Heat and desiccation are the predominant factors affecting inactivation of Bacillus licheniformis and Bacillus thuringiensis spores during simulated composting

Aims: The suitability of composting for disposal of livestock mortalities due to Bacillus anthracis was assessed by measuring viability of surrogate spores from two strains each of Bacillus licheniformis and Bacillus thuringiensis after a heating cycle modelled on a cattle composting study. Methods and Results: Sporulation was attempted from 10 to 37 degrees C, but poor yields at lower temperatures resulted in 25, 30 and 37 degrees C being selected to generate sufficient spores (8 log(10) CFU ml(-1)) for experiments. Spores were inoculated into 3 g autoclaved dried-ground compost rehydrated with 6 ml water or silica beads in a factorial design for each strain, sporulation temperature, matrix and sampling day (0, 25, 50, 100, 150). Maximum incubation temperature was 62 degrees C, but spores were maintained at >= 55 degrees C for 78 of 150 days. Although significant differences existed among Bacillus strains and sporulation temperatures, numbers of viable spores after 150 days averaged 1.3 log(10) CFU g(-1), a 5.2 log(10) reduction from day 0. Conclusions: Spore inactivation was likely due to heat and desiccation as matrices were autoclaved prior to incubation, negating impacts of microflora. Significance and Impact of Study: Results support composting for disposal of anthrax mortalities, provided long-term thermophillic heating is achieved. Due to limited sporulation at 10 degrees C, livestock mortalities from anthrax at this or lower ambient temperatures would likely be of lower risk for disease transmission.