Isolation and characterization of enterotoxin-deficient mutants of Escherichia coli.

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Abstract
The genes controlling the production of 2 types of enterotoxin of E. coli, 1 heat-labile (LT) and the other heat-stable (ST), are found on plasmids. The absence of a direct selection procedure made it difficult to isolate mutants affecting toxin production. The availability of a naturally occurring recombinant plasmid, carrying genes for LT and ST formation and also for resistance to tetracycline, streptomycin and sulfonamides, made it possible to use comutagenesis and N-methyl-N''-nitro-N-nitrosoguanidine to enrich for such mutants. Seven ST- and 58 LT- mutants were isolated and characterized. Among the LT- group, amber mutants, temperature-sensitive mutants (most of which produce unusually heat-labile LT) and leaky mutants with reduced LT activity were found. The majority of the tested LT- mutants produced immunologically crossreacting material, in most cases in wild-type amounts. Among all 17 of the LT- mutants that could be transferred, the mutation was on the plasmid. Only 1 of 4 transferrable ST- mutants appeared to be a plasmid mutant.