Bacillus thuringiensis Spores and Vegetative Bacteria: Infection Capacity and Role of the Virulence Regulon PlcR Following Intrahaemocoel Injection of Galleria mellonella

Abstract

<i>Bacillus thuringiensis</i> is an invertebrate pathogen that produces insecticidal crystal toxins acting on the intestinal barrier. In the <i>Galleria mellonella</i> larvae infection model, toxins from the PlcR virulence regulon contribute to pathogenicity by the oral route. While <i>B. thuringiensis</i> 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 ∆<i>plcR</i> mutant are inoculated directly into <i>G. mellonella</i> 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 ∆<i>plcR</i> 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.