Flavonoids exhibit a broad range of biological activities including antibacterial activity. However, the mechanism of their antibacterial activity has not been fully investigated. The antibacterial activity and membrane interaction of 11 flavonoids (including 2 polymethoxyflavones and 4 isoflavonoids) against Escherichia coli were examined in this study. The antibacterial capacity was determined as flavonoids>polymethoxyflavones>isoflavonoids. Using fluorescence, it was observed that the 5 flavonoids rigidified the liposomal membrane, while the polymethoxyflavones and isoflavonoids increased membrane fluidity. There was a significant positive correlation between antibacterial capacity and membrane rigidification effect of the flavonoids. A quantitative structure-activity relationship (QSAR) study demonstrated that the activity of the flavonoid compounds can be related to molecular hydrophobicity (CLogP) and charges on C atom at position3 (C3). The QSAR model could be used to predict the antibacterial activity of flavonoids which could lead to natural compounds having important use in food and medical industry.

Flavonoids are potential sources of natural preservatives. The inhibitory activities of three polymethoxylated flavones (PMFs), three flavones, and four flavonols against Escherichia coli were determined using the microbroth dilution method. Flavonoid inhibitory activities against DNA gyrase from E. coli were estimated by DNA supercoiling. Kaempferol exhibited the greatest antibacterial activity [minimal inhibitory concentration (MIC) = 25 μg/mL], while nobiletin showed the lowest activity (MIC = 177 μg/mL). A good correlation was found between the pIC50 values and the corresponding pMIC values for the purified DNA gyrase (r = 0.9582). The structure-activity relationship analysis suggests that, for a good inhibitory effect, the hydroxyl group substitution at C-5 in the A ring and C-4' in the B ring and the methoxyl group substitution at C-3 and C-8 in the A ring are essential. The presence of the hydroxyl group at C-6 in the A ring, C-3' and C-5' in the B ring, and C-3 in the C ring and the methoxyl group at C-3' in the B ring greatly reduced inhibition of bacteria. These findings provide a theoretical basis for the development of high-bioactive and low-toxicity natural preservatives.

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Flavonoids are potential antibacterial agents. However, key substituents and mechanism for their antibacterial activity have not been fully investigated. The quantitative structure-activity relationship (QSAR) and molecular docking of flavonoids relating to potent anti-Escherichia coli agents were investigated. Comparative molecular field analysis (CoMFA) and comparative molecular similarity indices analysis (CoMSIA) were developed by using the pIC50 values of flavonoids. The cross-validated coefficient (q2) values for CoMFA (0.743) and for CoMSIA (0.708) were achieved, illustrating high predictive capabilities. Selected descriptors for the CoMFA model were ClogP (logarithm of the octanol/water partition coefficient), steric and electrostatic fields, while, ClogP, electrostatic and hydrogen bond donor fields were used for the CoMSIA model. Molecular docking results confirmed that half of the tested flavonoids inhibited DNA gyrase B (GyrB) by interacting with adenosine-triphosphate (ATP) pocket in a same orientation. Polymethoxyl flavones, flavonoid glycosides, isoflavonoids changed their orientation, resulting in a decrease of inhibitory activity. Moreover, docking results showed that 3-hydroxyl, 5-hydroxyl, 7-hydroxyl and 4-carbonyl groups were found to be crucial active substituents of flavonoids by interacting with key residues of GyrB, which were in agreement with the QSAR study results. These results provide valuable information for structure requirements of flavonoids as antibacterial agents.

Higher levels of pepsin in sputum were associated with higher RSI and RFS in cases of laryngitis. There was no relationship between pepsin levels and RSI in cases of OSA. There were no differences of pepsin concentration in sputum collection methods or in collection timing.