Purification and chemical characterisation of larvicidal compound (s) of extracts prepared from submerged cultures of basidiomycetes against Aedes aegypti

Abstract

The diseases transmitted by mosquito continue to be rampant and in most case fatal especially in the developing countries. Mosquito is the main vector for malaria. Malaria is a scourge and according to the World Health Organisation (WHO) estimations, over 40% of the world population remain exposed to malaria. In Kenya, about 25,000 children die of malaria each year; although the actual number of deaths is unknown since most of them occur at home. Since prevention is better than cure, there is an urgent and immediate demand by the society to manage the growing population of mosquito. The chemicals used in managing the mosquito are natural and synthetic. The “miracle” of chemical technology five decades ago has not, however, provided a viable solution. In addition to inefficacy, chemical pesticide can cause mutant strains of fauna and flora. They can also produce potent toxic chemicals to the human body. In this regard, there is need to search for natural larvicides, especially from the unexplored fungal genera, as they are known to produce biologically active compounds with great diversity. Natural compounds are known to be more selective and biodegradable. This study involved evaluation of two basidiomycetes JO4012 and JO4014 as sources of larvicides against mosquito Aedes aegypti. These two basidiomycetes were collected as fruiting bodies of mushrooms from within the precincts of Egerton University. They were selected after preliminary screening and isolated as pure cultures on solid media and then cultured in liquid media. Extraction of culture filtrate and mycelia were done using ethyl acetate and acetone solvents. Bioassay of the crude extract of the culture filtrate and mycelia of both species exhibited significant larvicidal activity. Higher concentrations of the crude extract were required to kill larvae within a shorter period of time. In this study, the mycelium crude extract from JO4012 had the highest larvicidal activity. Chromatographic techniques; thin layer chromatography (TLC) and column chromatography (CC) were used to purify the crude extracts. CC yielded fractions that were subjected to bioassay to verify the activity observed in the crude extract. Further purification of the active fractions was done using Sephadex LH20. A glycosidic moiety was proposed as part of the compound responsible for the larvicidal activity of the culture filtrate extract from JO4012. However, based on the NMR experiments, the aglycone moiety signals were too weak to be observed. This study needs to be pursued to address the problem of mosquito.