Isolating, Screening, and Optimizing Antioxidant Production of Endophytic Bacteria From Zingiber officinale Roscoe (Ginger).

Zingiber officinale Roscoe (ginger) is widely used as a herb in traditional cuisines and medicines. This study aimed to identify endophytic bacteria with high antioxidative potential in ginger plants and determine the optimum conditions for antioxidant fermentation from the strain. The bacteria from ginger were isolated and the growth conditions were surveyed. The secondary metabolites of the endophytes extracted using ethyl acetate were used to determine the total phenolic content (TPC), total flavonoid content (TFC), and antioxidant activities. Six bacterial strains were selected for identification using 16S rRNA gene sequencing. Kosakonia sp. ZO-Rh4 exhibited the highest activity, and the fermentation conditions were optimized. The results showed that 32 endophytic strains isolated from ginger tissues exhibited the significant activity. The strains ZO-Rh4, ZO-L2, ZO-S7, ZO-S8, ZO-L7, and ZO-L8, revealed the highest ones, belonged to the genera Kosakonia, Pantoea, Enterobacter, and Klebsiella. Under optimum fermentation conditions of 32.58 °C, 71.49 h, 14.45 g/L glucose, and a pH of 7.58, the Kosakonia sp. ZO-Rh4 produced a TPC of 285.74 mg gallic acid equivalents/mL and TFC of144.49 mg quercetin equivalents/mL. Furthermore, the antioxidant activity significantly increased, as evidenced by the EC values for the 2,2-diphenyl-1-picrylhydrazyl scavenging activity, total antioxidant capacity, and reducing power assays. These findings highlight the potential of endophytes, particularly Kosakonia sp. ZO-Rh4 is a natural antioxidant with various applications.