Investigation of antibacterial activity of plant infusions against pathogenic bacteria

Abstract

The rapid global rise in antimicrobial resistance (AMR) among pathogenic microorganisms necessitates the exploration of alternative therapeutic strategies. Medicinal plants, widely used in traditional medicine, represent a promising source of antimicrobial agents and antibiotic adjuvants. This study investigated the antibacterial, antibiofilm, synergistic, toxicological, and mechanistic properties of medicinal plant infusions as potential alternatives or adjuncts to conventional antibiotics. Twenty-two plant infusions were selected based on ethnomedicinal relevance and scientific evidence and evaluated against Salmonella enterica (NCTC 13349), methicillin-resistant Staphylococcus aureus (MRSA; NCTC 12493), and two Escherichia coli strains (NCTC 13352 and NCTC 11560). Antibacterial activity was determined using broth microdilution assays to establish minimum inhibitory and bactericidal concentrations (MIC and MBC). The infusions exhibited antibacterial activity with MIC values ranging from 0.091 to 0.625 mg/mL, compared with ciprofloxacin (0.0005 mg/mL). These findings confirmed the antibacterial potential of the selected plants and expanded current knowledge regarding their activity against clinically relevant pathogens. The antibiofilm activity of six candidate infusions was evaluated. All infusions effectively inhibited early biofilm formation; however, mature biofilms were generally resistant. Notably, infusions of Smilax china, Bombax speciosa, Rheum palmatum, and Paeonia suffruticosa disrupted established MRSA biofilms, highlighting their potential in preventing biofilm-associated infections. Synergistic interactions between eleven plant infusions and antibiotics were assessed using the checkerboard method. Strong synergy was observed with Sanguisorba officinalis, Arctium lappa, and P. suffruticosa against S. enterica, and with multiple infusions against E. coli and MRSA, resulting in 128–512-fold reductions in antibiotic MICs. These combinations demonstrated that plant infusions can resensitise resistant bacteria and enhance antibiotic efficacy. Safety evaluation using hemolysis assays and the Galleria mellonella model confirmed low toxicity, with 100% larval survival at 32MBC. Mechanistic studies revealed membrane disruption as the primary antibacterial mode of action, supported by biochemical assays and transmission electron microscopy. Overall, this study provides strong evidence supporting medicinal plant infusions as safe and effective antibacterial agents and antibiotic adjuvants for addressing antimicrobial resistance.

Thesis is embargoed until 31 July 2027.

Date of Award	Jul 2026
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	The Tertiary Education Trust Fund
Supervisor	Chen Situ (Supervisor) & Seamus Fanning (Supervisor)