The dlt operon in Bacillus thuringiensis confers resistance to cationic antimicrobial peptides and virulence to insect

Abstract:

[Objective] The dlt operon in Gram-positive bacteria encodes enzymes that are necessary for the modification of D-alanylation of teichoic acids in cell wall. D-alanylation generates net positive charge on cell surface and, as a consequence, repulses the positively charged molecules, such as cationic antimicrobial peptides, thereby confers resistance to host animal. Here, we investigated the impact of dlt operon on phenotypic traits of Bacillus thuringiensis and role in virulence to insect. [Methods] We constructed the loss-of-function mutant of dlt by homologous recombination technique, and performed its morphological observation, surface charge difference analysis, stress resistance analysis and cell experiment.[Results] The results revealed that inactivation of dltA significantly decreased net negative charge of cell wall, drastically impaired the resistance of Bacillus thuringiensis to cationic antimicrobial peptides (polymyxin B and lysozyme) and alkaline. ΔdltA_Bt mutant displayed an obviously altered profile of growth curve, irregular shape and rough surface of cell, decreased biofilm formation and increased swarming motility. Moreover, inactivation of dltA significantly decreased adhesion ability to mid-gut epithelial cell of insect, and greatly attenuated virulence to Bombyx mori. [Conclusion] These findings provide evidence that D-alanylation of TAs mediated by dlt operon is closely correlated to many phenotypic traits of Bt, and has putative roles in the pathogenicity of B. thuringiensis to insect and the protection of B. thuringiensis from insect humoral immunity.