Restoration of meropenem efficacy against multidrug-resistant Acinetobacter baumannii by green tea polyphenol EGCG: Dual targeting of efflux and porin pathways

BACKGROUND: Acinetobacter baumannii is a Gram-negative bacterial pathogen responsible for severe nosocomial infections. Carbapenems, including meropenem and imipenem, are β-lactam antibiotics commonly used as a last-resort treatment for infections caused by A. baumannii. While it is recognized that green tea polyphenols can enhance the efficacy of carbapenems against A. baumannii, the underlying mechanisms of this synergy remain undefined. PURPOSE: This study aims to investigate the synergistic actions of green tea aqueous extract and its key bioactive component, epigallocatechin gallate (EGCG), in combination with meropenem against A. baumannii BAA-1605 and to elucidate the associated mechanisms. METHODS: The synergism with meropenem was assessed using antibiotic susceptibility testing and fractional inhibitory concentration index (FICI) analysis. The mechanism study included β-lactamase activity, Hoechst dye accumulation, and DiOC₂(3) membrane potential measurements. The effect of meropenem and/or EGCG on purified penicillin-binding protein 2 (PBP2) was also evaluated by nuclear magnetic resonance (NMR) spectroscopy and scanning electron microscopy (SEM). Transcriptomic sequencing and bioinformatic analyses were performed to elucidate global gene expression changes of A. baumannii BAA-1605 following exposure to meropenem and/or EGCG. RESULTS: Green tea aqueous extract synergized with meropenem against A. baumannii BAA-1605 by lowering the minimum inhibitory concentration (MIC) by 4-fold (128 to 32 μg/mL). EGCG was determined as the primary synergistic component in the extract (FICI = 0.375). The mechanism study showed that EGCG does not influence β-lactamase production or meropenem-mediated PBP2 inhibition but contributes to overcoming resistance by inhibiting efflux pump activity. Furthermore, transcriptomic analysis revealed that EGCG may suppress the AdeABC efflux system and enhance meropenem influx via a YiaD-dependent porin pathway. CONCLUSION: This study presents a dual role of efflux inhibition and influx facilitation by EGCG in restoring meropenem efficacy, supporting its potential as a promising adjuvant to restore β-lactam potency against A. baumannii or other Gram-negative bacteria harbouring homologs of AdeABC and YiaD proteins.