Summary: Enterotoxigenic are known to secrete several types of toxins including STb, a heat-stable enterotoxin. STb enterotoxin production was studied in wild-type strains. Using a quantitative STb-specific inhibition ELISA, the amount of toxin present in the culture supernatant fractions of various strains was determined. Variation in the production of STb toxin was observed for the wild-type strains. For strain 82-4247 grown in trypticase soy broth, the toxin was produced after 4 h of growth and was maximal after about 57 h of growth. The amount of toxin in the culture supernatant fraction increased concomitantly with bacterial growth. Using the rat loop assay, the biological activity of STb was retained even after the logarithmic phase of growth when STb production levelled off (i.e. from 24 to 74 h). STb production by strain 82-4247 varied with the culture medium used. In particular, addition of 1-0% (w/v) compared to 0-1 % glucose to Davis minimal medium decreased STb production, whereas addition of 1-0% (w/v) glycerol did not affect STb production. Addition of exogenous cAMP reversed the repressive effect of glucose. Using mutant strains, STb production was shown to be subject to catabolite repression.


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