Bacillus megaterium strains derived from water and soil exhibit differential responses to the herbicide mesotrione.

The intense use of herbicides for weed control in agriculture causes selection pressure on soil microbiota and water ecosystems, possibly resulting in changes to microbial processes, such as biogeochemical cycles. These xenobiotics may increase the production of reactive oxygen species and consequently affect the survival of microorganisms, which need to develop strategies to adapt to these conditions and maintain their ecological functionality. This study analyzed the adaptive responses of bacterial isolates belonging to the same species, originating from two different environments (water and soil), and subjected to selection pressure by herbicides. The effects of herbicide Callisto and its active ingredient, mesotrione, induced different adaptation strategies on the cellular, enzymatic, and structural systems of two Bacillus megaterium isolates obtained from these environments. The lipid saturation patterns observed may have affected membrane permeability in response to this herbicide. Moreover, this may have led to different levels of responses involving superoxide dismutase and catalase activities, and enzyme polymorphisms. Due to these response systems, the strain isolated from water exhibited higher growth rates than did the soil strain, in evaluations made in oligotrophic culture media, which would be more like that found in semi-pristine aquatic environments. The influence of the intracellular oxidizing environments, which changed the mode of degradation of mesotrione in our experimental model and produced different metabolites, can also be observed in soil and water at sites related to agriculture. Since the different metabolites may present different levels of toxicity, we suggest that this fact should be considered in studies on the fate of agrochemicals in different environments.

Metabolic Interference of sod gene mutations on catalase activity in Escherichia coli exposed to Gramoxone® (paraquat) herbicide.

Herbicides are continuously used to minimize the loss of crop productivity in agricultural environments. They can, however, cause damage by inhibiting the growth of microbiota via oxidative stress, due to the increased production of reactive oxygen species (ROS). Cellular responses to ROS involve the action of enzymes, including superoxide dismutase (SOD) and catalase (CAT). The objective of this study was to evaluate adaptive responses in Escherichia coli K-12 to paraquat, the active ingredient in the herbicide Gramoxone®. Mutant bacterial strains carrying deletions in genes encoding Mn-SOD (sodA) and Fe-SOD (sodB) were used and resulted in distinct levels of hydrogen peroxide production, interference in malondialdehyde, and viability. Mutations also resulted in different levels of interference with the activity of CAT isoenzymes and in the inactivation of Cu/Zn-SOD activity. These mutations may be responsible for metabolic differences among the evaluated strains, resulting in different patterns of antioxidative responses, depending on mutation background. While damage to the ΔsodB strain was minor at late log phase, the reverse was true at mid log phase for the ΔsodA strain. These results demonstrate the important role of these genes in defense against oxidative stress in different periods of growth. Furthermore, the lack of Cu/Zn-SOD activity in both mutant strains indicated that common metal cofactors likely interfere in SOD activity regulation. These results also indicate that E. coli K-12, a classical non-environmental strain, constitutes a model of phenotypic plasticity for adaptation to a redox-cycling herbicide through redundancy of different isoforms of SOD and CAT enzymes.

GST activity and membrane lipid saturation prevents mesotrione-induced cellular damage in Pantoea ananatis.

Callisto(®), containing the active ingredient mesotrione (2-[4-methylsulfonyl-2-nitrobenzoyl]1,3-cyclohenanedione), is a selective herbicide that controls weeds in corn crops and is a potential environmental contaminant. The objective of this work was to evaluate enzymatic and structural changes in Pantoea ananatis, a strain isolated from water, in response to exposure to this herbicide. Despite degradation of mesotrione, probably due a glutathione-S-transferase (GST) pathway in Pantoea ananatis, this herbicide induced oxidative stress by increasing hydrogen peroxide production. Thiol fragments, eventually produced after mesotrione degradation, could be involved in increased GST activity. Nevertheless, there was no peroxidation damage related to this production, as malondialdehyde (MDA) synthesis, which is due to lipid peroxidation, was highest in the controls, followed by the mesotrione- and Callisto(®)-treated cultures at log growth phase. Therefore, P. ananatis can tolerate and grow in the presence of the herbicide, probably due an efficient control of oxidative stress by a polymorphic catalase system. MDA rates depend on lipid saturation due to a pattern change to a higher level of saturation. These changes are likely related to the formation of GST-mesotrione conjugates and mesotrione degradation-specific metabolites and to the presence of cytotoxic adjuvants. These features may shift lipid membrane saturation, possibly providing a protective effect to bacteria through an increase in membrane impermeability. This response system in P. ananatis provides a novel model for bacterial herbicide tolerance and adaptation in the environment.