Inhibitory activity of blueberry extract against extended spectrum β-lactamase-producing Klebsiella pneumoniae

Abstract:

[Background] With the enhancement of drug resistance, the extended spectrum beta-lactamase-producing Klebsiella pneumoniae (ESBL-KP) seriously threatens food safety and human health. Blueberry, rich in polyphenols and anthocyanins, is a potential natural antibacterial candidate. [Objective] To analyze the inhibitory activity and mechanism of blueberry extracts against three ESBL-KP strains and test the effect of food matrix (cow milk) on the antibacterial activity of the extracts. [Methods] Fruit extracts of Vaccinium corymbosum L. and V. ashei were prepared with methanol, and the content of total phenols and anthocyanins in the extracts was determined. The antibiotic resistance of K. pneumoniae was detected by disk diffusion test. The antibacterial activity of blueberry extracts was evaluated based on minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), and growth curves of strains. The effect of blueberry extracts on cell membrane integrity was detected by confocal laser scanning microscopy, and its antibacterial activity in milk was detected. [Results] The total phenol content in fruit extracts of V. ashei and V. corymbosum L. was 2.3 mg/g and 3.5 mg/g, respectively, and the anthocyanin content was 67.5 mg/100 g and 92.5 mg/100 g respectively. The MIC and MBC of two blueberry extracts against K. pneumoniae KP106, KP305, and KP408 were 25 mg/mL and 50 mg/mL, respectively. The growth curve showed that the two extracts at 2xMIC could kill K. pneumoniae within 6–8 h. Confocal laser scanning microscopy demonstrated that blueberry extracts could destroy the integrity of cell membrane. The simulated cow milk sample test showed that 100 mg/mL fruit extract of V. corymbosum L. could completely inhibit the growth of ESBL-KP in milk. [Conclusion] Blueberry extract exerts inhibitory effect on K. pneumoniae by destroying the cell membrane of the bacteria, and has the potential to prevent and control K. pneumoniae in milk. This study provides a theoretical reference for the application of blueberry in the prevention and control of drug-resistant K. pneumoniae in milk.