ANTIBACTERIAL PROPERTIES OF TANNIC-ACID AND RELATED-COMPOUNDS AGAINSTTHE FISH PATHOGEN CYTOPHAGA-COLUMNARIS

Tannic acid, gallic acid, and propyl gallate exhibited inhibitory acti vity as demonstrated by the agar dilution assay against Cytophaga colu mnaris is (= Bacillus columnaris, Chondrococcus columnaris, Flexibacte r columnaris, or Flavobacterium columnare), a ubiquitous gliding fish pathogen, at 150, 275, and 300 mu g/mL, respectively, in modified Shie h medium, at a low-bacterial-inoclum density of 10(3-4) colony-forming units/mL. Methyl gallate was not effective at the highest concentrati on tested (500 mu g/mL). The minimum inhibitory concentrations (MICs) of gallic acid, methyl gallate, and propyl gallate were lower in natur al pond water than in modified Shieh medium, whereas the MIC of tannic acid was the same in both. Tannic acid, a polymeric compound with mul tiple hydroxyl groups, had a greater capacity for binding protein and glycogen by at least nine times that of the other test compounds. The results suggest that the hydroxyl group availability of tannins is ess ential for antibacterial activity.