Recycling of Bacillus sphaericus 2362 in mosquito larvae: a laboratory study

After ingestion by Culex pipiens and Anopheles stephensi fourth instar larvae, spores of Bacillus sphaericus strain 2362 rapidly germinated inside live mosquito midgut. Bacterial counts and electron microscopic observations on intoxicated larvae revealed that the number of viable spores rapidly decreased during the first 12 h, with a minimum between 12 and 24 h. In cadavers, the number of heat-resistant spores quickly increased between the first and second day post-feeding. After one week, the number of spores inside dead larvae reached approximately 20 times the number of ingested spores for both mosquito species (ca. 4 X 10(5) spores/larva). Ultrathin sections of recycled spores showed the presence of a crystalline inclusion identical to that initially present in spores before ingestion. Bioassay on C. pipiens fourth instar larvae showed a similar toxicity between in vivo recycled spores (LC50 = 1.1 +/- 0.3 X 10(5) spores/ml after 24-h exposure) and culture-medium-grown spores of B. sphaericus strain 2362 (LC50 = 1.7 +/- 0.4 X 10(5) spores/ml).