Thermal resistance of Bacillus larvae spores in honey.

Considerable attention has been given to the effect of heating in various media on the viability and virulence of the spores of Bacillus larvae, the causative agent of the honeybee disease, American foul brood (White, 1920). Few data have been reported, however, concerning temperatures above the boiling point of honey (White, 1920; Burnside, 1940a, 1945). Under a project on sterilization of honey to permit feeding it to bees without danger of spreading disease, thermal death-time studies were conducted on spores of this organism in diluted honey over a temperature range of 100 to 140 C. The results reported here have been used in the development of a process for treatment of honey to permit feeding it to bees without danger of transmission of American foul brood. Although Burnside (1945) reported that "boiling for 30 minutes can be depended upon to destroy the virulence of Bacillus larvae under ordinary conditions", the authors have been advised (Hambleton, 1950) that any processing of honey for bee feeding must guarantee absence of viable spores of this organism, not simply destruction of virulence. Burnside (1940a) has reviewed the literature on the heat resistance of B. larvae spores and investigated the thermal resistance of the spores in diluted honey, water, and beeswax and in the dry state. He reported growth in a culture of spores boiled in water or exposed to flowing steam for 7 hours, boiled for 5 hours in 50 per cent honey, autoclaved in water at 15 pounds per square inch for 25 minutes or dry for 40 minutes, exposed to 98 C dry heat for 2 days or in beeswax for 5 days at 100 C. He stressed the importance of heavy inocula in thennal death studies and the use of media favolable to the germination of heated spores.