Valorisation of Citrus sinensis wastes: antibacterial potential of peel and seed ethanolic extracts against Staphylococcus aureus and Escherichia coli

Abstract Background Bacterial infections caused by Staphylococcus aureus and Escherichia coli remain difficult to treat with conventional antibiotics, renewing interest in antibacterial agents derived from plants. The peels and seeds of sweet orange ( Citrus sinensis ) are abundant agro-processing residues that are rich in bioactive compounds but are usually discarded. This study compared the in vitro antibacterial activity of ethanolic peel and seed extracts of C. sinensis against a Gram-positive and a Gram-negative organism. Methods Dried peels (189.1 g) and seeds (41.5 g) were extracted by ethanolic maceration in a fixed solvent volume, and the crude extracts were reconstituted in 2% dimethyl sulphoxide (DMSO) to give concentrations of 20, 40, 60 and 80% w/v (200–800 mg/mL). Antibacterial activity against S. aureus (ATCC 43300, methicillin-resistant) and E. coli (ATCC 25922) was determined in triplicate by the agar well diffusion method, with gentamicin and DMSO-matched sterile distilled water as positive and negative controls. Zones of inhibition were analysed by one-way and two-way analysis of variance (ANOVA) and independent-samples t-tests. Results Both extracts inhibited the two organisms in a concentration-dependent manner. The largest zones were produced by the 80% peel extract against S. aureus (12.33 ± 0.58 mm) and E. coli (12.33 ± 1.53 mm). One-way ANOVA confirmed a significant effect of concentration for every extract-organism combination ( p < 0.05). The peel produced numerically larger zones than the seed at most concentrations, but because the two extracts were prepared at different solvent-to-material ratios, this difference did not reach statistical significance ( S. aureus , p = 0.091, Cohen’s d = 0.85; E. coli , p = 0.840) and is reported as exploratory rather than confirmatory. S. aureus was generally more susceptible than E. coli . Conclusions Ethanolic peel and seed extracts of C. sinensis showed concentration-dependent antibacterial activity against both test organisms. The findings support the potential of sweet orange processing residues as a low-cost source of antibacterial compounds, while indicating the need for a solvent-to-material-ratio-matched comparison of peel and seed, determination of minimum inhibitory and bactericidal concentrations, and phytochemical characterisation in future work.