The emergence of multidrug-resistant (MDR) bacteria due to the overuse and misuse of antibiotics in the medical and agricultural sectors has now become a critical global healthcare issue. Antimicrobial peptides (AMPs) and synthetic mimics thereof have shown promise in combating MDR bacteria effectively, mainly because of their mechanism of action that disrupts bacteria cell membrane, consequently hindering resistance development in bacteria. However, these antimicrobials also exhibit toxicity to healthy mammalian cells at high dosage. To overcome this toxicity issue, the application of combination therapy alongside traditional antibiotics could enable the administration of these membrane-disrupting antimicrobials at lower dosage. Herein, this thesis investigates the synergetic effects of new tri-systems for combination therapy against Gram-negative bacteria which contain: i) an AMP (colistin methanesulfonate); ii) an antimicrobial polymer as AMP mimic and; iii) commercial antibiotics. It was found that colistin and the antimicrobial polymer could combine synergistically with any of the three antibiotics, doxycycline, rifampicin, and azithromycin, against wild type and MDR strains of Pseudomonas aeruginosa. Crucially, given the lower dosage of antimicrobial polymer used in these combination systems, the therapeutic index (also known as selectivity index), which is an indicator of an antimicrobial system to preferentially target bacteria over mammalian cells, is higher than the standalone agents. Furthermore, in this thesis, other selected antimicrobial polymers that are active toward mycobacteria instead of Gram-negative were also investigated as potential adjuvants or synergists to potentiate the antimicrobial activity of antibiotics against Mycobacterium smegmatis via a two-component system. Among the different families of antibiotics screened, it was found that these polymers only act as adjuvants of aminoglycosides. Overall, this thesis yields valuable new insights on combination therapy that will be useful toward combating MDR bacteria in clinical settings.