A Radioisotope Technic for the Rapid Detection of Coliform Organisms

Abstract

The fermentation of lactose with the production of gas, the standard presumptive test for coliform organisms, is detected rapidly by the use of 1-C14 lactose in the medium. The gas produced by the organisms contains Cl4O2 which is trapped as BaC14O3 and measured for radioactivity. The culture apparatus, designed by the authors, consists of a small plastic unit containing a 0.5-inch diameter cup in which a membrane filter through which an unknown sample of water has been passed is immersed in broth or synthetic medium containing the 1-C14 lactose. An air stream is directed across the surface of the culture and forced to exhaust through a porous paper pad impregnated with a saturated solution of Ba(OH)2. C14O2 evolved by the coliform organisms, if present, is picked up in the air stream and deposited on the porous pad as BaC14O3. The pad is dried and measured for radioactivity in a gas-flow Robinson counter connected to a binary scaler with a built-in amplifier. Counting is in the proportional range. A level of radioactivity significantly above that of a sterile control indicates the presumptive test for coliform organisms is positive. As few as approximately 5 Escherichia coli cells have been detected after 10 minutes of incubation. Organisms in lag phase seem to produce much more CO2 per cell than organisms in exponential growth. Work is in progress to control biological vagaries which sometimes delay the test. The test has not been made quantitative.