Frequency and expression of st313-td gene and virulence of Salmonella Dublin strains isolated from humans and animals in Brazil

Abstract Salmonella Dublin is a strongly cattle-adapted serovar that is also responsible for severe invasive infections in humans. Although invasive infections by non-typhoid Salmonella are rare, in developing areas such as sub-Saharan Africa, these infections are frequent and generally related to Salmonella Typhimurium strains from Sequence Type (ST) 313, that harbor a possible virulence marker, the st313-td gene, broadly detected in S. Dublin strains. The aims of this study were to verify the frequency of st313-td by PCR and the expression by RT-PCR of st313-td, sopE2 and fliC in Salmonella Dublin strains isolated in Brazil. Moreover, the invasion capacity in Caco-2 human epithelial cells, survival within human macrophages (U937), in vivo virulence analysis in Galleria mellonella and the motility of the strains were verified. All studied strains presented the st313-td. The relative expression rates ranged from 0.1 to 2.3 fold change of st313-td and from no expression to 16.6 fold change of sopE2, while no expression was detected of fliC. The invasion in Caco-2 cells ranged from 54.0 to 88.9% and the intra-macrophage survival from 72.9 to 98.1%. In addition, most of the strains studied presented a highly virulent profile in vivo and a non-motile profile in the motility test. In conclusion, the presence of st313-td in all the S. Dublin strains studied besides its expression among strains from invasive cases suggest that this gene might be related to the invasive characteristic of this serovar. The low expression of sopE2 in strains from human invasive cases suggests that the expression of this gene may not be a limiting factor to the invasion of S. Dublin strains. Moreover, the absence fliC expression and the low motility rates observed suggest that the flagella absence may favor the host immune system evasion by S. Dublin and the establishment of invasive infection. The high invasion to human epithelial cells and survival within human macrophages, besides the high mortality rates observed in vivo, reinforce the invasive potential of S. Dublin strains.