Mango kernels contain gallotannins which exhibit antimicrobial actitivity. - GreenMedInfo Summary

This study investigated the antimicrobial activity and mode of action of penta-, hexa-, hepta-, octa-, nona-, and deca-O-galloylglucose (gallotannins) isolated from mango kernels. The minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) against foodborne bacteria and fungi were determined using a critical dilution assay. Gram-positive bacteria were generally more susceptible to gallotannins than Gram-negative. The MIC of gallotannins against Bacillus subtilis, B. cereus, Clostridium botulinum, Campylobacter jejuni, Listeria monocytogenes and Staphylococcus aureus was 0.2 g L(-1) or less; enterotoxigenic E. coli and Salmonella enterica were inhibited by 0.5 - 1 g L(-1) and lactic acid bacteria were resistant. The use of lipopolysaccharide mutants of S. enterica indicated that the outer membrane confers resistance towards gallotannins. Supplementation of LB media with iron eliminated the inhibitory activity of gallotannins against S. aureus, and siderophore deficient mutants of S. enterica were less resistant towards gallotannins compared to the wildtype strain. Hepta-O-galloylglucose sensitized Lactobacillus plantarum TMW1.460 to hop extract, indicating inactivation of hop resistance mechanisms, e.g. the multi-drug resistance (MDR) transporter HorA. Carbohydrate metabolism of L. lactis MG1363, a conditionally respiring organism, was influenced by hepta-O-galloylglucose when grown under aerobic conditions and in the presence of heme but not under anaerobic conditions, indicating that gallotannins influence the respiratory chain. In conclusion, the inhibitory activity of gallotannins is attributable to their strong affinity to iron, and likely additionally relates to the inactivation of membrane-bound proteins.