Summary: strain 569B Inaba harbouring P plasmid produced less toxin than the parent strain. To examine the effect of plasmid loss on toxin production, temperature-sensitive (ts) mutants of P, unable to replicate at 42 °C, were isolated. One ts plasmid was unstable at 42 °C and its loss yielded a cured strain that resumed a normal level of toxin biosynthesis characteristic of the plasmid-free parent strain. Toxin production was again suppressed in the cured strain after reacquisition of P plasmid. This suggested a role for plasmid-borne genes in the regulation of toxin biosynthesis. A mutant of strain 569B Inaba that produced mutant toxin was isolated by transfer of P and V plasmids. The mutant toxin was similar to choleragenoid because it did not give rise to symptoms of cholera but induced antitoxin immunity in rabbits.


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