Synergistic activity of luteolin and amoxicillin combination against amoxicillin-resistant Escherichia coli and mode of action.

The purpose of this research was to investigate whether luteolin has antibacterial and synergistic activity against amoxicillin-resistant Escherichia coli (AREC) when use singly and in combination with amoxicillin. The primarily mode of action is also investigated. The susceptibility assay (minimum inhibitory concentration and checkerboard determination) was carried out by the broth macrodilution method's in Müeller-Hinton medium. MIC and checkerboard determination were carried out after 20 h of incubation at 35°C by observing turbidity. The MICs of amoxicillin and luteolin against all AREC strains were >1000 and ≥ 200 μg/ml respectively. Synergistic activity were observed on amoxicillin plus luteolin against these strains. Viable count of this combination showed synergistic effect by reducing AREC cell numbers. The results indicated that this combination altered both outer and inner membrane permeabilisation. Enzyme assay showed that luteolin had an inhibitory activity against penicillinase. Fourier Transform-Infrared (FT-IR) spectroscopy exhibited that luteolin alone and when combined with amoxicillin caused increase in fatty acid and nucleic acid, but decrease in amide I of proteins in bacterial envelops compared with control. These results indicated that luteolin has the potential to reverse bacterial resistance to amoxicillin in AREC and may operate via three mechanisms: inhibition of proteins and peptidoglycan synthesis, inhibition of the activity of certain extended-spectrum β-lactamases and alteration of outer and inner membrane permeability. These findings offer the potential to develop a new generation of phytopharmaceuticals to treat AREC.