mechanisms of tannins can be summarized as follows. (i) The astringent property of the tannin may induce complexation with enzymes or substrates. Many microbial enzymes in raw culture filtrates or in purified forms are inhibited when mixed with tannins. (ii) A tannin's toxicity may be related to its action on the membranes of the microorganisms. (iii) Complexation of metal ions by tannins may account for tannin toxicity.3 Tannic acid, but not gallic acid, was found to be inhibitory to the growth of intestinal bacteria such as Bacteroides fragilis, Clostridium perfringens, Escherichia coli and Enterobacter cloacae amongst others. Tannic acid has a much greater relative binding efficiency to iron than gallic acid. Tannic acid may work like a siderophore to chelate iron from the medium and make iron unavailable to microorganisms. Microorganisms growing under aerobic conditions need iron for a variety of functions, including reduction of the ribonucleotide precursor of DNA, formation of haem, and other essential purposes.17 Chung et al.17 reported that the inhibitory effect of tannic acid on the growth of intestinal bacteria may be caused by its strong iron-binding capacity. Chung et al.4 also reported that tannic acid inhibited the growth of all 15 of the bacteria tested, but gallic acid and ellagic acid did not inhibit any of them. They concluded that the ester linkage between gallic acid and glucose (to form tannic acid) was important to the antimicrobial potential of these compounds.4 Based on the serum iron concentration seen in the present study (see Results), we suggest that inhibition of the growth of S. aureus cells in the media with tannic acid was not due to a decrease in iron.