Method for determining <i>Bacillus thuringiensis</i> var. <i>thuringiensis</i> Berliner in soil

Abstract

A plate dilution frequency technique was developed for determination of Bacillus thuringiensis var. thuringiensis Berliner in soil. Confirmation of B. thuringiensis colonies was achieved by microscopic examination of stained smears for the presence of characteristic parasporal crystals. Competitive growth of B. thuringiensis relative to soil microorganisms was promoted by use of nutrient agar amended to contain 5 p.p.m. polymyxin B sulfate and 4 p.p.m. penicillin G, and incubation at 37 °C for 48 hours. Limits of detection were 600 to 1000 B. thuringiensis propagules/g soil, depending on the soils and the ratio of total soil microorganisms to B. thuringiensis present.