Plasmid-associated sensitivity of Bacillus thuringiensis to UV light

Bacillus thuringiensis HD-73 Spores Have Surface-Localized Cry1Ac Toxin: Physiological and Pathogenic Consequences

Spores from Cry(sup+) strains of Bacillus thuringiensis bound fluorescein isothiocyanate-labeled antibodies specific for the 65-kDa activated Cry 1Ac toxin, whereas spores from Bacillus cereus and Cry(sup-) strains of B. thuringiensis did not. The Cry(sup+) spores could be activated for germination by alkaline conditions (pH 10.3), whereas Cry(sup-) spores could not. Once the surrounding exosporia had been removed or permeabilized, Cry(sup+) spores were able to bind the toxin receptor(s) from insect gut brush border membrane vesicle preparations, and their germination rates were increased ca. threefold in the presence of brush border membrane vesicles. A model is presented whereby in the soil the Cry toxins on the spore surface are protected by the exosporium while in the gut they are exposed and available for binding to the insect receptors. This model explains why the disulfide-rich C terminus of the cry genes is so highly conserved even though it is removed during the processing of the protoxin to the activated toxin. It also highlights the trade-off resulting from having Cry toxins located on the spore surface, i.e., decreased spore resistance versus enhanced insect pathogenesis.

Mosquitocidal toxins, genes and bacteria: the hit squad

Certain entomopathogenic species of bacilli and Clostridium produce one or more toxins that kill mosquito larvae even at concentrations in the picomolar range. Altogether, 19 distinct genes are known that encode mosquitocidal toxins, which vary in their potency, species specificity and mode of action. Unlike chemical insecticides, mosquitocidal bacilli used as larvicides are safe for animals and the environment, and do not affect non-pest insects. Mosquitocidal bacteria are effective to varying degrees against Culex, Anopheles and Aedes mosquito larvae, but their rapid sedimentation from the larval feeding zone, UV-light sensitivity and narrow host range have hampered their development. New genetic engineering approaches are being investigated that could overcome these limitations and allow stable expression of broad host range combinations of toxins in UV-resistant, buoyant recombinant bacteria, as discussed here by Alan Porter.

Method for purification of bacterial endospores from soils: UV resistance of natural Sonoran desert soil populations of Bacillus spp. with reference to B. subtilis strain 168

Endospores of Bacillus spp. were purified from three Sonoran desert soil samples by Chelex extraction and NaBr density gradient centrifugation and their UV resistances compared with that of B. subtilis strain 168. Natural spore populations exhibited tight adherence to soil particles which was not readily overcome by the extraction and purification procedure. It was observed that spores purified from soil exhibited 2-3 fold higher resistance to UV (as measured by the 90% lethal dose, LD90) than did B. subtilis strain 168 grown on NSM, a standard laboratory sporulation medium, and purified by the same extraction procedure. Cultivation of spore-forming bacteria isolated from soil on NSM resulted in production of spores with essentially identical UV resistance as strain 168, suggesting that spore UV resistance is influenced by the environment in which spores are produced.

ssp genes and spore osmotolerance in Bacillus thuringiensis israelensis and Bacillus sphaericus

It was shown previously that spores and vegetative cells of Bacillus sphaericus (Bf) and Bacillus thuringiensis israelensis (Bti) are very sensitive to osmotic variations. Since spore osmotolerance has been associated with their SASP (small acid soluble spore proteins) content coded by ssp genes, hybridization assays were performed with sspE and sspA genes from B. subtilis as probes and showed that Bti and Bf strains could lack an sspE-like gene. The B. subtilis sspE gene was then introduced into Bti 4Q2 strain; spores were obtained and showed a 65 to 650 times higher level of osmotolerance to NaCl, without affecting other important properties: hypoosmotic resistance in vegetative cells, spore UV resistance, and larvicidal activity against diptera larvae.