Effects of Plectranthus barbatus leaf extract on survival, digestive proteases, midgut morphophysiology and gut microbiota homeostasis of Aedes aegypti larvae

Abstract

Aedes aegypti can transmit dengue, chikungunya, zika and yellow fever. Its control represents the main strategy for reducing the spreading of these diseases. In the present work, Plectranthus barbatus leaf saline extract (SE), decoction and infusion were studied for their effects on the survival of A. aegypti third instar larvae (L3), as well as for the presence of secondary metabolites, lectins, and trypsin inhibitors. The effects of SE (non-heated or heated) on digestive proteases, permeability of peritrophic membrane, morphophysiology, microbiota growth and melanogenesis of L3 are reported. SE killed L3 (LC50 of 0.48%, w/v), while decoction and infusion were not larvicides. HPLC analysis identified caffeic acid and flavonoids in SE, decoction, and infusion. SE also contained a ribose/galactose-binding lectin, trypsin inhibitor activities and polypeptides (34, 63 and 66 kDa) resistant to hydrolysis by L3 gut proteases. The incubation of SE with larval proteases did not alter lectin and trypsin inhibitor activities. Treatment of L3 with SE (at LC50) increased the permeability of peritrophic membrane and prevented the larvae from passing into the fourth instar. The morphology of larval midgut was not affected by SE, but the content of neutral polysaccharides in epithelial cells was reduced. SE stimulated in vitro the proliferation of L3 midgut microbiota and increased in vivo the production of melanin by larvae. The heating (100°C, 5 h) of SE increased lectin, trypsin inhibitor and larvicidal activities. In conclusion, SE larvicidal activity may be linked to the presence of lectin and trypsin inhibitor and involves the inhibition of digestive proteases, damage to larval midgut morphophysiology and induction of dysbiosis.