An in-vitro study investigating current and novel antimicrobial agents as potential topical treatments for cutaneous infections caused by bacteria, fungi and Leishmania

Introduction Guanidino, amidoamine, and quaternary ammonium-based compounds are inherently cationic, finding broad applications in the personal care sector, including hair conditioners and skin creams. Additionally, these compounds serve crucial roles in combating pathogenic diseases as treatments, antiseptics, disinfectants and prophylactics. Their antimicrobial efficacy stems from their structure, characterised by a long hydrophobic hydrocarbon chain attached to a positively charged hydrophilic head, enabling them to disrupt cellular membranes. However, limited research exists on how variations in hydrocarbon chain length and saturation levels influence their antimicrobial activity. The potential of cationic compounds to act as antimicrobial agents for cutaneous infections caused by bacteria, fungi and Leishmania will be investigated in this study. Cationic compounds do not easily penetrate the skin barrier; therefore, preliminary investigations were conducted to explore encapsulation strategies and their impact on antimicrobial activity. Methods Cationic compounds underwent structure-activity relationship (SAR) analysis against several pathogens, including Staphylococcus aureus (S. aureus), Pseudomonas aeruginosa (P. aeruginosa), Candida albicans (C. albicans) and Leishmania mexicana (L. mexicana). The bacterial and fungal species are often associated with multi-drug resistance. L. mexicana, a causative agent of cutaneous Leishmaniasis, is linked to costly, toxic treatments and rising resistance. Amidopropyl dimethylamine (APD) and amidopropyl quaternary ammonium (APQ) compounds were synthesised with varying chain lengths and saturation levels and compared to store bought cationic compounds. The compounds were evaluated through minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), time-kill and effective concentration 50 (EC50) assays, investigating how structural modifications impacted activity against the varying membranes of each organism. Chlorhexidine and paromomycin were encapsulated in Lipodisq®, a specialised lipid nanoparticle. The efficacy of these compound-Lipodisq® formulations was assessed using MIC, MBC, time-kill, and EC50 assays. Results Polyhexamethylene biguanide (PHMB) is a widely used antiseptic and treatment agent. MIC/MBC and time-kill assays revealed no significant difference in activity between PHMB, polyamidopropyl biguanide (PAPB), and polyhexamethylene guanide (PHMG) (p > 0.05) for both bacterial species. However, at the 6-hour timepoint, PHMG showed significantly greater efficacy against C. albicans than PHMB and PAPB (p < 0.001). Myristamidopropyl dimethylamine (MAPD), a disinfectant in the contact lens solution Aldox®, was consistently outperformed by palmitamidopropyl dimethylamine (PAPD), elaidamidopropyl dimethylamine (EAPD), and oleamidopropyl dimethylamine (OAPD) across both bacterial and fungal species (p < 0.001). The introduction of a methyl group to amidopropyl dimethylamines formed permanently charged amidopropyl quaternary ammoniums, though these quaternary ammoniums did not show significantly improved activity against S. aureus and P. aeruginosa compared to their dimethylamine counterparts. However, they did display significantly greater activity against C. albicans (p < 0.001). Miltefosine is a current treatment for cutaneous Leishmaniasis, but many cationic compounds tested exhibited significantly better activity (p < 0.05) against L. mexicana amastigotes. Notably, alexidine and benzylhexadecyltrimethyl ammonium demonstrated EC50 values below 0.25 μg/ml, in contrast to miltefosine's EC50 of 6.5 μg/ml. The compounds displayed markedly different efficacy between the promastigote and extracellular amastigote forms of L. mexicana. Benzyl ammonium compounds were more effective against amastigotes, whereas polymeric biguanides performed better against promastigotes. Chlorhexidine nanoparticles demonstrated no improvement in activity against the bacterial and fungal species in MIC, MBC and time-kill experiments. Against L. mexicana promastigotes and amastigotes, both the chaperone and unloaded Lipodisq® demonstrated significant activity (p < 0.05) in comparison to the control. Summary This study is the first to identify several cationic compounds with superior in-vitro antibacterial and antifungal activity compared to currently used antiseptics and treatments. PHMG exhibited significantly greater efficacy than PHMB against C. albicans, while PAPD, EAPD, and OAPD demonstrated significantly better activity than the commonly used disinfectant MAPD against both bacterial and fungal species. These compounds show promise for further investigation as potential topical treatments for cutaneous diseases. Moreover, this is the first study to report the activity of these cationic compounds against L. mexicana. Guanidino, amidoamine, and quaternary ammonium compounds show potential for use in topical applications to treat cutaneous Leishmaniasis. Encapsulating these compounds in Lipodisq® technology demonstrates promise for topical delivery. However, further modification and study is needed to ensure consistent activity.

Citation

Ansah, M. (2025) An in-vitro study investigating current and novel antimicrobial agents as potential topical treatments for cutaneous infections caused by bacteria, fungi and Leishmania. University of Wolverhampton. https://wlv.openrepository.com/handle/2436/625991

Publisher

University of Wolverhampton

URI

https://wlv.openrepository.com/handle/2436/625991

Type

Thesis or dissertation

Language

en

Description

A thesis submitted in partial fulfilment of the requirements of the University of Wolverhampton for the degree of Doctor of Philosophy.

Collections

Theses and Dissertations