Inhibitory effects of black tea (Camellia sinensis) extracts on Streptococcus pyogenes: A comparison between black and green teas in vitro

Although there are reports in the literature on the antimicrobial effects of green tea, the effect of black tea, the most common beverage in Iran, on growth of bacteria, such as Streptococcus pyogenes, has not been studied extensively. This study was undertaken to determine the inhibitory effect of black tea on the growth of Streptococcus pyogenes in vitro and compare it with green tea.

Extracts from both green and black teas were prepared using the percolation method and dried. This was followed by assessment of the total anti-oxidant capacity (TAOC) of solutions of the dried extracts with different concentrations. The assessment was done by determining percentage inhibition of oxidation of ABTS and reduction of its radical by 1 mg/ml extracts of black and green teas at 37°C. Viability of the bacteria was determined by exposing them to different concentrations of the tea extracts in an aqueous medium and then transferring the cultures to a solid medium, followed by colony counting at certain intervals. In the next step, the growth inhibitory effects of black and green tea extracts were determined on the basis of interactions between the extracts with some of the antibiotics routinely used against Gram-positive by the disc diffusion method (11 times on different days).The data were analyzed using the Wilcoxon sign and Kruskal Walis tests.

The data showed that the antioxidant capacity of green tea extract is significantly higher than that of black tea extract (p<0.001). Conjugation of the standard antibiotics disk with 1.25 mg green tea extract had a synergistic effect on ampicilin in a dose-dependent manner, it had no effect on amoxicillin, but an inhibitory effect on cephalexin. Increasing the green tea extract dose to 2.5 mg caused the zone of inhibitory growth of cephalexin to return to the basal limit and that of amoxicillin even beyond that. In the case of black tea, the increased doses resulted in increases in the antibacterial effect of ampicilin in vitro, reaching statistical significance (P<0.001) at the 2.5mg dose. Although a 1.25 mg-dose of black tea extract inhibited the antibacterial effect of amoxicilin and cephalexin, the inhibitory effect decreased significantly (p<0.001) when the dose was increased to 2.5 mg; however, the zone of inhibition was still less than the basal limit.

It seems that the inhibitory effects of tea on bacterial growth is directly related to its TAOC. The mechanism involved is probably through production of hydrogen peroxide by the tea polyphenols acting as pro-oxidants. Our findings indicate the possibility of using proper amounts of tea or polyphenols as nutritional supplements as an adjunct nutritiona therapy in certain infections.