Antibacterial Activity of Two Medicinal Mangroves plants against Selected Human Pathogens

Abstract

Mangrove ecosystems are rich reservoirs of structurally diverse secondary metabolites with potential antimicrobial applications. This study evaluated the in vitro antibacterial efficacy of  leaf and bark extracts of two true mangrove species, Excoecaria agallocha L. (Euphorbiaceae) and Rhizophora mucronata Lam. (Rhizophoraceae), collected from Valapattanam, Kannur, Kerala. Air-dried plant tissues were extracted sequentially using solvents of increasing polarity: chloroform, petroleum ether, ethyl acetate, and methanol. Antibacterial activity was assessed against four human pathogens (Staphylococcus aureus, Proteus mirabilis, Pseudomonas aeruginosa, and Klebsiella pneumoniae) using the agar well diffusion method. Across all tested organisms, polar solvents (methanol and ethyl acetate) yielded significantly larger inhibition zones compared to non-polar solvents (chloroform and petroleum ether), which exhibited baseline activity (8.0 to 9.8mm). Peak antibacterial potency against Gram-positive S. aureus was recorded for the ethyl acetate bark extract of E. agallocha ($30.3±0.57 mm), followed by the methanol bark extract of R. mucronata (27.3± 0.57 mm). Moreover, the methanol leaf extract of E. agallocha displayed broad-spectrum inhibition across all four bacterial strains. These results demonstrate that mid-to-high polarity constituents in E. agallocha and R. mucronata hold potential as natural therapeutic leads. Further investigation into bio-guided fractionation, compound isolation, and minimum inhibitory concentrations (MIC) is recommended.