Evaluation of the Synergistic Effect of Thymol/Ampicillin on Escherichia coli strains

Abstract Introduction: The purpose of this study was to investigate the synergistic effect of thymol/ampicillin on Escherichia coli (E. coli). Material and methods Antimicrobial effect of thymol, and ampicillin individually and in combination with different concentrations on E. coli ATCC25922 were performed by MIC-MBC method. The antimicrobial effect of a compound that had synergistic effect on twenty clinical strains of E. coli bacteria was performed by the MIC-MBC method, and the identification of chemical bonds, functional groups, and molecular interactions of this compound was investigated with an FTIR device. Checkered method, time-killing curve and biofilm inhibition on E. coli ATCC25922 bacteria, and investigation of cytotoxicity on human red blood cells (RBCs) by hemolysis method and human skin fibroblast cells (Ffk) with MTT method were performed. F1 compound had a synergistic effect. Results The results of this study showed that the antimicrobial effects of thymol, ampicillin, and thymol/ampicillin (F1 compound), on E. coli ATCC25922 were 256, 8, 8/2 µg/ml (FICI: 1) respectively. The antimicrobial effects of the F1 compound on clinical strains of E. coli were 128–512/32–64 µg/ml respectively. The time curve for killing E. coli was higher in the combined mode than in the individual modes. These compounds had chemical bonds OH carbohydrates proteins, polyphenols, C = O Amide I band, C-O-Cpolysaccharide, and C-Namide III band, but C = C conjugated, C ≡ C compound, shows the connection between thymol/ampicillin. The biofilm inhibition rate of thymol, ampicillin, and thymol/ampicillin against E. coli bacteria was 29.69%, 60%, and 62% respectively. The toxicity of thymol, ampicillin, and F1 compound against human RBCs were 36.12, 8.38, and 9.54, and against human Ffk cells were 9.92, 6.83, and 9.33 respectively. Discussion Considering the resistance of ampicillin in the treatment of diseases caused by E. coli, the results of this study proved that the thymol/ampicillin has high anti-bacterial and anti-biofilm properties. Toxicity on several cells was the same, and this compound can be tested later.