The green tea extract was shown to have an inhibitory effect on the growth of E. coli strains isolated from UTIs. The MIC results can be adjusted to reflect the EGC content, using a value of 18% EGC as the content in total green tea polyphenols (Vuong et al., 2011). That makes the adjusted results as follows: 40% of strains tested were susceptible at a concentration of EGC at ≤0.45 mg/ml (450 μg/ml); 36% susceptible at ≤0.54 mg/ml (540 μg/ml); 18% susceptible at ≤0.63 mg/ml (630 μg/ml); and 5% susceptible at ≤0.72 mg/ml (720 μg/ml). Since all of the strains tested (99%) were susceptible at a concentration of ≤0.72 mg/ml, this suggests that EGC might be a good inhibitor of bacterial growth. Table 2 shows the MIC data for total green tea extract and for the EGC component.
The data was collected by in vitro experiments, but the effect can be described using information that is known about the metabolism of EGC from green tea. The amount of green tea polyphenols that would be present in the urine, including the amount of EGC, will vary according to the origin of the tea. It has been found, for instance, that Japanese tea (an average of 15 teas) contains approximately 20 mg of EGC per gram of dry tea (Vuong et al., 2011). An average cup of Japanese green tea is made with one tablespoon of dry tea (instructions from package of tea) which is equivalent to approximately 7.5 g of dry tea (a package of 60 g of dry tea makes eight cups). That equates to approximately 150 mg of EGC per cup of Japanese green tea.
Urinary excretion after a single ingested dose of EGC has been shown to peak at 8 h after ingestion, with levels in the urine reaching to 3.0–5.0 mg (Yang et al., 1998). The amount of EGC excreted after ingestion of one cup of tea (as described above) should equal approximately 3.5 mg. Since the projected MICs for EGC are well below 3.5 mg, this suggests that even one cup of green tea could have an effect on a urinary tract pathogen, and drinking multiple cups over the course of a day could possibly provide a prolonged effect. Additional studies testing the in vivo effect of drinking green tea on UTIs could be useful for determining if the effects observed in this study have medical significance.
Studies have shown that concentrations of 500 μg of tea polyphenols can inhibit the growth of E. coli, and that concentrations of ≥5000 μg are considered bactericidal. This effect is believed to be due to the fact that tea polyphenols down regulate the production of proteins such as EF-2 (elongation factor for protein translation); proteins involved in phospholipid, carbon, and energy metabolism; and production of proteins involved in amino acid biosynthesis (Cho et al., 2007).
There was no correlation between MIC values in this study and the antimicrobial resistance of the isolates. Each level of MIC contained a variety of antimicrobial resistance patterns.
Another set of studies that would be interesting and potentially useful would be to test EGC and several of the standard antimicrobial agents used to treat UTIs to determine if there might be synergism between EGC and any of the antimicrobial agents. Studies have been done that show the ability of green tea to act synergistically with gentamicin and amikacin against E. coli (Neyestani et al., 2007).