A defined growth medium (designated AP), in which the concentrations of Na and K could be altered independently of one another, was developed for ATCC 25975. The addition of 100 mM-Na to AP-medium containing 25 mM-K initially reduced the rate of expression of extracellular glucosyltransferase (GTF). However, once had adaptated to grow in the presence of 100 mM-Na, the rate of GTF expression was stimulated. In fact once adapted to the presence of Na in the environment the same increase in the rate of enzyme expression was observed in all batch cultures irrespective of the K concentration (2-50 mM). At 2 mM-K there was no change in the level of saturation of the membrane lipids when the Na concentration was increased from 0 mM to 100 mM. Na-stimulation of GTF expression was confirmed in non-proliferating cell suspensions at different K concentrations. In non-proliferating cell suspensions, GTF expression outlined a rectangular hyperbola with respect to K concentration when the K concentration was stepped up from 2 mM. The increase in GTF synthesis and secretion was transient and was similar to that previously reported by us in Na-rich medium, though it did not reach the same high levels. The reduced transient stimulation of GTF expression correlated both with an enrichment in the saturated fatty acids of the membrane lipids of , and with the fact that the degree of saturation was only slightly reduced when the K concentration was stepped up from 2 mM to 50 mM. Needless to say, the final octadecenoic to octadecanoic (C/C) fatty acid ratio retained its direct correlation with the transient increase in GTF production following the step up in K concentration, giving rise to an apparent biphasic plot when combined with that previously reported.


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