Survival of Bacillus thuringiensis and Bacillus cereus spore inocula in soil: Effects of pH, moisture, nutrient availability and indigenous microorganisms

The influence of soil pH, moisture, nutrient availability and indigenous microorganisms on the survival and growth of Bacillus thuringiensis and B. cereus spore inocula in soil was investigated. The factor of greatest importance was nutrient availability. B. thuringiensis could not grow under most natural soil conditions, whilst B. cereus grew only slowly. Supplementing soil with additional nutrients, or autoclaving, stimulated growth of the Bacillus populations. Growth was reduced by a low soil pH (5.2 compared to 7.3), whilst both Bacillus species grew faster and survived better in wetter (at 0, −0.01 MPa) than drier (at −0.10, −1.00 MPa) soils. The death of B. thuringiensis in natural soil probably accounts for its rarity in the outdoor environment. It is suggested that this demise is attributable to a failure of B. thuringiensis to germinate in soil.