Antibacterial, Bactericidal and Membrane-Disruptive Activities of Essential Oils from Callistemon salignus, Croton gratissimus and Melaleuca bracteata against Reference Enterococcus Species

Abstract

The increasing occurrence of antimicrobial-resistant Enterococcus species has encouraged the investigation of plant-derived products with antibacterial potential. This study evaluated the antibacterial, bactericidal, and membrane-disruptive activities of essential oils obtained from the leaves of Callistemon salignus, Croton gratissimus, and Melaleuca bracteata against five reference Enterococcus species: Enterococcus avium, Enterococcus casseliflavus, Enterococcus hirae, Enterococcus gallinarum, and Enterococcus faecalis. Antibacterial activity was assessed by agar disc diffusion, while minimum inhibitory concentration and minimum bactericidal concentration values were determined using broth microdilution-based methods. Bactericidal potential was further interpreted using MBC/MIC ratios, and membrane-disruptive activity was evaluated using membrane-damage indices. All tested essential oils produced measurable inhibition against at least some of the Enterococcus species, with inhibition zones ranging from 9.0 to 12.0 mm, compared with 25 to 47 mm for ciprofloxacin. Melaleuca bracteata essential oil showed the strongest overall activity, with MIC values ranging from 2.5 to 5.0 mg/mL and MBC values of 5.0 to >5.0 mg/mL. Callistemon salignus showed moderate activity, with MIC values from 2.5 to >5.0 mg/mL and MBC values of 5.0 to >5.0 mg/mL. Croton gratissimus showed comparatively weaker activity, with MIC values of 5.0 to >5.0 mg/mL and no detectable bactericidal activity within the tested concentration range. Membrane-damage indices were highest for M. bracteata against E. avium and E. faecalis. Overall, the findings indicate that M. bracteata and C. salignus possess measurable antibacterial effects against reference Enterococcus species, with membrane disruption likely contributing to their activity.