The MLST results are in concordance with a study reported by Armand-Lefevre and colleagues, who compared S. aureus isolates from healthy pig farmers, human controls, and pigs . They recovered methicillin-susceptible S. aureus exhibiting ST9, 398, and 433 from pig farmers and swine; only one ST 398 isolate of a pig farmer was methicillin resistant. ST398 was first recognised by our group, and reported to the international MLST database in 2004. At that time no correlation between S. aureus with ST398 and pig farming had been reported. In Hong Kong, two ST398 strains were described to have been isolated from patients with bacteremia . No relation with pig farming was reported. Typing results of the French ST398 strains (4 pig-related MSSA and 1 pig-related MRSA isolate) revealed the same typing result as the Dutch ST398 strains. At our lab the French strains were PFGE non-typeable, spa type t034 and t1250, and were PVL negative. Spa type t108, t034, and t1250 are related to each other, indicating to have a common ancestor.
Voss and colleagues reported for the first time the isolation of PFGE non-typeable MRSA strains from pig care-takers . The strains were closely related to each other as shown by spa typing. They screened a total of 26 farmers of whom 6 were colonized with MRSA. The authors identified three different MRSA strains by spa typing, type t108, t567, and t943. Spa type t108 was also found in the present study, indicating the relatedness of this spa type with pig-farming. Only one pig was found to be MRSA positive, carrying the same strain type as the farmer. In contrast, we found MRSA in 8 out of 10 randomly chosen pigs. The difference in prevalence could perhaps be explained by sampling differences, MRSA transmission among pigs or to differences in risk factors between the farms.
All pig MRSA isolates were PFGE non-typeable by PFGE and had the same typing characteristics as the human MRSA isolates. Furthermore, the pig-related MRSA isolates were related to PFGE non-typeable MRSA strains from the national MRSA database. It seems that the PFGE non-typeable MRSA strains are not only transmitted between human and pigs but also between humans. The human to human transmission was elucidated by the fact that among the PFGE non-typeable MRSA isolates from the national institute of public health (RIVM) MRSA database in at least 3 cases a family member was colonized with an MRSA strain with identical typing characteristics. Furthermore, the child of the pig farmer's family had no contact with pigs and was colonized with the same strain as the parents.
An earlier report of a significant association between pig farming and resistant commensal bacteria was published by Aubry-Damon et al. . The authors showed that levels of commensal bacteria with antimicrobial resistance were higher among pig farmers than among controls, including a higher isolation rate of S. aureus in pig farmers. The cause of the higher S. aureus isolation rate in pig farmers remained unclear.
More research on a larger scale is necessary to further address the prevalence of MRSA among pigs, pig farmers and their contacts. Furthermore, it would be interesting to what extent the PFGE non-typeable MRSA isolates were associated with pig farming, which may elucidate the importance of the clonal cluster.