Many studies on the evaluation of the antimicrobial activity of natural products have been conducted against various microorganisms, but the exploit of natural products meditated antimicrobial activity against Enterobacter hormaechei has not been found in the literature. In this study, we set up to investigate vanillic acid (VA)-mediated antimicrobial activities and its modes of action against carbapenem-resistant E. hormaechei (CREH). Minimum inhibitory concentration (MIC) of VA against CREH was examined by the agar diffusion method. Antibacterial mode of VA against CREH was elucidated by measuring variations in intracellular ATP concentration, intracellular pH, membrane potential, and cell morphology. Moreover, anti-biofilm formation of VA and its damage to CREH cells embedded in biofilms were further explored. Our results show that VA was effective against CREH with MIC of 0.8 mg/mL. VA could rupture cell membrane integrity of CREH, as measured by a decrease of intracellular ATP, pH and membrane potential, along with distinctive alternations in cell morphology. In addition, VA exerted a remarkable inhibitory effect on the biofilm formation of CREH and killed CREH cells within biofilms. These findings display that VA has a potent antibacterial and anti-biofilm activity against CREH, and hence has the potential to be exploited in clinic as a novel candidate agent to treat CREH infections and in the food industry as food preservatives and surface disinfectants.

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