1 June 3-4, 2015 Istanbul (Turkey) EFFECTS OF MALATHION ON THE MICROBIAL FLORA IN INDIGENOUS ACTIVATED SLUDGE Djaber TAZDAÏT a,b* , Rym SALAH a,b , Nadia ABDI b , Hakim LOUNICI b and Nabil MAMERI b a Department of Biochemistry and Microbiology, Faculty of Biological and Agronomical Sciences, Mouloud Mammeri University of Tizi-Ouzou, P.O. Box 17 RP 15000 Hasnaoua, Tizi-Ouzou, Algeria. b Engineering and Environment Research Unit, Ecole Nationale Polytechnique, El- Harrach, Algeria. * Author to whom all correspondence should be addressed: Mr. Djaber TAZDAÏT (djabertazdait@mail.ummto.dz) Abstract- Malathion S-[1, 2- di (ethoxycarbonyl) ethyl] dimethyl phosphorothiolothionate; CAS No 121-75-5; C10 H19 O6 PS2] is one of the most widely used organophosphate insecticides throughout the world. It is commonly used to control mosquitos and a variety of insects that attack fruits and vegetables. Removal of this pesticide can be attained by physical- chemical and biological processes. Several studies have examined the degradation of malathion by microbes such as bacteria, but little information is available on its effects on bacteria and fungi in mixed cultures. Therefore, in this study, the identification and enumeration of dominant aerobic, heterotrophic flora of a mixed culture of indigenous activated sludge were achieved before and after the addition of malathion. The results showed that malathion affected the composition of the microbial flora of the activated sludge used. Indeed, the findings revealed that Pseudomonas aeruginosa, yeasts and molds counts increased after addition of malathion. On the other hand, total coliforms, faecal coliforms, Escherichia coli and faecal streptococci numbers were not affected as a consequence of the addition of the insecticide. This feature might, in the future, lead the way to predict biodegradation efficiencies of the pesticide in detoxification processes. Keywords: activated sludge; malathion; microbial flora; Pseudomonas aeruginosa.