Quantitative measurements of cholera enterotoxin in cultures of toxinogenic wild-type and nontoxinogenic mutant strains of Vibrio cholerae by using a sensitive and specific reversed passive hemagglutination assay for cholera enerotoxin

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Abstract
A sensitive and specific reversed passive hemagglutination (RPHA) assay for cholera enterotoxin has been developed. Equine anti-choleragenoid antibodies purified by immunoadsorption were covalently coupled to formalinized sheep erythrocytes, using bis-diazotized benzidine, and the antitoxin-sensitized erythrocytes were shown to agglutinate specifically in the presence of cholera enterotoxin. In a microtiter RPHA assay system, the smallest quantity of enterotoxin that caused hemagglutination was approximately 20 pg. A sensitive assay for antibodies to enterotoxin was also developed, based on inhibition of RPHA. Using such assays, we demonstrated that several nontoxinogenic (tox-) strains of Vibrio cholerae produced small but detectable yields of enterotoxin, 4 to 16 ng/ml, under conditions where the highly toxinogenic strain 569B Inaba produced approximately 16 microgram of enterotoxin per ml. The enterotoxin produced in small quantities by these tox- strains was found to be identical to the enterotoxin from V. cholerae 569B Inaba iv its immunological and biological activities. Strains of V. cholerae that produce intermediate yields of enterotoxin have been obtained by two techniques: (i) as less toxinogenic mutants derived from highly toxinogenic strains and (ii) as more toxinogenic mutants derived from tox- strains. Thus, the yield of enterotoxin in cultures of V. cholerae grown under standardized conditions is is a genetically controlled trait that can be altered by mutation.