Occurrence and genetic diversity of Salmonella in organic and conventional pig productions in France

Summary

The objectives of this study were 1) to assess the occurrence of Salmonella in organic pig production, in comparison with conventional pig production, 2) to evaluate the genetic diversity of strains isolated from these two productions and 3) to estimate the cross-contamination on slaughter line between conventional pig and organic pig.
In one slaughterhouse, 26 organic herds and 31 conventional herds (2 pigs per herd) were sampled for Salmonella detection. Analyses were realized on colon content and swabs of carcass for each pig. Two isolates by positive samples were serotyped and typed by PFGE using XbaI enzyme. All S. Typhimurim and monophasic variant of serovar Typhimurium were subtyped by MLVA.
Prevalence of Salmonella in colon content was higher for organic pigs, 37.9% IC95than for conventional pigs, 32.7% but difference was not significant (p=0.563). Salmonella prevalence was lowest on carcasses and very close between the two productions: 10.7% for organic and 10.3% for conventional.
The 104 isolates were distributed in 7 serovars: Derby (46 isolates), Brandenburg (18), Typhimurium (13), monophasic variant of Typhimurium 4,12:i:- (11) and 4,5,12:i:- (10), Infantis (2) and Mbandaka (2). Sixteen PFGE profiles were obtained: 1 per serovar for serovars Mbandaka, Infantis, and Brandenbrug, 3 for Derby, 4 for Typhimurium and 4 for monophasic variant 4,12:i:-.
Seven PFGE profiles, representing 84% of the isolates, were common between organic and conventional pigs. A major profile gathered 79% of the S. Derby strains. S. Brandenburg strains were also very clonal, all presented the same PFGE profiles whereas they came from 5 different herds. With 20 isolates from 12 carcasses, it has not been possible to show with certainty Salmonella cross-contamination between organic and conventional pigs during the process.
For S. Typhimurium, MLVA gave a better discrimination than PFGE, 8 patterns against 4; particularly for 6 isolates with the same PFGE pattern which was subdivided into 5 MLVA patterns. While on the 21 monophasic isolates, MLVA and PFGE gave similar discrimination (7 patterns with MLVA and 6 with PFGE).