Nutritional Value and Antimicrobial Activity of Selected Edible Tubers of Namibia

Abstract

Indigenous, leafy vegetables have been extensively studied but edible, tuberous vegetables are relatively underexplored in terms of their nutritional and medicinal values. This study aimed to evaluate the macronutrient, total phenolic and flavonoid contents as well as the antioxidant, cytotoxicity, antimicrobial and antibiofilm activities of six edible tubers from Namibia.   Proximate analysis revealed that the tubers have a high total carbohydrate and energy content, with the highest values recorded for Walleria nutans. The high protein content of 13 g/100 g dry mass recorded for Coccinea rehmannii, supports the use of members of this genus as nutritional protein supplements. The best, albeit poor, antimicrobial activity was recorded for the dichloromethane (DCM) extract of Eulophia hereroensis with an MIC of 2.5 mg/mL against Eschericia coli. Evaluation of the antibiofilm activity against Staphylococcus aureus showed that the DCM extract of Walleria nutans displayed the best biofilm formation inhibitory activity, whereas E. hereroensis exhibited the best biofilm eradication activity. A high total phenolic content and strong antioxidant activity was recorded for the aqueous methanolic extract of E. hereroensis, with IC₅₀s of 0.056 ±0.000 and 0.041 ±0.003 mg/mL obtained with the DPPH and H₂O₂ scavenging assays, respectively. All tubers showed little to no cytotoxicity with CC₅₀ > 200 µg/mL against the VERO cell line. The macronutrient content, in vitro antioxidant and antimicrobial activities of the selected tuberous vegetables are reported here for the first time.  Eulophia hereroensis and W. nutans showed potential as functional foods and merit an in-depth phytochemical study to isolate and characterize the phenolic compounds and potential antibiofilm agents.