THE BACTERIOSTATIC ACTION OF ROSE BENGAL IN MEDIA USED FOR PLATE COUNTS OF SOIL FUNGI

Abstract

Acid media commonly used in making plate counts of soil fungi were compared with media of the same composition, except that the reaction was near neutral and rose bengal was added to make a conc. of 1 part in 15,000. The highest counts and the most satisfactory plates were obtained by using glucose nitrate soil extract agar with the dye. The ac-tinomyces were entirely eliminated and those bacterial colonies that developed were mucoid and easily differentiated from the fungi. Spreading of Trichoderma and other fungi was reduced to a minimum. A greater conc. of the dye (1 part in 10,000) reduced the counts somewhat; a weaker cone. (1 part in 25,000) allowed more spreading of the fungi and more bacterial colonies to develop. Media containing peptone and the dye gave counts that were a little lower than those on the glucose nitrate soil extract agar. More bacterial colonies developed and there was more spreading by the fungi on these media. Other inhibiting substances, such as boric acid, benzoic acid and chrysoidine Y, were entirely unsatisfactory when substituted for the rose bengal in the plating medium.