Abstract
Background. Black tea, the most popular drink in Iran, has several polyphenolic compounds with possible antibacterial effects. Although the antimicrobial properties of green tea have already been reported, the microbiological effects of black tea have been less widely investigated. In this study, the anti‐streptococcal effects of black tea extract were evaluated and compared with those of green tea.
Design. In vitro evaluation of antibacterial effects.
Methods. Both black and green tea extracts were analysed using high‐performance liquid chromatography to compare their major polyphenol profiles. Different concentrations of the extracts or gallic acid, the abundant phenolic compound found in black tea, were used for bacterial sensitivity tests in both pour plate and disc diffusion methods. Disc diffusion was then used to evaluate the interactions between the extracts and certain anti‐streptococcal antibiotics.
Results. Both black and green teas, at a concentration of 12.5 mg ml−1 after 7 hours and at 25 mg ml−1 after 3 hours, completely inhibited Streptococcus pyogenes growth. However, gallic acid at a concentration as low as 5 µg ml−1 after 5 hours and at 10 µg ml−1 after 2 hours inhibited streptococcal growth. Both black tea and green tea extracts were found to have either synergistic or antagonistic effects at different concentrations on the selected antibiotics, whereas gallic acid strengthened the antibacterial effects of all antibiotics in a dose‐dependent manner.
Conclusion. Both black tea and green tea extracts may have synergistic or antagonistic effects on certain anti‐streptococcal antibiotics. These effects are more prominent with black tea.