Mechanism of honokiol in inhibiting Escherichia coli biofilm formation

Abstract:

[Objective] To investigate the mechanism of honokiol at different concentration in inhibiting Escherichia coli 10389 biofilm (BF) formation. [Methods] We employed triphenyltetrazolium chloride (TTC) method to determine the minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) of honokiol for the test strain and tetrazolium salt (XTT) reduction assay to investigate the influence of honokiol concentration on BF formation of the test strain and the influence over time. Through qRT-PCR, we examined the effect of honokiol concentration on the expression of genes related to BF formation and quorum sensing of the test strain. We detected the effect of honokiol at sub-MIC on the expression of furanosyl borate diester (AI-2) in E.coli 10389 and its regulated downstream genes associated with BF formation by bioluminescence and qRT-PCR. [Results] Honokiol inhibited the BF formation of E.coli 10389, but the mechanism was different for different concentration of honokiol. Among them, honokiol at MIC significantly increased the mRNA expression of toxin gene hha and bacterial acid regulator ariR involved in the BF formation of E.coli 10389 and significantly decreased the mRNA expression of toxin overexpression-modulating gene ybaJ compared with the control group. Honokiol at sub-MIC can suppress AI-2 secretion by E.coli 10389 and decrease the mRNA expression of its regulated downstream genes related to BF formation. Compared with the control, 16 mg/mL honokiol reduced the mRNA expression of colanic acid gene mqsR, mucoid gene mcbR, and flagellum formation-related genes csrA, flhD, flhC, and flic by 65.21%, 55.01%, 73.16%, 62.01%, 60.30%, and 59.71%, respectively. [Conclusion] Honokiol suppresses the BF formation of E.coli 10389, but the mechanism is different for honokiol of different concentration. Among them, honokiol at MIC mainly inhibits BF formation by affecting the expression of related genes, and honokiol at sub-MIC by suppressing the expression of the AI-2 synthase luxs gene of Luxs/AI-2 system, decreasing AI-2 secretion, and further influencing the synthesis of capsular polysaccharide, mucoid, and flagellin.