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The effect of various culture media on K99 antigen production by enterotoxigenic Escherichia coli strains of bovine origin was investigated by slide agglutination tests, enzyme-linked immunosorbent assays and in vitro attachment to intestinal villi. l-Alanine at concentrations higher than 0·7 g l−1 was responsible for the inhibition of K99 synthesis observed on media rich in amino acids. The increased inhibitory activity of l-alanine in glucose-rich media after autoclaving suggested the formation of inhibitory products via Maillard’s reaction. Of various l-alanine derivatives tested, only those that hydrolysed to l-alanine were inhibitory. l-Alanine analogues were without effect and the addition of 10 mm-cyclic AMP did not overcome the repression of K99 biosynthesis by l-alanine. Enzymes involved in cell wall synthesis such as l-alanine racemase or d-alanyl-d-alanine synthetase were evidently unaffected by l-alanine.
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