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


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