Evaluation of in-vitro antibacterial activity of methanol extract of the Sri Lankan endemic plant, Loxococcus rupicola (Thwaites) H.Wendl. & Drude

Abstract

Loxococcus rupicola (Thwaites), is an endemic palm species of Sri Lanka, traditionally valued for its medicinal properties; however, its antimicrobial potential remains largely unexplored. This study aimed to evaluate the phytochemical profile and antibacterial activity of methanolic leaf extracts of L. rupicola against standard susceptible and drug-resistant bacterial strains. Fresh, healthy leaves of L. rupicola were collected from the Ratnapura District, authenticated at the Bandaranaike Memorial Ayurveda Research Institute, shade-dried, powdered, and extracted with methanol using ultrasound-assisted extraction. Phytochemical screening for phenolic compounds, tannins, flavonoids, alkaloids, saponins, and glycosides was performed using standard qualitative tests. Antibacterial activity was assessed against Staphylococcus aureus ATCC 25923, methicillin-resistant S. aureus (MRSA) ATCC 43300, Escherichia coli ATCC 25922, Klebsiella pneumoniae ATCC 1706, ESBL-producing K. pneumoniae ATCC 700603, and carbapenemase-producing K. pneumoniae ATCC 1705 by the agar well diffusion method on Mueller-Hinton agar, following the Clinical and Laboratory Standards Institute guidelines where applicable. Extracts at 75%, 50%, and 25% (w/v) in DMSO were tested alongside positive and negative controls. Zone diameters were measured, and statistical analysis was performed using ANOVA and Student’s t-test (p ≤ 0.05). The extraction yielded 1.48 g of crude methanolic extract from 10 g of dried leaf powder (14.8% w/w). Phytochemical screening revealed the presence of phenolic compounds, tannins, flavonoids, alkaloids, and glycosides, while saponins were absent. The extract exhibited antibacterial activity against both susceptible and resistant Gram-positive strains, with the highest inhibition zone for S. aureus (20 mm) observed at 75%, and the highest inhibition zone for MRSA (21 mm) observed at 25%, showing significant differences among concentrations (p < 0.05). MRSA demonstrated greater susceptibility at lower extract concentrations (p < 0.01). The highest inhibition zone for E. coli (20 mm) was also observed at the highest concentration (75%). Limited activity was observed against K. pneumoniae ATCC 1706 (11 mm), and no inhibition was detected against ESBL producing or carbapenemase producing K. pneumoniae. Methanolic leaf extracts of L. rupicola contain bioactive compounds with selective antibacterial activity, effectively inhibiting both susceptible and resistant Gram-positive bacteria, particularly MRSA. The limited efficacy against multidrug-resistant Gram-negative bacteria may be due to cell wall structural barriers. These findings provide preliminary evidence that L. rupicola may serve as a natural source of antibacterial agents, warranting further studies on bioactive compound isolation, mechanisms of action, and potential synergistic applications.