Investigation of β-Lactamase Enzyme Inhibitory, Anti-Bacterial and Anti-Fungal Bio-Activities of Dillenia retusa Thumb: An Endemic Medicinal Plant to Sri Lanka

Dillenia retusa (Dilleniacea) is a medicinal plant endemic to Sri Lanka with an immense bioactive potential. To date, no studies are reported on anti-microbial potential of this plant. The present study investigates anti-bacterial, b-lactamase enzyme inhibitory and anti-fungal bio-activities of the ethanol extracts of D. retusa.

The leaves (DRL), fruits (DRF) and bark (DRB) of D. retusa were extracted with ethanol using cold extraction technique. Anti-bacterial properties of the extracts were investigated against five sensitive bacterial strains namely, Shigella flexneri (ATCC®12022), Pseudomonas aeruginosa (ATCC® 10145), Escherichia coli (ATCC® 25922), Salmonella typhi (ATCC® 14028), Staphylococcus aureus (ATCC® 6538) and four resistant bacterial strains namely, Klebsiella pneumoniae (MDR ATCC® 700603), Escherichia coli (MDR ATCC® 35218), Salmonella enterica (MDR ATCC® 700408) and Staphylococcus aureus (ATCC® MDR MRSA 252) using alamar blue micro-plate based assay. Anti-b-lactamase activity of the extracts were determined using b-lactamase enzyme inhibitory assay. Anti-fungal activity of the extracts was determined against five sensitive fungal strains namely, Candida albicans (ATCC® 14053), Candida glabrata (ATCC® 2001), Aspergillus niger (ATCC® 1015), Microsporum canis (ATCC® 10214) and Trichophyton rubrum (ATCC ®MYA-4438)) using agar-tube dilution method. Further, polyphenols and flavonoid contents of the extracts were determined. The bio-activities were statistically analyzed at p<0.05.

The extracts showed anti-bacterial activity against sensitive bacterial strains within the MIC range of 1.8-2.8 mg/mL and MBC range of 3-4.8 mg/mL and were found to be bactericidal. The extract of DRL showed the highest anti-b-lactamase activity (IC50: 226.25 ± 10.53 µg/mL) in a dose-dependent manner. The antifungal activity of the extracts was detected within the range of 1.6-3.2 mg/mL and the highest anti-fungal activity was observed for DRB against C. albicans which may be attributed to the high polyphenol content of DRB.

The extracts were identified as novel potential sources of antimicrobial compounds which may impart even higher antimicrobial activities upon bio-activity guided fractionation and purification of extracts.