The effect of malathion on a bacterial population

An organophosphate insecticide, malathion (technical grade, 95%), which is used for mosquito fogging operations by the Harris County Mosquito Control (Houston, Texas), was found to be stimulatory or inhibitory to the bacterial population of Hermann Park Lake. The greatest stimulatory effect was observed with 0.5% and 1.0% concentration of malathion. Marked inhibition was observed with 5.0% concentration of malathion. The growth of Houston Isolates #1, #2, and #6 was stimulated by 0.5% and 1.0% concentrations of technical grade malathion (95%). Houston Isolate #10 was inhibited by 0.5 % and 1.0% concentrations. All pure culture isolates were inhibited by 5.0% concentration. Houston Isolates #1, #2, and #6 utilized malathion monoacid to support growth. Houston Isolate #11 utilized malathion and malathion monoacid to support its growth. The growth of Houston Isolate #11 was inhibited by malathion diacid. The effect of a commonly used pesticide on certain microorganisms was revealed in this investigation. The fact that bacteria utilized malathion and its products is strongly suggestive that these bacteria can degrade this pesticide.