Hydrolysable tannins from Pelargonium graveolens overcome bacterial virulence and resistance through membrane and motility disruption.

Abstract

ETHNOPHARMACOLOGICAL RELEVANCE: Pelargonium graveolens L'Hér. (Geraniaceae) is traditionally used for the treatment of microbial infections. Its flowers exhibit antibacterial activity, but the bioactive constituents and their modes of action against multidrug-resistant (MDR) pathogens remain insufficiently characterised. AIM OF THE STUDY: This study aimed to identify the antibacterial compounds from P. graveolens flowers and to elucidate their in vitro effects on virulence-related bacterial phenotypes against clinically relevant MDR bacteria. MATERIALS AND METHODS: ), biofilm formation, motility (swimming, swarming), efflux pump activity, membrane permeability and depolarisation were assessed using biochemical, fluorescence-based and microscopic assays including cryogenic transmission electron microscopy (cryo-TEM). RESULTS: Penta-, hexa-, hepta-, octa- and nonagalloylglucose were identified as the main antibacterial constituents with broad-spectrum activity observed for pentagalloylglucose and higher-order gallotannins. Mechanistic investigations showed that at a concentration of 250 μg/mL, gallotannins significantly impaired bacterial motility, reduced biofilm formation under the tested experimental conditions in a strain-dependent manner, enhanced membrane permeability and depolarisation, and strongly inhibited efflux pump activity, exceeding the response induced by the reference proton motive force disruptor Carbonyl Cyanide 3-Chlorophenylhydrazone (CCCP) in some strains. CONCLUSIONS: Gallotannins from P. graveolens exhibited multiple in vitro antibacterial and anti-virulence-related effects, including disruption of membrane-associated functions, motility and efflux-mediated resistance, highlighting their potential as complementary antimicrobial candidates against toxin-producing MDR pathogens and setting the stage for further studies on cytotoxicity towards mammalian cells and selectivity to confirm their therapeutic relevance.