Figure 4

Survival and replication of Salmonella within infected cells. Following the identification of macrophages as the primary cell type infected by SD3246-GFP in tissues of orally-challenged calves, the survival and replication of Salmonella in peripheral blood-derived macrophages was studied in vitro. Invasion assays were performed in triplicate for S. Dublin (A), S. Typhimurium (B) and S. Gallinarum (C) with gentamicin treatment following an hour of incubation with macrophages and maintenance of gentamicin at a sub-inhibitory concentration thereafter. It was found that all three serovars could survive within bovine macrophages for up to 24 h (black line), with a drop in viability between 6 and 24 h. To study the replication of the serovars macrophages were similarly infected with strains containing the fluorescence dilution plasmid pFCcGi. It was observed that the rate of replication of intracellular S. Dublin (A), S. Typhimurium (B) and S. Gallinarum (C) declined slightly over the first 6 h of infection and then plateaued until 24 h despite differences in their in vivo phenotype and disease presentation.