Author: Janine Bolo
Bolo, Janine, 2025 Quaternary Ammonium Compound as Antibiotic Potentiators, Flinders University, College of Medicine and Public Health
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Antibiotics are still the most important and powerful tool in healthcare system to combat infection from pathogens. However, the rising emergence of antimicrobial resistance (AMR) is concerning and has become a serious health care system threat, resulting to the declining efficacy of antibiotics, therefore causing deaths. One possible approach to addressing the problem is to reinvigorate the activity of existing antibiotics. Quaternary ammonium compounds (QACs) are organic cationic compound with positively charge ammonium atom head that is hydrophilic in nature, binding to the negatively charged surface of bacteria via coulomb’s law, attached to a hydrophobic alkyl chain. They are known to have broad-spectrum antimicrobial properties with less cytotoxic effects with low concentration. It is widely used as disinfectant to hospitals and other establishments and have also been used as antiseptic solution. However, despite the antimicrobial property of QAC, previous studies have not been through profound investigation of the potential of QAC to improve the activity of antibiotics.
This study aims to improve the antimicrobial activity of the existing conventional antibiotics by permeabilizing the bacterial cell membrane through QACs, hence, promoting the influx of the antibiotics. To test this, we measure the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of the QACs; cetyltrimethylammonium bromide (CTAB) and didecyldimethylammonium chloride (DDAC) and use the subinhibitory concentration to then combine with antibiotics against two clinically relevant strain of Gram-positive bacteria which is Staphylococcus aureus (S. aureus) and a Gram-negative bacteria which is Escherichia coli (E. coli) then measure the MIC and MBC value to see if there is any reduction as compared to the value by itself. The antibiotics that were used for this study are the following: Cefazolin; Colistin; Daptomycin; Gentamicin; Kanamycin; Levofloxacin; Tetracycline; Vancomycin. The choice of the antibiotics was based on their different mode of action against Gram-positive and Gram-negative pathogens.
We tested a wide range of combinations and observed that in some cases there was no change in antibiotic activity, while in other cases the activity was increased significantly, that indicates improved antibiotic activity. We investigated further to measure whether these interactions were synergistic in nature and reported 2 synergistic interactions such as kanamycin combined with CTAB against E. coli and tetracycline combined with CTAB against S. aureus. We further characterized the activity of those two synergistic interactions by looking at time-kill kinetics and resulted with a dramatically decrease in bacterial growth after 2 hrs in both combination and completely eradicated after 24 hours. Lastly, we aimed to elucidate the mechanism of enhanced antibiotic activity by evaluating the permeability triggered by QACs, and we did not report a clear trend between permeability and antibiotic activity, suggesting other factors are contributing to the effect. These results validated the combination approach, and the investigation justifies further research into how this can be applied in a biomedical application.
Keywords: Quaternary ammonium compounds, antibiotic potentiation, potentiator, CTAB, DDAC, MIC, MBC, Time kill kinetics, Checkerboard assay, Flow cytometry, S. aureus, E. coli
Subject: Biotechnology thesis
Thesis type: Masters
Completed: 2025
School: College of Medicine and Public Health
Supervisor: Dr. Andrew Hayles