespread among CC398 MRSA isolates. This suggests that the use of zinc in feed might have contributed to the emergence of MRSA.