Heat-stable enterotoxin from Escherichia coli: factors involved in growth and toxin production.

Six enterotoxigenic strains of Escherichia coli produced variable levels of heat-stable enterotoxin (ST) when grown under pH control at 8.5 in a simple synthetic medium containing neither amino acids nor vitamins. Bacterial growth and ST production were at levels as high as or higher than those observed in complex media. ST elaboration was detectable in the early logarithmic phase of growth and appeared to be related to disappearance of glucose in the growth medium. The results of this study did not suggest pH-dependent release of ST. Imposition of pH control in complex media resulted in increased growth rates, earlier detectable ST synthesis, and elevated levels of ST. In synthetic medium, attainment of the stationary growth phase was followed by a significant decrease in culture density and a concomitant increase in ST. Cellular autolysis experiments revealed that as much as 20% of the total ST activity was present in a cell-associated form.