Activation and germination of Bacillus thuringiensis spores in Manduca sexta larval gut fluid

Incubation of Bacillus thuringiensis HD-1 spores in the larval gut fluid of Manduca sexta (tobacco hornworm) resulted in increased viable counts, conversion to phase-dark spores, and a loss of absorbance in spore suspensions, indicative of spore germination. Heat-activated and untreated spores incubated in water did not exhibit these changes. Only when spores were heat activated and incubated in germinants L-alanine and adenosine did changes in the spores approximate those observed in gut fluid. These data suggest that M. sexta larval gut fluid induces the activation and germination of B. thuringiensis spores.