Complementary cooperation between two syntrophic bacteria in pesticide degradation.

Interactions between microbial species, including competition and mutualism, influence the abundance and distribution of the related species. For example, metabolic cooperation among multiple bacteria plays a major role in the maintenance of consortia. This study aims to clarify how two bacterial species coexist in a syntrophic association involving the degradation of the pesticide fenitrothion. To elucidate essential mechanisms for maintaining a syntrophic association, we employed a mathematical model based on an experimental study, because experiment cannot elucidate various conditions for two bacterial coexistence. We isolated fenitrothion-degrading Sphingomonas sp. TFEE and its metabolite of 3-methyl-4-nitrophenol (3M4N)-degrading Burkholderia sp. MN1 from a fenitrothion-treated soil microcosm. Neither bacterium can completely degrade fenitrothion alone, but they can utilize the second intermediate, methylhydroquinone (MHQ). Burkholderia sp. MN1 excretes a portion of MHQ during the degradation of 3M4N, from which Sphingomonas sp. TFEE carries out degradation to obtain carbon and energy. Based on experimental findings, we developed mathematical models that represent the syntrophic association involving the two bacteria. We found that the two bacteria are characterized by the mutualistic degradation of fenitrothion. Dynamics of two bacteria are determined by the degree of cooperation between two bacteria (i.e., supply of 3M4N by Sphingomonas sp. TFEE and excretion of MHQ by Burkholderia sp. MN1) and the initial population sizes. The syntrophic association mediates the coexistence of the two bacteria under the possibility of resource competition for MHQ, and robustly facilitates the maintenance of ecosystem function in terms of degrading xenobiotics. Thus, the mathematical analysis and numerical computations based on the experiment indicate the key mechanisms for coexistence of Sphingomonas sp. TFEE and Burkholderia sp. MN1 in syntrophic association involving fenitrothion degradation.