Inhibitory Spectra and Modes of Antimicrobial Action of Gallotannins from Mango Kernels (Mangifera indicaL.)

Abstract

ABSTRACTThis study investigated the antimicrobial activities and modes of action of penta-, hexa-, hepta-, octa-, nona-, and deca-O-galloylglucose (gallotannins) isolated from mango kernels. The MICs and minimum bactericidal concentrations (MBCs) against food-borne bacteria and fungi were determined using a critical dilution assay. Gram-positive bacteria were generally more susceptible to gallotannins than were Gram-negative bacteria. The MICs of gallotannins againstBacillus subtilis,Bacillus cereus,Clostridium botulinum,Campylobacter jejuni,Listeria monocytogenes, andStaphylococcus aureuswere 0.2 g liter−1or less; enterotoxigenicEscherichia coliandSalmonella entericawere inhibited by 0.5 to 1 g liter−1, and lactic acid bacteria were resistant. The use of lipopolysaccharide mutants ofS. entericaindicated that the outer membrane confers resistance toward gallotannins. Supplementation of LB medium with iron eliminated the inhibitory activity of gallotannins againstStaphylococcus aureus, and siderophore-deficient mutants ofS. entericawere less resistant toward gallotannins than was the wild-type strain. Hepta-O-galloylglucose sensitizedLactobacillus plantarumTMW1.460 to hop extract, indicating inactivation of hop resistance mechanisms, e.g., the multidrug resistance (MDR) transporter HorA. Carbohydrate metabolism ofLactococcus lactisMG1363, 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 activities of gallotannins are attributable to their strong affinity for iron and likely additionally relate to the inactivation of membrane-bound proteins.