Aedes albopictus gut symbiotic bacterium Bacillus cereus improves its deltamethrin resistance

Abstract Background Aedes albopictus is a highly invasive vector for a variety of pathogens. The intensive use of insecticides has led to the widespread insecticide resistance in Ae. albopictus populations worldwide, compromising disease vector control efforts. We investigated whether the mosquito gut symbiotic bacterium Bacillus cereus reduces deltamethrin susceptibility in Ae. albopictus and elucidated the underlying mechanisms. Methods World Health Organization (WHO) standard tube bioassays were conducted to assess deltamethrin resistance status in both laboratory and field Ae. albopictus populations before and after oral infection with Bacillus cereus_HL4.2 (B. cereus_HL4.2). We measured enzymatic activities of three major detoxification enzyme families (cytochrome P450 monooxygenases, glutathione S-transferases [GSTs], and carboxylesterases) as metabolic markers. Transcriptomic profiling via RNA sequencing (RNA-seq) identified genes differentially expressed upon B. cereus infection, with subsequent validation by quantitative reverse-transcription PCR. In vitro assays assessed the direct deltamethrin-degrading capacity of B. cereus_HL4.2, and green fluorescent protein (GFP)-labeled bacterial strains tracked bacterial persistence and transmission through mosquito developmental stages. Results Oral infection with B. cereus_HL4.2 significantly increased the survival rate of laboratory-susceptible Ae. albopictus after deltamethrin exposure (from 7.6 ± 2.0% to 31.3 ± 4.3%) upon lethal insecticide exposure. B. cereus_HL4.2 infection elevated detoxification enzyme activities: cytochrome P450s increased 1.39-fold and GSTs increased 1.21-fold. Transcriptomic analysis revealed upregulation of genes related to the cAMP signaling pathway and purine metabolism following B. cereus_HL4.2 infection, while genes associated with ABC transporter and sensory signaling pathways were primarily downregulated. In vitro studies demonstrated that B. cereus_HL4.2 possesses direct deltamethrin-degrading capacity. GFP-tracking confirmed that B. cereus_HL4.2 colonizes the mosquito gut during larval development and persists through adult emergence. Conclusions Bacillus cereus_HL4.2 infection reduces deltamethrin susceptibility in Ae. albopictus primarily through two complementary mechanisms: (i) metabolic upregulation of detoxification enzymes and related genes, and (ii) direct enzymatic degradation of deltamethrin. Genetically modifying B. cereus_HL4.2 may offer a potential strategy for managing insecticide resistance in mosquitoes. Graphical Abstract