Antimicrobial activity of mangrove bark tannins and copper ions on marine bacteria

Abstract

A novel sealant or antifouling agent is desperately needed since numerous antifouling biocides, including tributyltin and diuron, have been banned due to their hazardous effects on the marine environment. Mixed-tannin extracted from Rhizophora apiculata was selected as the natural source to be combined with copper ions to form a copper-tannate (Cu-T) complex. The Cu-T complex was tested for antimicrobial properties via disc diffusion assay against a series of identified marine fouling bacteria. The zone of inhibition obtained from the effect of Cu-T on test bacteria ranged from 10.3 mm-21.0 mm. Meanwhile, the range for minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) varied from 0.25 mg/mL-1.00 mg/mL and 0.50 mg/mL-2.00 mg/mL, respectively. The Cu-T complex affected the growth of eight marine fouling bacteria, with bacteriostatic effect on Bacillus aquimaris IBRL FB13 and Vibrio alginolyticus IBRL FB6. Time kill assay which was performed at the extract concentrations of 1/2MIC, MIC and 2MIC revealed that the antibacterial activity of Cu-T complex was concentration-dependent. From the photomicrographic observation, the Cu-T complex deteriorated the cell walls of the Gram-positive and Gram-negative bacteria. This study highlights the potential of the Cu-T complex as a sealant for mitigating biofouling formation on aquaculture equipment.