Nutrition and enterotoxin synthesis by enterotoxigenic strains of Escherichia coli: defined medium for production of heat-stable enterotoxin

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Abstract
A defined medium has been developed which supports synthesis of heat-stable enterotoxin (ST) by porcine and bovine strains of enterotoxigenic (ENT+) Escherichia coli in levels equivalent or better than a complex Casamino Acids-salts medium. The medium components did not support production of heat-labile enterotoxin (LT) but were similar for ST synthesis by ENT+ strains producting only ST and those which produced ST in addition to LT. The amino acids in Casamino Acids found to be necessary for growth and enterotoxin synthesis were proline, serine, aspartic acid, and alanine. Maximal growth and toxin levels were obtained after 8 h of incubation. Improved growth, but not an increase in synthesis of ST, was observed in the presence of Mg2+, Mn2+ and Fe3+ compared with Mg2+ alone. A chelator, tricine, was necessary for maximal cell densities,, probably to solubilize trace ions and make them more available to the bacteria. Increased growth was observed upon addition of glucose to both complex and defined media; however, glucose as well as gluconate and pyruvate appeared to cause repression of toxin synthesis. Addition of vitamins, oleic acid, or DL-lactic acid to the defined medium slightly increased levels of ST.