Abstract
Bacillus thuringiensis is an invertebrate pathogen that produces insecticidal crystal toxins acting on the intestinal barrier. In the Galleria mellonella larvae infection model, toxins from the PlcR virulence regulon contribute to pathogenicity by the oral route. While B. thuringiensis is principally an oral pathogen, bacteria may also reach the insect haemocoel following injury of the cuticle. Here, we address the question of spore virulence as compared to vegetative cells when the wild-type Bt407cry- strain and its isogenic ∆plcR mutant are inoculated directly into G. mellonella haemocoel. Mortality dose-response curves were constructed at 25 and 37 °C using spores or vegetative cell inocula, and the 50% lethal dose (LD50) in all infection conditions was determined after 48 h of infection. Our findings show that (i) the LD50 is lower for spores than for vegetative cells for both strains, while the temperature has no significant influence, and (ii) the ∆plcR mutant is four to six times less virulent than the wild-type strain in all infection conditions. Our results suggest that the environmental resistant spores are the most infecting form in haemocoel and that the PlcR virulence regulon plays an important role in toxicity when reaching the haemocoel from the cuticle and not only following ingestion.
Highlights
Bacillus thuringiensis strains are among the most widely used larvicidal entomopathogen [1].B. thuringiensis is a member of the Bacillus cereus group, composed of several genetically closely relatedGram positive and sporulating bacterial species [2,3], including the human opportunistic pathogenB. cereus, which is involved in food intoxication or more severe systemic infections [4]
[13], we found that the PlcR regulon is required for full Bt407WT virulence when spores or vegetative we found that the PlcR regulon is required for full Bt407WT virulence when spores or vegetative cells cells are mechanically introduced into the haemocoel
Our study shows that environment-resistant spores of B. thuringiensis are the most efficient at infecting G. mellonella, even in the haemocoel
Summary
Bacillus thuringiensis strains are among the most widely used larvicidal entomopathogen [1].B. thuringiensis is a member of the Bacillus cereus group, composed of several genetically closely relatedGram positive and sporulating bacterial species [2,3], including the human opportunistic pathogenB. cereus, which is involved in food intoxication or more severe systemic infections [4]. Bacillus thuringiensis strains are among the most widely used larvicidal entomopathogen [1]. B. thuringiensis is a member of the Bacillus cereus group, composed of several genetically closely related. The major difference between B. cereus strains and B. thuringiensis is the presence of larvicidal crystal toxins in latter. Cry toxins are produced as inclusion bodies (“crystals”) during the bacterial sporulation phase. Insecticidal action is initiated after ingestion of inclusion bodies by a susceptible larva, in which they are solubilized by the alkaline pH of the midgut, resulting in release of the protoxin, which is processed by digestive enzymes to its active form. The activated toxin binds to several receptors on the gut epithelial surface resulting in pore formation, gut paralysis, decreased food uptake, and final death of the larva (for review [5,6])
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