Among the strategies aiming to reduce the use of pesticides, the increase in active ingredient efficiency by formulation design or pesticide mixture (each one being spread at a lower dose) is a well-established approach. Nevertheless, the effect of chemical mixture on the fate, behaviour and toxicity of each compound has not been widely studied. The impact of formulation and mixture of a herbicide, mesotrione, and a fungicide, tebuconazole, on their own biodegradation, in terms of kinetics and metabolic pathway, but also on microbial growth of several microbial strains degrading- or not was studied. A biodegradative pathway for mesotrione common to several Gram-positive and Gram-negative bacteria was proposed, leading under our conditions, to the accumulation of AMBA metabolite. The comparison with the formulation Callisto® showed a high effect of additives on the kinetics of mesotrione biodegradation, ranging from a strong inhibition for the Gram positive strains to stimulation for a strain of E. coli. For tebuconazole, only three bacteria were found capable of dissipating this fungicide and new metabolites were identified. The use of the formulation Balmora® lead to variable effects on tebuconazole biodegradation according to the strain and its concentration. Nevertheless, the additives always had an inhibitory effect on microbial growth, in particular on Gram positive bacteria. Studies on mixtures of both pesticides, pure or formulated, showed the complexity of the processes involved: variable effects but often increase of the pesticide biodegradation although the microorganism studied did or did not degrade them; a synergic toxic effect of pure or formulated pesticide mixture on microbial growth.
