Evaluation of the antimicrobial activity of Plectranthus amboinicus leaf extracts on planktonic and biofilm forms of Staphylococcus aureus and Pseudomonas aeruginosa

Abstract

This investigation of the antimicrobial properties of Plectranthus amboinicus (Indian borage) leaf extracts was initiated due to the growing global concern of antimicrobial resistance (AMR) and the scarcity of antimicrobial drugs currently in development. Plectranthus amboinicus is known for its antimicrobial properties due to the phytochemical-rich essential oil contained within the glandular trichomes present on its leaves. The study presented focused on the phytochemical yield and bioactivity of leaf extracts from different stages of leaf development, namely the lag, log, and stationary phases of growth. These extracts were evaluated for their antimicrobial and antibiofilm activity on Pseudomonas aeruginosa PA01 (Gram-negative pathogen) and Staphylococcus aureus NCTC8325 (Gram-positive pathogen). Extracts from the log phase exhibited the highest concentration of bioactive compounds, with the lowest minimum inhibitory concentration (25mg/ml). Stationary phase extracts were observed to produce the highest level of biofilm dispersal activity against P. aeruginosa (80%), while log phase extracts were shown to effectively disperse S. aureus (59%) biofilms. It is of interest to note that the impact of the extracts was more pronounced when applied to treat P. aeruginosa biofilms compared to those of S. aureus. The effect of the leaf extracts on bacterial virulence factors was also investigated. The extracts were shown to reduce alginate production, catalase activity, bacterial motility and induce the production of reactive oxygen species (ROS) and lipid peroxidation. Moreover, the leaf extracts were observed to enhance membrane permeability by 58% in P. aeruginosa and 83% in S. aureus, while also decreasing efflux pump activity by 25.5% and 24.2%, respectively. In addition, the extracts were observed to alter cell surface hydrophobicity in both pathogens. Furthermore, as the concentration of Indian Borage extracts increased, the cell viability of HeLa cells decreased, indicating a concentration-dependent cytotoxic effect. This study was the first to explore the influence of P. amboinicus on bacterial antioxidant systems and cell membranes, paving the way for the development of resistance-modifying agents derived from this plant.