Farm-specific lineages of methicillin-resistant Staphylococcus aureus clonal complex 398 in Danish pig farms. 
The objective of this study was to investigate the genetic diversity of methicillin-resistant Staphylococcus aureus (MRSA) clonal complex (CC) 398 using pulsed-field gel electrophoresis (PFGE).  Dust and pigs at five age groups were sampled in six Danish MRSA-positive pig farms.  MRSA CC398 was isolated from 284 of the 391 samples tested, including 230 (74%) animal and 54 (68%) environmental samples.  PFGE analysis of a subset of 48 isolates, including the six strains previously isolated from farm workers, revealed the existence of farm-specific pulsotypes.  With a single exception, human, environmental and porcine isolates originating from the same farm clustered together in the PFGE cluster analysis, indicating that spread of MRSA CC398 in Danish pig farms is mainly due to clonal dissemination of farm-specific lineages that can be discriminated by PFGE.  This finding has important implications for planning future epidemiological studies investigating the spread of CC398 in pig farming.