A Rapid and Sensitive Microbiological Assay for Phaseolotoxin

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Abstract
A rapid and sensitive bioassay for phaseolotoxin, a phytotoxic compound produced by Pseudomonas phaseolicola [P. syringae], was developed. Apparently phaseolotoxin inhibits the growth of Escherichia coli strain K-12 in minimal-glucose medium by inhibiting L-ornithine carbamyltransferase (OCT), thereby creating a phenotypic requirement for arginine. Inhibition is reversed by citrulline and arginine, but not by ornithine. A dose-response curve was established with purified phaseolotoxin preparations and as little as 10-12 pg of phaseolotoxin was detected. Growth inhibition also was observed with cell-free culture filtrates and around colonies of P. phaseolicola. Maximum zones of inhibition were observed when P. phaseolicola was grown at 18.degree. C, the optimum temperature for toxin production and induction of chlorosis in plants. There was a 100% correlation between the ability of P. phaseolicola strains to induce chlorosis in bean and to inhibit E. coli. This assay technique greatly facilitates screening for nontoxigenic mutants of P. phaseolicola.