Biosurfactants Induce Antimicrobial Peptide Production through the Activation ofTmSpatzles inTenebrio molitor

Biosurfactant immunomodulatory activities in mammals, nematodes, and plants have been investigated. However, the immune activation property of biosurfactants in insects has not been reported. Therefore, here, we studied the defense response triggered by lipopeptides (fengycin and iturin A), glycolipids (rhamnolipid), and cyclic polypeptides (bacitracin) in the coleopteran insect, mealwormTenebrio molitor. The in vitro antimicrobial activities against Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) bacteria and fungi (Candida albicans) were assessed by mixing these pathogens with the hemolymph of biosurfactant-immune-activated larvae.E. coligrowth was remarkably inhibited by this hemolymph. The antimicrobial peptide (AMP) induction results also revealed that all biosurfactants tested induced several AMPs, exclusively in hemocytes. The survivability analysis ofT. molitorlarvae challenged byE. coli(10(6)CFU/mu L) at 24 h post biosurfactant-immune activation showed that fengycin, iturin A, and rhamnopid significantly increased survivability againstE. coli. Biosurfactant-inducedTmSpatzlesactivation was also monitored, and the results showed thatTmSpz3andTmSpz-likewere upregulated in the hemocytes of iturin A-injected larvae, whileTmSpz4andTmSpz6were upregulated in the fat bodies of the fengycin-, iturin A-, and rhamnolipid-injected larvae. Overall, these results suggest that lipopeptide and glycolipid biosurfactants induce the expression of AMPs inT. molitorvia the activation of spatzle genes, thereby increasing the survivability ofT. molitoragainstE. coli.