Sistema de respostas não específicas de linhagens de Escherichia coli K-12 à toxicidade induzida pelos herbicidas paraquat, 2,4-D e atrazina

Abstract

Microorganisms are essential components in the maintenance of biogeochemical cycles and ecosystems. In agricultural environments, continuous use of herbicides to minimize the loss in productivity could damage growth inibition of micro-organisms. One cause of this inhibition is increased production of ROS (reactive oxygen species), such as superoxide radicals (O2 -) and hydroxyl (OH-) and hydrogen peroxide (H2O2), causing oxidative stress. The magnitude of this stress can be conditioned by the chemical group and mode of action of the herbicide. The cell responses against ROS involve an increase in the expression of enzymes such as superoxide dismutase and catalase, responsible for the dismutation of O2- and H2O2, respectively. The objective of this study was to evaluate the relationship between the effect of herbicides with different modes of action and answers of superoxide dismutase (SOD) isoenzymes in the bacterium Escherichia coli K-12 bacteria. For this, 2,4-D, paraquat and atrazine herbicides were used, and knockout strains of E. coli K-12 strains carrying mutations in the gene encoding Mn-SOD (sodA) and Fe-SOD (sodB). Herbicides were shown to be capable of promoting the imbalance of redox potential, increasing the production of H2O2 and malondialdehyde (MDA) above the rates observed in controls and differently between strains. Exposed to paraquat, which indices in vivo redox cycling, strains of E. coli wt and ΔsodB showed increased production of H2O2, and increase in the Mn-SOD activity, probably as a result from activation of the SoxR, which promotes the transcription of the gene sodA. In ΔsodA, the toxicity rates with paraquat were not higher than the control, indicating a possible regulation of the Fe-SOD expression by OxyR transcriptional factor. Our results indicated that deletion of genes encoding SOD enzymes originated patterns of antioxidative responses, according to the tested periods of time, regardless the herbicides. To ΔsodB, the damage was minor in time 9pm; but in ΔsodA, the damage was minor in time 5pm, demonstrating the important role of these genes in the defense against oxidative stress in different stages of growth. Cellular responses demonstrated despite the observed toxicity indices, the strains were able to grow at rates close to those verified in control, including an increase in the fitness value in ΔsodA, which indicates a wide plasticity of responses, and a potential for a quick fitting of E. coli K-12. Extending this hypothesis, considering an environment containing a toxic molecule, such as an herbicide, and a bacterium having a polymorphic system for SOD, if a mutation appears in a gene coding for isoforms, it may show an increase in cell viability and still maintain a functional antioxidative enzyme. We suggest that E. coli K-12, a strain created in a laboratory, and with probably low survivability in a natural environment, developed mechanisms of in vitro herbicide tolerance, even without previous selection. This phenotypic plasticity model might be found in other bacteria of agricultural land, with high turnover of cultures and intense use of herbicides, which could cause a considerable impact on the diversity and functionality of soil microbiota.