The influence of cultural conditions on the expression in Salmonella typhimurium of an antigen related to cholera toxin

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Abstract
The choice of strain, culture conditions, composition of medium and size of inoculum all affected the expression of a cholera-toxin-related antigen (CTRA) in Salmonella typhimurium. A previous study had shown that the number of organisms expressing CTRA in Casamino acid Yeast Extract (CYE) medium decreased between 4 h and 6 h in uninterrupted culture. In the present experiments, organisms harvested at 4-5 h were subcultured into fresh CYE medium and incubated for a further 2 h; the total number of organisms increased, and the decrease in the proportion of organisms expressing CTRA was reduced. Use of Hartley Digest Broth in place of CYE medium increased the proportion of organisms expressing CTRA in all strains tested, in both the uninterrupted and the subculture procedures. The higher the initial inoculum, the lower was the proportion of organisms expressing CTRA. The presence of the antigen in cells remained constant for about 18 h after transfer from 37°C to 4°C. These data have important implications for the production and purification of CTRA: they show that it was expressed during log-phase of growth, and they suggest that expression was regulated by a non-growth-limiting factor. Moreover, some avirulent strains were better producers of the antigen than virulent ones. The significance of the data is discussed in relation to the in-vivo situation.