Ultrastructural Analysis of Spores and Parasporal Crystals Formed by Bacillus sphaericus 2297

Bacillus sphaericus 2297, growing from a boiled, relatively nontoxic spore inoculum, increased about 30-fold in toxicity for mosquito larvae during early exponential growth but showed an approximately 1,000-fold toxicity increase during the late-exponential phase, as spores began to appear in the culture. The development of spores in the bacterial cells was accompanied by the formation of parasporal crystals. These parasporal crystals appeared during stage III as the forespore septum engulfed the incipient forespore. The paraspores were separated from the forespores by a branch of the exosporium across the cell. Measurements of the parasporal substructure revealed a 6.3-nm distance between the striations. When spores and paraspores were fed to mosquito larvae and the larvae were fixed 15 min after feeding, it was found that the spores remained relatively unchanged but that the matrix of the paraspores was dissolved. After dissolution of the paraspore matrix, a meshlike envelope remained which retained the paraspore shape and which was often in contact with the cross-cell portion of the exosporium. The parasporal crystals may be a source of the mosquito larval toxin in this strain of B. sphaericus, but proof will require their isolation from other cellular components.