Study of the Inhibitory Effect of Myrtle (Myrtus communis L.) Leaf Powder Extracts on Escherichia coli and Pseudomonas aeruginosa

• Alaa Rafea jamal yaqub

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With the emergence of antimicrobial resistance of Gram-negative pathogens worldwide, the need for plant-derived antibacterials has increased. Myrtle (Myrtus communis L., Myrtaceae) is a shrub native of the Mediterranean area, with leaves reported as antimicrobial due to their high content in phenolics and terpenes. In this study, the inhibitory potential of M. communis leaves extracts on two Gram-negative pathogens namely E. coli and P. aeruginosa which are of clinical and food importance was evaluated in vitro. Three extraction methods namely cold aqueous maceration, hot aqueous extraction and 70% ethanolic extraction were used to extract air-dried powdered leaves and the extraction yields were determined gravimetrically (13.4, 19.2 and 20.6% w/w, respectively). Qualitative phytochemical screening revealed the presence of tannins, phenolics, flavonoids and glycosides in all extracts with terpenoids and resins being enriched in ethanolic fraction. The agar well diffusion method was used to evaluate each extract at five concentrations (1.2, 2.5, 5, 12 and 24% w/v) against both organisms in three independent replicates (a 3 × 5 × 2 factorial arrangement; 90 extract determinations in total), and the data were analysed by factorial ANOVA and by non-parametric comparison with the reference standards. Sterile distilled water was used for the negative control and gentamicin (10 µg) and ciprofloxacin (5 µg) as the positive reference standards. Extracts at all concentrations showed no zone of inhibition against either organism (6.0 ± 0.0 mm including the well; 0 mm net inhibition), whereas the reference antibiotics produced clear and reproducible zones (17.3-28.3 mm; P < 0.001 versus all extract treatments) confirming that the assay was sufficiently sensitive to detect inhibition. Because only a diffusion format was used, minimum inhibitory concentrations were not determined and low-level activity detectable by dilution methods cannot be excluded. The absence of detectable activity is most plausibly attributable to the Gram-negative outer-membrane permeability barrier, the limited agar diffusion of mainly non-polar constituents of myrtle and also by the known limitations of agar diffusion assays, and is consistent with previous studies on myrtle that reported no diffusion-detectable activity against E. coli and only weak, dilution-detectable activity against Pseudomonas.

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