Invasion of Vero cells by Salmonella species

The invasiveness of Salmonella strains for Vero cells was studied by quantitative bacteriology; the technique was more sensitive than phase contrast microscopy. All of 59 Salmonella strains, of 19 different serotypes, were more invasive than Escherichia coli K12. Three strains of Shigella were as invasive as most of the Salmonella strains whereas 29 strains of E. coli, two of Proteus, three of Klebsiella and one of Serratia were much less invasive. Two Citrobacter strains exhibited intermediate invasiveness. Eleven Salmonella strains were also shown to be invasive in HeLa, int 407, bovine kidney, chick kidney and chick embryonic fibroblast cells. The difference between invasive and non-invasive organisms was apparent irrespective of the numbers of bacteria in contact with Vero cells or the duration of bacteria-cell contact. There was little intracellular multiplication of S. typhimurium in Vero cells. Unlike the situation with Shigella, incubation of Salmonella or Salmonella-cell mixtures at 41 degrees C, 22 degrees C or 0 degree C had little effect on invasiveness. Non-viable Salmonella organisms were non-invasive. Incubation of Vero cells with cholera toxin, dinitrophenol, iodoacetic acid, cytochalasin B or D-mannose did not substantially reduce invasiveness. Virulence-associated plasmids were not essential to invasion by S. typhimurium, S. gallinarum or S. pullorum. Neither somatic antigens nor mannose-sensitive haemagglutinins were essential to the invasiveness of an S. infantis strain, but an additional factor, eliminated by N-methyl, N-nitro, N-nitrosoguanidine mutagenesis did contribute to invasiveness.