Comparison of antimicrobial properties of two spices commonly consumed in Nigeria and effect of temperature on their antioxidant properties

The study compared the antimicrobial activities of essential oils (EOs) from Piper guineense (Uziza) and Xylopica aethiopica (Uda) using diffusion and dilution assays, and evaluated their antioxidant properties at different temperatures. Comparison of the antimicrobial activities was determined using well-in-agar diffusion, disc diffusion and broth dilution methods against Escherichia coli, Salmonella sp and Bacillus cereus as test isolates. The antioxidant activities at different temperatures (28, 50 and 70°C) were determined using 1,1- diphenyl-2-picrylhydrazyl (DPPH), ferric ion reducing antioxidant power (FRAP) and Fe2+ chelation methods. Their antimicrobial activities increased with increase in concentration. From the diffusion assays, B. cereus was more susceptible to the EOs than other isolates (19.5 and 18.0 mm). MIC values obtained from the broth dilution assay (8.0 – 16.0 mg/mL) were generally much lower than those obtained from disc diffusion and well-in-agar diffusion assays. Percent inhibition of isolates using broth dilution method ranged from 91.28 to 96.86%. Their antioxidant activities decreased with increase in temperature. P. guineense EO yielded higher DPPH scavenging activity (43.25, 46.64 and 33.36%, respectively) and Fe2+ chelating ability (25.60, 12.58 and 8.97 mg/g, respectively) at the different temperatures, while FRAP elicited the strongest antioxidant ability of the EOs (70.10, 37.51 and 26.55 mg/g, respectively). The EOs exhibited their strongest antioxidant abilities at 28°C.   Key words: Microbial isolates, antimicrobial susceptibility testing, antioxidant capacity, spices, essential oils.