## 5 Combination of Antimicrobial Cationic Polymers with Antibiotics  
Although nowadays there are many efforts in developing new effective antibiotics, the number of such drugs that are available in the market is somewhat limited due to the rising costs for the development and approval of medications [156]. The derivatization of existing antibiotics and their combinations represent promising alternatives to shorten the time of expansion for new antimicrobial agents. Additionally, to further improve the antimicrobial activities of antibiotics, considerable research has been focused on complex or chemically attach these drugs to biocidal polymers through their functional groups. By this co-administration, it is possible to take advantage of antimicrobial properties of polymers in combination with the lethal action of antibiotics that involves the inhibition of DNA replication [157], among other mechanisms. The application of these dual systems results in preventing bacterial infection with a reduction of the dose of the drug which acquires relevance due to the antibiotic dissemination in the environment that could lead to the development of bacterial resistance. One of the first attempts to achieve this additive effect was reported by [PERSON] et al. who combined the bioadhesive properties of CS derivatives with the antibacterial activity of chlorheximide showing an increase in the antiplaque effect [158]. Concerning this chitin derivative, other studies have demonstrated that the combination of antimicrobial CS derivatives with different antibiotics such as tobramycin, clarithromycin and sulfamethoxazole result in a notable synergistic performance against _P. aeruginosa_[159; 160]. Likewise, CS, and other cationic polymers have been combined with antibiotics. For example, [PERSON] et al. [161] reported the copolymerization of a methacrylate monomer containing ciprofloxacin with a cationic monomer containing protonated primary amine and hydrophobic methyl acrylate to prepare a series of water-soluble amphiphilic statistical copolymers. The evaluation against E. coli showed that a low antibiotic content (4.1% mol) was sufficient to enhance the antimicrobial properties of the copolymer as confirmed by the decrease of MIC values from 40 to 10 \(\upmu\)g mL\({}^{-1}\). This group reported a similar synergic behavior with the incorporation of ciprofloxacin into the backbone of an amphiphilic copolymer based on QAS and butyl acrylate [162]. In both cases, the deactivation of bacteria was the result of the cell disruption caused by the cationic and hydrophobic moieties of the copolymer, plus the inhibition of the activity of the bacterial enzyme DNA gyrase induced by ciprofloxacin. In this context, cationic polyacrylamide has also been employed to potentiate the action of daptomycin against S. aureus biofilms, improving their antimicrobial performance due to the formation of electrostatic interactions between cationic polymers and antibiotics that increase drug accumulation in the bacterial cell [163]. Besides displaying antimicrobial activity, dendrimers can be considered as agents that enhance the therapeutic effectiveness of existing antibiotics due to their well-defined globular structure and their ability to attach drugs onto their surface. The attachment of antibiotics in the dendrimers surface presents some advantages as they may enhance drug solubility or increase their time of release as well as effectively helping to deliver antibiotics. Likewise, the modification of dendrimers with non-toxic derivatives such as maltose or PEG chains may decrease their toxicity to eukaryotic cells. In this regard, [PERSON]'s group released some contributions related to the conjugation of commonly used antibiotics such as nadifloxacin [164], ciprofloxacin [164] and amoxicillin [165] with modified poly(propylene imine) (PPI) dendrimers. In these studies, the simultaneous administration of both agents significantly displayed higher antimicrobial efficacy against _S. aureus_, _E. coli_ and _P.