[Bactericidal activity of the dye Erio acid red XB 400 towards Bacillus thuringiensis]

Laboratory and field tests revealed that the addition of Erio acid red XB 400 dye (EAR) to Bacillus thuringiensis formulations inhibited spores of the bacillus. In the laboratory, 74% of the spores present in a suspension containing 16 X 10(9) viable spores/mL, and 0.25 gm/l of EAR, were inhibited after 28 h. Spore inactivation in a physiological solution containing 1 X 10(7) viable spores/mL was 75% after the same period of exposure to the same EAR concentration. Field tests showed a reduction in the number of viable spores in a suspension exposed to sunlight; a suspension of 75 000 viable spores/mL yielded 2000 and 400 viable spores/mL after 2 and 4 h of exposure to sunlight, while the same suspension added with 2.5 ppm EAR yielded 1000 and 100 viable spores/mL after the same periods of exposure. The photodynamic action of sunlight on the dye provokes a chemical reaction (oxidation) and the inactivation effect of EAR increases with temperature. Consequently, use of EAR is incompatible with B. thuringiensis formulations and methods used for deposit assessment, based on the use of EAR, should be modified accordingly.