ABSTRACTSince its discovery in the early 2000s, methicillin-resistantStaphylococcus aureus(MRSA) clonal complex 398 (CC398) has become a rapidly emerging cause of human infections, most often associated with livestock exposure. We applied whole-genome sequence typing to characterize a diverse collection of CC398 isolates (n= 89), including MRSA and methicillin-susceptibleS. aureus(MSSA) from animals and humans spanning 19 countries and four continents. We identified 4,238 single nucleotide polymorphisms (SNPs) among the 89 core genomes. Minimal homoplasy (consistency index = 0.9591) was detected among parsimony-informative SNPs, allowing for the generation of a highly accurate phylogenetic reconstruction of the CC398 clonal lineage. Phylogenetic analyses revealed that MSSA from humans formed the most ancestral clades. The most derived lineages were composed predominantly of livestock-associated MRSA possessing three different staphylococcal cassette chromosomemecelement (SCCmec) types (IV, V, and VII-like) including nine subtypes. The human-associated isolates from the basal clades carried phages encoding human innate immune modulators that were largely missing among the livestock-associated isolates. Our results strongly suggest that livestock-associated MRSA CC398 originated in humans as MSSA. The lineage appears to have undergone a rapid radiation in conjunction with the jump from humans to livestock, where it subsequently acquired tetracycline and methicillin resistance. Further analyses are required to estimate the number of independent genetic events leading to the methicillin-resistant sublineages, but the diversity of SCCmecsubtypes is suggestive of strong and diverse antimicrobial selection associated with food animal production.IMPORTANCEModern food animal production is characterized by densely concentrated animals and routine antibiotic use, which may facilitate the emergence of novel antibiotic-resistant zoonotic pathogens. Our findings strongly support the idea that livestock-associated MRSA CC398 originated as MSSA in humans. The jump of CC398 from humans to livestock was accompanied by the loss of phage-carried human virulence genes, which likely attenuated its zoonotic potential, but it was also accompanied by the acquisition of tetracycline and methicillin resistance. Our findings exemplify a bidirectional zoonotic exchange and underscore the potential public health risks of widespread antibiotic use in food animal production.

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Antimicrobials are important tools for the therapy of infectious bacterial diseases in companion animals. Loss of efficacy of antimicrobial substances can seriously compromise animal health and welfare. A need for the development of new antimicrobials for the therapy of multiresistant infections, particularly those caused by Gram-negative bacteria, has been acknowledged in human medicine and a future corresponding need in veterinary medicine is expected. A unique aspect related to antimicrobial resistance and risk of resistance transfer in companion animals is their close contact with humans. This creates opportunities for interspecies transmission of resistant bacteria. Yet, the current knowledge of this field is limited and no risk assessment is performed when approving new veterinary antimicrobials. The objective of this review is to summarize the current knowledge on the use and indications for antimicrobials in companion animals, drug-resistant bacteria of concern among companion animals, risk factors for colonization of companion animals with resistant bacteria and transmission of antimicrobial resistance (bacteria and/or resistance determinants) between animals and humans. The major antimicrobial resistance microbiological hazards originating from companion animals that directly or indirectly may cause adverse health effects in humans are MRSA, methicillin-resistant Staphylococcus pseudintermedius, VRE, ESBL- or carbapenemase-producing Enterobacteriaceae and Gram-negative bacteria. In the face of the previously recognized microbiological hazards, a risk assessment tool could be applied in applications for marketing authorization for medicinal products for companion animals. This would allow the approval of new veterinary medicinal antimicrobials for which risk levels are estimated as acceptable for public health.

The temporal increase in antimicrobial resistance and in MDR bacteria causing UTI in dogs and cats creates important therapeutic limitations in veterinary medicine. Furthermore, the detection of MDR high-risk clonal lineages raises public health concerns since companion animals with UTI may contribute to the spread of such bacteria.

Please cite this article as: Cavaco, L.M., Hasman, H., Aarestrup, F.M., on behalf of the MRSA collaborating group (MRSA-CG), Zinc resistance of Staphylococcus aureus of animal origin is strongly associated with methicillin resistance, Veterinary Microbiology (2010Microbiology ( ), doi:10.1016Microbiology ( /j.vetmic.2011 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. The test population consisted of 476 porcine MRSA isolates from ten European 30 countries, 18 porcine MRSA isolates from Canada and seven MRSA from China, 92 31 MRSA and 60 methicillin-susceptible S. aureus (MSSA) isolates from veal calves in the 32Netherlands and 88 porcine MSSA isolates from four European countries. Most porcine 33 MRSA (n=454) and all bovine MRSA belonged to clonal complex (CC) 398 whereas 37 34 of the pig MRSA from Europe and the seven Chinese isolates belonged to other CCs 35 and 3 isolates were not classified into a CC. 36All isolates were tested for susceptibility to zinc chloride and copper sulphate using 37 agar dilution and tested by PCR for the czrC gene encoding zinc resistance. 38Phenotypic zinc resistance (MIC>2mM) was observed in 74% (n=324) and 42% (n=39) 39 of European MRSA CC398 from pigs and veal calves respectively, and in 44% of the 40 Canadian isolates (n=8), but not among the Chinese isolates. Almost all (99%) zinc-41 resistant MRSA carried czrC. Of the 37 European non-CC398 MRSA, 62% were 42 resistant to zinc, but only 46% of them carried czrC,. The MICs of the MSSA isolates to 43 zinc chloride ranged from 1 to 4 mM and none carried czrC. The MICs of copper 44 sulphate were neither associated with methicillin resistance nor with the detection of 45 czrC. 46This study showed that zinc resistance and the czrC gene is widespread among 47 CC398 MRSA isolates. This suggests that the use of zinc in feed might have 48 contributed to the emergence of MRSA. 49 50