To determine the virulence patterns in humans of the three most prevalent Salmonella serovars in pig in France, Derby (SD), Typhimurium (ST) and its monophasic variant (vmST), we tested 5 strains per serovar in vitro, on cells, and in vivo, in an insect model. For the in vitro assay, Caco-2 cells were inoculated with 107 CFU of Salmonella. A percentage of adhesion and a percentage of invasion were determined for each strain. For the in vivo insect model, each strain was inoculated in 20 larvae of Galleria mellonella in a concentration of 104 UFC per larvae. After 48 hours at 37°C, dead larvae were counted to calculate the mortality percentage.
In Caco-2, the percentage of adhesion varied from 3.93% to 54.60% and was respectively in average 9.5%, 12.4% and 30.9% for SD, ST and vmST. Significant difference was found between vmST and SD. Moreover, the highest difference between strains was observed for the vmST serovar with a % of adhesion varying from 8.63% to 54.6%. The invasion in Caco-2 of all our strains was low, under 2.4%.
Average mortality was respectively 89%, 48% and 58% for SD, ST and vmST. Significant difference was found between SD and the two other serovars. All SD strains were highly virulent while an intra-serovars difference of virulence patterns was observed for the two other serovars.
Our study showed different pattern of virulence between- and intra-serovars. We showed for the first time data on virulence on S. Derby, and surprisingly, our strains isolated from pig were highly virulent on larvae while its adhesion on Caco-2 cells was the lowest. This serovar is described as having a lower virulence potential. The two models showed that some monophasic S. Typhimurium were highly virulent suggesting that this serovar could be more pathogen for humans.