| Summary: | The article investigates the expression of <i>oxyR</i> and <i>soxS</i> oxidative stress genes in <i>E. coli</i> under the effect of pristine multi-walled carbon nanotubes (MWCNTs) and pristine single-walled carbon nanotubes (SWCNTs), MWCNTs and SWCNTs functionalized with carboxyl groups (MWCNTs-COOH and SWCNTs-COOH, respectively), SWCNTs functionalized with amino groups (SWCNTs-NH<sub>2</sub>) and SWCNTs functionalized with octadecylamine (SWCNTs-ODA). Significant differences were found in the expression of the <i>soxS</i> gene, while no changes were observed in the expression level of the <i>oxyR</i> gene. The pro-oxidant effect of SWCNTs, SWCNTs-COOH, SWCNTs-NH<sub>2</sub>, and SWCNTs-ODA is presented, and the contrary antioxidant effect of pristine MWCNTs and MWCNTs-COOH in the presence of methyl viologen hydrate (paraquat) is shown. The article shows that SWCNTs-COOH, SWCNTs-NH<sub>2</sub>, and SWCNTs-ODA added to the medium generate the production of reactive oxygen species (ROS) in bacterial cells. SWCNTs-COOH intensified the <i>E. coli</i> biofilm formation, and the biofilm biomass exceeded the control by 25 times. Additionally, it is shown that the <i>rpoS</i> expression increased in response to MWCNTs-COOH and SWCNTs-COOH, and the effect of SWCNTs-COOH was more significant. SWCNTs-COOH and SWCNTs-NH<sub>2</sub> initiated an increase in ATP concentration in the planktonic cells and a decrease in the biofilm cells. The atomic force microscopy (AFM) method showed that the volume of <i>E. coli</i> planktonic cells after the exposure to carbon nanotubes (CNTs) decreased compared to that without exposure, mainly due to a decrease in cell height. The absence of a strong damaging effect of functionalized SWCNTs on <i>E. coli</i> K12 cells, both in suspension and in biofilms, is shown. Contact with functionalized SWCNTs initiated the aggregation of the polymeric substances of the biofilms; however, the cells did not lyse. Among the studied CNTs, SWCNTs-COOH caused an increased expression of the <i>soxS</i> and <i>rpoS</i>, the formation of ROS, and stimulation of the biofilm formation.
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