Background:
Methicillin-resistant Staphylococcus aureus (MRSA) CC398 is non-typeable by standard pulsed-field gel electrophoresis (PFGE) due to methylation of the SmaI site. This makes it difficult to study the epidemiology of this livestock-associated MRSA clone. In this study, we employed a recently developed PFGE protocol using Cfr9I, a neoschizomer of SmaI, to investigate the diversity of MRSA CC398 in Danish pig farms. The PFGE profiles displayed by isolates from pigs, environmental samples and farm workers were compared in order to understand whether farms are contaminated with multiple MRSA CC398 sublineages and whether specific sublineages may occur on different farms.
Methods:
A cross sectional study was performed in five Danish pig farms where farm workers had been shown to carry MRSA CC398 in the previous year. A total of 75 environmental and 308 animal samples were collected from three production sites: the farrowing unit (dry sows =7 d prepartum, farrowed sows and piglets =7 d postpartum), the weaning unit (21 d after weaning) and the fattening unit (=7 d before slaughter). After selective enrichment in Mueller Hinton containing 6.5% NaCl, presumptive MRSA CC398 colonies were isolated on Brilliance MRSA agar (Oxoid®) and confirmed by multiplex PCR. PFGE using Cfr9I was performed on all human isolates and a representative from each age group and environmental site according to the protocol described by Bosch et al (2010).
Results:
MRSA CC398 was identified in all six farms at frequencies between 50 and 82%. Two PFGE subtypes were identified: subtype A was detected among isolates from farms 1, 3 and 5, and subtype B in farms 2 and 4. Within each farm, the same PFGE profile was observed among humans, animals and environmental isolates.
Discussion and conclusions:
The results indicate that Danish pig farms generally harbour a single MRSA CC398 sublineage. As the human isolates from farm workers were obtained approximately one year prior to this study, the results also show that specific MRSA CC398 sublineages may persist for long periods within the farm environment without undergoing major changes in their PFGE profiles. No geographical patterns were observed in the distribution of the two PFGE subtypes.