Biodegradation of malathion by Bacillus licheniformis strain ML-1

Malathion, a well-known organophosphate pesticide, has been used in agriculture over the last two decades for controlling pests of economically important crops. In the present study, a single bacterium, ML-1, was isolated by soil-enrichment technique and identified as Bacillus licheniformis on the basis of the 16S rRNA technique. The bacterium was grown in carbon-free minimal salt medium (MSM) and was found to be very efficient in utilizing malathion as the sole source of carbon. Biodegradation experiments were performed in MSM without carbon source to determine the malathion degradation by the selected strain, and the residues of malathion were determined quantitatively using HPLC techniques. Bacillus licheniformis showed very promising results and efficiently consumed malathion as the sole carbon source via malathion carboxylesterase (MCE), and about 78% malathion was degraded within 5 days. The carboxylesterase activity was determined by using crude extract while using malathion as substrate, and the residues were determined by HPLC. It has been found that the MCE hydrolyzed 87% malathion within 96 h of incubation. Characterization of crude MCE revealed that the enzyme is robust in nature in terms of organic solvents, as it was found to be stable in various concentrations of ethanol and acetonitrile. Similarly, and it can work in a wide pH and temperature range. The results of this study highlighted the potential of Bacillus licheniformis strain ML-1 as a biodegrader that can be used for the bioremediation of malathion-contaminated soil.

Malathion resistance conferred by a carboxylesterase in Anopheles culicifacies Giles (species B) (Diptera: Culicidae)

A strain of Anopheles culicifaciesGiles (species B) originally collected from Aurangabad, Maharashtra State, India, showed highly specific resistance to malathion. Homogenates of one-day-old adults metabolized 14C-labelled malathion at a level comparable with strains of A. stephensi Liston and A. arabiensis Patton known to possess the malathion carboxylesterase resistance mechanism and at a higher rate (6.4 and 7.8 times, respectively) than malathion-susceptible strains of these species. The major metabolite obtained was malathion dicarboxylic acid, a carboxylesterase product, which suggests that this may be the principal resistance mechanism in this strain. Levels of non-specific esterase activity in larvae and adults were similar to those shown by both the susceptible and resistant strains of A. stephensi, implying that no quantitative increase in esterases had occurred. Microtitre plate assays on individual mosquitoes provided no evidence for the presence of the insensitive acetylcholinesterase resistance mechanism.