## 2 Co-Administration of Two Different Antimicrobial Polymers
### Nitric Oxide co-Administration Systems
For example, an interesting three-component antibacterial compound was reported by [PERSON] and coworkers [108], who investigated the synergy in the bactericidal potency of a CS-AgNPs composite in the presence of molecular iodine. Iodine is a well-documented broad-spectrum agent that inactivates bacteria by affecting functions of enzymes and cell proteins and altering the membrane structure [116]. It was demonstrated that the iodinated composite presented higher antimicrobial activity at a lower dose compared to the effective dose of the individual components. Figure 8 shows the proposed mechanism of action, where the positively charge CS chains attach to the negative cell wall, while the AgNPs dispersed in the matrix turn the cell wall porous and activate the formation of ROS. Furthermore, the AgNPs also produced iodine atoms from iodine molecules deposited on their surface. These iodine atoms induced an enhanced production of ROS, which causes further damage to the cell. Overall, CS, AgNPs and iodine (which, in turn, forms an iodine atom) work in tandem for superior antibacterial activity in comparison to either of the components, and at lower concentrations of each of the species in the composite than their individual potency. This composite had the advantage that the concentration of AgNPs could be minimized. Similarly, this group studied CS-CuNPs composite in the presence of molecular iodine which provided stability to NPs in the media [117]. The bactericidal activity of this composite was determined on _E. coli_, where the MIC was found to be 130.84 \(\upmu\)g mL\({}^{-1}\), which consisted of 127.62 \(\upmu\)g mL\({}^{-1}\) of CS-CuNP composite and 3.22 \(\upmu\)g mL\({}^{-1}\) of iodine. The high effectiveness of this biocidal composite was attributed to polycationic CS which interacts with the negatively charged cell envelope. Furthermore, CuNPs possibly became attached to sulfur groups contained in the proteins of the bacterial cell membrane. This interaction can cause permeability of the membrane leading to leakage of proteins and other intracellular constituents and the death of bacteria. Another investigation regarding ternary antimicrobial systems was performed by [PERSON] and coworkers who involved CS, AgNPs and some antibiotics [118]. Besides AgNPs, in the last decade, recent studies on ZnO/CS composites have demonstrated that the presence of ZnO NPs alone or in combination with other biocidal NPs remarkably enhances the antibacterial properties of CS [119]. For example, [PERSON] and coworkers prepared a novel CS/Ag/ZnO blend to prepare films where Ag and ZnO NPs with spherical and granular morphology were uniformly distributed into the CS polymer [120]. The test of antimicrobial activities on a broad range of bacteria, including _B. subtilis_, _E. coli_, _S. aureus_, _Penicillium_, _Aspergillus_ and yeast colonies showed that CS/Ag/ZnO films had higher antimicrobial activities than films derived from CS/Ag and CS/ZnO blends, indicating that the composite of Ag and ZnO enhanced the biocidal properties of CS. In the case of ZnO NPs, when these are under light irradiation, electron-hole pairs are generated. The holes (h+) react with OH- on the surface of NPs, creating hydroxyl radicals (OH-), superoxide anion (O\({}_{2}\)\({}^{-}\)) and perhydroxyl radicals (HO\({}_{2}\)-). These highly active free radicals damage the cells of microorganisms as a result of decomposition and ultimately destruction. With the presence of AgNPs, charge transfer was improved, reducing the chance of electron-hole pairs to recombine and promote the generations of perhydroxyl radicals and other active oxidizing materials. Therefore, the presence of Ag and ZnO significantly enhanced the antimicrobial ability of CS. Additional selected examples of CS-based antimicrobial systems working in combination with biocidal NPs as ZnO, AgNPs, etc., are summarized in Table 3.  
Figure 8: Schematic representation of the proposed mechanism of antibacterial activity of the iodinated CS-AgNPs composite.