Determination of flavonoid content and antibacterial activity of crude ethanolic extract from calamansi (Citrus macrocarpa) peels against Staphylococcus aureus

The rising problem of antimicrobial resistance (AMR) has substantially decreased the effectiveness of conventional antibiotics, highlighting the urgent necessity to identify new antibacterial agents from natural sources. Staphylococcus aureus, including methicillin-resistant strains, is still a prominent cause of community-acquired and nosocomial infections, particularly in developing countries such as the Philippines. Calamansi (Citrus microcarpa), a fruit traditionally used in traditional medicine, contains many bioactive comopunds, including flavonoids and terpenoids, which have been proven to have antibacterial properties. This study seeks to test the antibacterial potential of a crude ethanolic extract produced from calamansi peels against S. aureus, and to investigate the impact of extraction process on antibacterial property. Fresh calamansi peels were collected, oven-dried, ground, and macerated with 80% ethanol prior to Soxhlet extraction. Phytochemical testing confirmed the presence of flavonoids. Disk diffusion assay was performed to assess its antibacterial activity at 20% w/v concentration. 1% DMSO was utilized as the negative control, and doxycycline (30 mcg/disc) as the positive control. The positive control significantly inhibited S. aureus (32.00 ± 5.80 mm), but the crude ethanolic extract did not exhibit any measurable inhibition. These data emphasize the need of improving extraction procedures to increase the stability and recovery of bioactive ingredients such as limonene. The study highlights the scientific importance of negative findings and lays the groundwork for future research using different solvents or essential oil-based extraction methods to better assess the antibacterial properties of calamansi.