The invasiveness of strains for Vero cells was studied by quantitative bacteriology; the technique was more sensitive than phase contrast microscopy. All of 59 strains, of 19 different serotypes, were more invasive than K12. Three strains of were as invasive as most of the strains whereas 29 strains of , two of , three of and one of were much less invasive. Two strains exhibited intermediate invasiveness. Eleven 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 in Vero cells. Unlike the situation with , incubation of or -cell mixtures at 41°C, 22°C or 0°C had little effect on invasiveness. Non-viable organisms were non-invasive. Incubation of Vero cells with cholera toxin, dinitrophenol, iodoacetic acid, cytochalasin B or -mannose did not substantially reduce invasiveness. Virulence-associated plasmids were not essential to invasion by or Neither somatic antigens nor mannose-sensitive haemagglutinins were essential to the invasiveness of an strain, but an additional factor, eliminated by -methyl, -nitro, -nitrosoguanidine mutagenesis did contribute to invasiveness.


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