In Vitro Antibacterial Activity of Theaflavin against Ralstonia solanacearum

Abstract

Ralstonia solanacearum, a soil-borne bacterium, is responsible for bacterial wilt in a variety of solanaceous crops, leading to significant yield losses. Using antibacterial agents to control bacterial wilt is expensive and may produce antimicrobial resistance. Theaflavin, a polyphenolic compound obtained from tea (Camellia sinensis), has shown antimicrobial effects previously. Hence, this study was carried out to evaluate the activity of theaflavin against R. solanacearum. A 100mg/mL stock solution was prepared using theaflavin powder and sterile distilled water as the solvent. A series of dilutions was prepared using the above to obtain the concentrations: 50mg/mL, 25mg/mL, 12.5mg/mL, and 6.25mg/mL. Tetracycline was used as the positive control, and sterile distilled water as the negative control. R. solanacearum was cultured on Muller-Hinton agar overnight, and a fresh bacterial suspension (McFarland 0.2) was prepared in Phosphate Buffer Solution. A well diffusion assay was performed on Muller-Hinton agar for the aforementioned concentrations against R. solanacearum. Antibacterial effect of theaflavin was measured by deducting the Diameter of the Inhibition zone by sterile distilled water and from the diameter of the inhibition zone due to theaflavin/ tetracyclin. All theaflavin concentrations tested inhibited R. solanacearum. Along with the reducing concentration gradient, the size of the inhibition zone decreased. The ZOIs were 11.50±0.7mm for 100mg/ml, 9.13±0.6mm for 50mg/mL, 7.50 ±0.6mm for 25mg/mL, 5.38 ±0.3mm for 12.5mg/mL and 3.75±0.5mm for 6.25mg/mL. ZOI for tetracycline was 16.75±0.5. These findings indicate that theaflavin has a concentration-dependent antibacterial effect against R. solanacearum, suggesting that theaflavin may be an alternative to control bacterial wilt.