"It is generally believed that possible beneficial health effects of tea polyphenols are due to their anti-oxidant activity," wrote lead author Hui Cheng Lee from the National University of Singapore.
"Evidence from our study indicates that phenolics are likely to benefit the host by inhibiting pathogen growth and regulating commensal bacteria, including probiotics, and could therefore be considers as prebiotics."
The health benefits of tea ranging from a lower risk of certain cancers to weight loss, and protection against Alzheimer's, have been linked to the polyphenol content of the tea. Green tea contains between 30 and 40 per cent of water-extractable polyphenols, while black tea (green tea that has been oxidized by fermentation) contains between 3 and 10 per cent. Oolong tea is semi-fermented tea and is somewhere between green and black tea.
The four primary polyphenols found in fresh tealeaves are epigallocatechin gallate (EGCG), epigallocatechin, epicatechin gallate, and epicatechin.
The new study, published in the Elsevier journal Research in Microbiology, looked at the effects of 31 different phenolics extracted from Yunnan Chinese tea on the growth of 28 different bacteria, including pathogenic, commensal (normal), and probiotics found in the intestine. These included strains of the aerobic pathogens E. coli, Salmonella, and Listeria, and probiotic Lactobacillus strains, and strains of the anaerobic pathogens Bacteroides and Clostridium, and probiotic Bifidobacterium strains.
Cells were cultured in the presence of 0.1 per cent polyphenols at 37 degrees Celsius for 24 hours.
"Our data demonstrate that phenolic compounds have general inhibitory effects on intestinal bacteria. The level of inhibition varies depending on the bacterial species and chemical structure of the compound," wrote Lee.
Indeed, growth of the pathogenic E. coli and Salmonella typhimurium were most strongly inhibited by the tea polyphenols and their metabolites, as were strains belonging to the Bacteroides and Clostridium genera.
However, the researchers report that the growth of the probiotic Bifidobacterium and Lactobacillus strains were less affected by the tea compounds.
"Since probiotic growth was relatively unaffected by most of the aromatic compounds tested, probiotic colonization in the intestine should continue in the presence of phenolics so as to improve the intestinal microbial balance and inhibit pathogen growth," said the researchers.
"Although not fully comprehensive, this in vitro study indicates a substantial number of complex interactions between intestinal bacteria, phenolics and their metabolism," they said.
They called for more research to further investigate the influence of the tea polyphenols on gut microflora, and the overall maintenance of human health and disease prevention, and said that the research suggests that the antioxidants may also be prebiotic.
Prebiotic ingredients, or those that boost the growth of beneficial probiotic bacteria in the gut, are worth about €90 million in the European marketplace but are forecast to reach €179.7 million by 2010, according to Frost & Sullivan.
The market has been largely created by three inulin producers, all based in Europe, but other ingredient manufacturers are increasingly looking to promote the prebiotic effect of their products as evidence suggests that prebiotics could be even more useful than the probiotic bacteria that they feed.
Source: Research in Microbiology 2006, Volume 157, Pages 876-884 "Effect of tea phenolics and their aromatic fecal bacterial metabolites on intestinal microbiota" Authors: H.C. Lee, A.M. Jenner, C.S. Low, Y. K. Lee