The incidence of mosquito-borne human diseases is increasing worldwide, with effective chemical control limited due to widespread insecticide resistance in the insect. Recent evidence also suggests that bacterial symbionts of mosquitoes, known to be essential in nutritional homeostasis and pathogen defense, may play a significant role in facilitating mosquitocide resistance. Here, I examined the metabolic detoxification and toxicity of two mosquitocides, propoxur and naled, and the capacity of bacterial symbionts to modify the detoxification of the mosquitocides and, thus, alter their toxic action in the yellow fever mosquito, Aedes aegypti. The insecticide synergists piperonyl butoxide (PBO), triphenyl phosphate (TPP), and S,S,S-tributyl phosphorotrithioate (DEF) were used to examine the metabolic detoxification and toxic action of the two mosquitocides in mosquito larvae. A significant increase in the toxicity of propoxur was observed when applied in combination with PBO; however, there was no corresponding decrease in AChE activity. Naled applied in combination with PBO resulted in a decrease in anticholinesterase activity (higher residual AChE activity) and a subsequent decrease in toxicity of the insecticide. This suggests that esterases play a major role in the metabolic detoxification of both insecticides in mosquito larvae. The acute toxicities of naled and propoxur to Ae. aegypti larvae were also studied following a reduction of bacterial symbionts with the broad-spectrum antibiotics gentamycin, penicillin, and streptomycin. Antibiotic-treated mosquito larvae showed increased susceptibility and a reduction in cytochrome P450 monooxygenase and general esterase activities when treated with naled and propoxur. A reduction of bacteria in mosquito larvae treated with broad-spectrum antibiotics, therefore, appears to affect the metabolic detoxification of standard-use mosquitocides, such as propoxur and naled. The results also suggest that the bacteria themselves may contain metabolic detoxification enzymes that are functionally similar to those in the mosquito larvae. Additional experiments, however, are needed to fully elucidate the contribution of bacterial symbionts in Ae. aegypti larvae in the metabolic detoxification of mosquitocides. / Master of Science in Life Sciences

Malaria continues to cause significant mortality in sub-Saharan Africa and elsewhere, and existing vector control measures are being threatened by growing resistance to pyrethroid insecticides.  With the goal of developing new human-safe, resistance-breaking insecticides we have explored several classes of acetylcholinesterase inhibitors.  In vitro assay studies have shown that trifluoromethyl ketones (TFK's) are potent inhibitors of An. gambiae AChE (AgAChE), that inhibit the enzyme by making a covalent adduct with the catalytic serine of the enzyme.  However research in the Carlier group has shown that trifluoromethyl ketones bearing benzene and pyrazole cores have shown very little toxicity to An. gambiae, perhaps due to hydration and rapid clearance. Focus was directed towards synthesis of oximes, oxime ethers, and hydrazones as potential prodrugs to prevent immediate hydration and reach the central nervous system.  The synthesis of various oximes, oxime ethers, and hydrazones has been shown to give cimpounds toxic to Anopheles gambiae within 3- to 4-fold of the toxicity of propoxur. However, thus far we have not been able to link the toxicity of these compounds to a cholinergic mechanism.  Pre-incubation studies suggest that significant hydrolysis of these compounds to TFKs does not occur or 22 h at pH 7.7 or 5.5.   Future work will be directed towards TFKs that have better pharmacokinetic properties.  Work will also be directed at synthesis of oxime and hydrazone TFK isosteres to determine the mechanism of action of these compounds. / Master of Science