Evaluation Of The Antibacterial Effect Of Colistin Combined With Isopropoxy Benzene Guanidine On Mcr-1 Positive Escherichia Coli

Escherichia coli is an important opportunistic pathogen affecting human health worldwide.Due to the intrinsic resistance and acquired resistance mechanisms,mostβ-lactam drugs are not effective in treating infections caused by E.coli.Thus,carbapenems play a very important role in the treatment.Many multi-drug resistant strains resistant to carbapenems have emerged,while relevant studies have not yet developed effective treatment options.There is evidence that polymyxins are the last line in the treatment of infections caused by carbapenem-resistant bacteria.With the increasing use of colistin,the sensitivity of E.coli to colistin is reducing,especially the presence of plasmid-mediated colistin resistance gene mcr-1,which may lead to the break of colistin as the last line of defense.Due to the difficulty in the development of new antibiotics and the long cycle,it is difficult to cope with the development of current bacterial resistance.Hence,many scholars suggest that a combination of multiple antibiotics should be used.In this study,we evaluated in vitro and in vivo antibacterial effects of colistin combined with isopropoxy benzene guanidine on mcr-1 positive Escherichia coli.In this experiment,the MIC of colistin monotherapy was performed on the standard strain E.coli ATCC 25922 and 19 pig-derived mcr-1 positive E.coli.The results showed that the MIC of E.coli ranged from 1 to 8μg/m L,and the range of MBC was 1 to 16μg/m L,MBC was about 1-4 times of the MIC,which showed that colistin is a bactericidal drug for E.coli.15 strains of mcr-1 positive E.coli were selected for the determination of MPC of colistin.The results showed that the MPCpr was around 8～32μg/m L,which was about 4 to 32 times of their MICs.This indicated that the colistin had a wide therapeutic window for E.coli suggesting that there is a possibility of causing drug resistance mutation if colistin cannot reach at the site of action in clinical use.In this test,the standard strain ATCC25922 and 18 strains of pig-derived mcr-1positive E.coli were tested for colisitin and isopropoxy benzene guanidine by a checkerboard method.For all strains,the FICI of the 19 E.coli strains was between 0.023and 0.282,which showed a synergistic effect.And most of the strains had a FICI value of less than 0.1.By adding different concentrations of isopropoxy benzene guanidine,the changes in MIC of colistin were measured.It was found that the MIC value of colistin of all strains decreased by more than 4 times when added with 80μg/m L isopropoxy benzene guanidine.Therefore,80μg/m L of isopropoxy benzene guanidine was used as the optimal concentration in combination with colistin,and used in the follow-up experimental study.In this experiment,the antibacterial effects of the monotherapy group and the combination group on the standard strain ATCC25922 and clinical strain GDM7P123 were evaluated by the in vitro sterilization curve and in vivo the mouse peritonitis model.In vitro sterilization curve,the absorbance of OD₆₀₀showed a downward trend in the combined group while an upward trend was observed in the single drug group.The in vivo therapeutic test showed that the mortality of the peritonitis mice in the co-administered group was higher than that in the colistin alone group.The isopropoxy benzene guanidine monotherapy group had a similar mortality rate to the model group in peritonitis mice.In summary,in vitro MIC of colistin combined with isopropoxy benzene guanidine showed good synergistic effect.Isopropoxy benzene guanidine as a synergist for colistin resistance has potential application value.In vivo efficacy trials of different dose regimens for the treatment of E.coli peritonitis in a mouse model,the survival rate of the combined group was not higher than that of the colistin monotherapy group.And the combination may not be suitable for the treatment of systemic infections.The appropriate disease model should be selected so that the combination using program can exert its maximum efficacy pending further research.