Quantity and genetic diversity of livestock-associated methicillin-resistant Staphylococcus aureus in pig

To assess quantity of livestock-associated methicillin-resistant Staphylococcus aureus (LA-MRSA) in nasal and skin samples of pigs, ten pigs were sampled three times during two weeks on two farms and nine additional pigs were sampled on farm and in a clean, controlled environment. Samples were processed both by direct plating and enrichment. PCR and whole-genome sequencing were used to determine spa types, multilocus sequence types, resistance and virulence genes. Core genome multilocus sequence typing (cgMLST) was performed to investigate the genetic diversity of the strains in comparison to national surveillance samples.LA-MRSA was detected in the nares of all pigs in all samplings on farm 1 and in one sampling in five pigs on farm 2. Nasal quantity varied between 10^1 and 10^3 CFU/swab and quantity on the skin between 10^1 and 10^2 CFU/swab. In the controlled facility MRSA was detected in the nares of all pigs at in at least one sampling. spa type t034 predominated. cgMLST analysis showed one cluster with minimum allele differences between 0 and 11. In this study, LA-MRSA was detected persistently at low levels in the farm environment. In the controlled environment a decline in nasal carriage but persistent skin carriage were observed. This may indicate the role of air contamination in nasal carriage and environmental contamination in skin carriage. cgMLST showed that strains of spa type t034 are closely related at the national level. Further studies on factors contributing to the stability of this successful clone may help to control LA-MRSA.