Summary: The effects of growth conditions on the production of 987P fimbriae by the enterotoxigenic strain 1592 were examined in steady state chemostat experiments at different specific growth rates. The amount of fimbriae produced by fimbriate cells (P) was dependent on the specific growth rate (μ). Under aerobic growth conditions fimbriae production increased with higher μ values till μ = 0·40 hand decreased again at μ values close to μ (0·48 h). Under anaerobic growth conditions the maximal production was comparable to that under aerobic growth conditions, and was also maximal close to μ (0·16 h). Phase variation, measured as the percentage of fimbriate cells in a particular population, was independent of μ. The composition of the growth medium influenced both phase variation and overall production of fimbriae. A shift from minimal to a complex medium induced a rapid reduction in the amount of fimbriae per Pcell and a slower reduction in the percentage of Pcells. A shift from complex to minimal medium resulted in an increase in the percentage of Pcells and a constant amount of fimbriae per Pcell. The frequency of the phase switch was calculated for different growth conditions. The frequency of the P→ Pswitch between two steady states was 2·7 x 10. In batch culture the frequency of the P→ Pswitch was minimally 2·9 x 10. The results indicate that phase variation and the production of 987P fimbriae by fimbriate cells are under independent physiological control.


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