Concentration-Dependent Antimicrobial Activity of Solvent-Partitioned Rhizome Fractions of Nigerian Curcuma longa L. against Bacterial and Fungal Pathogens

Curcuma longa L. is a rhizome rich in constituents with antimicrobial properties. This study evaluated the concentration-dependent antimicrobial activity of solvent-partitioned rhizome fractions of a Nigerian cultivar against Staphylococcus aureus, Escherichia coli, Microsporum audouinii, Trichophyton tonsurans, and Candida albicans. A 70% methanol extract was partitioned into n-hexane, dichloromethane (DCM), ethyl acetate, n-butanol, and aqueous fractions. Antimicrobial activity was determined in triplicate using agar-well diffusion. Inhibition zones were expressed as mean ± SEM (n = 3), and concentration effects were analysed for each microorganism using one-way ANOVA at p < 0.05. All fractions exhibited concentration-dependent activity, although their spectra differed. At 100 mg/mL, n-hexane produced the greatest inhibition of E. coli (24.67 ± 0.33 mm) and S. aureus (24.00 ± 0.58 mm). DCM was inactive against S. aureus but inhibited M. audouinii (19.67 ± 0.33 mm) at 41.48 mg/mL. Ethyl acetate best inhibited T. tonsurans (19.33 ± 0.67 mm) at 20.46 mg/mL but was inactive against C. albicans. The n-butanol fraction showed broad activity, including inhibition of C. albicans (17.33 ± 0.33 mm) at 100 mg/mL, whereas the aqueous fraction was most active against E. coli (20.00 ± 0.58 mm) at 100 mg/mL. Concentration had a highly significant effect (p < 0.0001). Solvent partitioning yielded distinct antimicrobial profiles: n-hexane showed antibacterial activity, while n-butanol had the broadest spectrum and strongest anti-Candida effect. These findings support bioassay-guided isolation and mechanistic evaluation of active constituents.