Abstract: Objective To explore the theoretical basis to prevent further drug resistance, we examined the effects of Colistin (COL) alone or in combination with ceftazidime-avibactam (CAZ-AVI) on the ability of extensively-drug resistant Pseudomonas aeruginosa (XDR-PA) to prevent COL resistant mutations in vivo and in vitro. Provide a. Methods The minimum inhibitory concentration (MIC) of CAZ-AVI and COL against 10 strains of XDR-PA were determined by agar plate doubling dilution method. The both method were used to enrich the bacteria at a concentration of 10¹⁰ CFU•mL^-1, and the agar plate doubling dilution method to determine the mutant prevention concentration (MPC) of COL alone or in combination with CAZ-AVI against XDR-PA. A rabbit tissue cage infection model was established. The three doses of COL (2.5, 3.75, 5 mg/（kg·d）) were used alone or the basic dose of COL (2.5 mg/（kg·d）) combined with CAZ-AVI ([50+12.5] mg/（kg·d）) for treatment to explore the correlation between drug concentration and the formation of drug-resistant mutants in rabbit tissue cage infection models. The COL resistant mutants were screened in vivo, and the coding genes of the two-component regulatory systems PmrAB, PhoPQ, ParRS, and CprRS were amplified by PCR and sequenced. Results COL is sensitive to 10 strains of XDR-PA, no intermediary and drug-resistant bacteria have been found. The MIC range is 0.25~2mg/L. CAZ-AVI is sensitive to 8 strains of XDR-PA and 2 strains are resistant. The MPC range of COL alone for 10 strains of XDR-PA is 64~256mg/L, when combined with CAZ-AVI, the MPC range is reduced to 4~16mg/L, and the reduction range is 8~16. In the rabbit tissue cage model, the concentration of COL reached its peak at 2h,and its concentration was all within the resistance mutation selection window (MSW) within the interval of one dosing. When given different concentrations of COL monotherapy, the bacterial growth of XDR-PA was obviously inhibited after one treatment, but it returned to the original level after 24 hours. When the basic dose of COL was combined with CAZ-AVI, the amount of P. aeruginosa bacteria in the tissue cage maintained a continuous downward trend, and there was no recovery of growth. Resistance mutants were screened in the COL single drug group, and 5 were randomly selected from each group. The MIC range for COL was 8~128mg/L. Gene mutations mostly occurred in PmrB. Conclusions The combined use of COL and CAZ-AVI can reduce its single-use anti-drug resistance mutation concentration of XDR-PA, reduce MSW, and effectively protect the antibacterial activity of COL.

Key words: COL, CAZ-AVI, Combination medication, Mutant prevention concentration, Mutation selection window