# Carbon Nanotubes-Based Hydrogels for Bacterial Eradiation and Wound-Healing Applications
## 4 CNT-Based Antibacterial Applications
### Antibacterial Activity of Pristine CNT
In addition, they announced that bacterial survival duration after the direct connection with CNTs enhanced with the increasing length-to-diameter ratios with a linear coefficient >0.79 for all examined doses. Moreover, they reported that the shape, in addition to the size, of a particle can impact on the particle phagocytosis by macrophages. The nanoscale size, shape, specific surface area, chemical composition, and surface structure of CNTs are the essential factors influencing its toxicity. It has been established that the larger the surface area, the higher the contact area with microbes, increasing the antimicrobial interaction and enhancing the killing mechanism. The dispersed CNTs exhibited significantly more toxic effects on bacterial cells than the aggregates. The inactivation of _S. aureus_, _E. coli_, and _B. subtilis_ was increased from 30-50% to 90-100% by adding the functional groups to the surface to improve the dispersibility of the nanomaterials [102]. Additionally, the geometry of CNTs can also affect antimicrobial activity. It has been observed that SWCNTs are more practical to kill _E. coli_ compared to MWCNTs. According to the authors, SWCNTs have better penetration capability into cell walls than MWCNTs due to their smaller diameter and high surface area, which helps for better interaction with cell surface [103]. SWCNTs with sharp diameters can easily penetrate the bacterial cell wall than MWCNTs [88]. Bacteria generate more stress-related gene products with small diameter CNTs [103]. [PERSON] et al., performed a gene expression experiment that revealed the pathways involving membrane damaging, repairing, and lipid recycling, such as fatty acid beta-oxidation and glycolysis [103]. The fatty acid biosynthesis pathways were highly expressed in the presence of SWCNTs than MWCNTs exposure to bacteria. More lipids from the membrane were released to the media due to membrane damage/cell death in the presence of SWCNTs. In the fatty acid biosynthesis pathway, the upregulation of the synthesis of new lipid molecules occurred and was incorporated into the cells for cell growth or repair of the broken membrane. The high expression of glycolysis with SWCNTs indicates that the cells are required to produce more energy to survive the stress-related conditions. The by-products of Acetyl-CoA pathways are typically used to synthesize fatty acids to repair the membranes. Figure 4a represents a schematic summary of _E. coli_ K21 gene expression stress response under exposure of CNT. The generation of ROS such as O\({}^{-}_{2}\) and OH\({}^{-}\) with CNTs is also responsible for killing the bacteria [105,106]. The reactive species oxidize the fatty acids in the cell membrane and damage the cell permeability, adversely affecting the cell functions [105,107]. A schematic mechanism of antibacterial activity by ROS generation is presented in Figure 4b. The greater expression of superoxide dismutase and catalase was observed in _E. coli_ in  
Figure 4: (**a**) Scheme showing antibacterial activity through cell membrane rupture in E. coli K12 bacteria [103], (**b**) Schematic diagram of bacteria killing through ROS mechanism in presence of long and short CNT [104]. the presence of CNTs. The DNA damage pathways and repair systems were activated with CNTs. A similar gene expression pattern has been previously observed in _E. coli_ cells exposed to hydrogen peroxide and superoxide. Physical membrane damage and oxidative stress are the fundamental mechanisms for the toxicity of all carbon-based nanomaterials [88; 103; 107; 108]. Cell contact time with CNT also influences antimicrobial activity [109]. Enhanced toxicity was observed in Gram-positive and Gram-negative bacteria in the presence of SWCNTs with increasing the contact time from 1 to 3 h [110]. Significant antimicrobial effects were observed in some Gram-positive bacteria after long exposure times (36h). The antimicrobial effects of CNTs also depend on bacterial cell wall characteristics.