Abstract: 【Objective】To investigate the effects of environmental stress factors on the survival adaptability of pig-origin multi-drug resistant Escherichia coli,so as to provide a theoretical basis for creating an environment where multidrug resistant strains could present a high cost of adaptability and for accelerating the reduction of resistance level in the population.【Method】Multi-drug resistant E. coli QL15 isolated from a large-scale pig farm in Guizhou Province was taken as the study object,and E. coli quality control(sensitive)strain ATCC25922 was taken as the control. The adaptability of drug-resistant strain QL15 and that of sensitive strain ATCC25922 under different environmental stress(medium concentration,carbon source,nitrogen source,p H and temperature)was determined by in vitro competition assay. The relationship between drug-resistant gene phenotype and gene expression of efflux pump and outer membrane pore protein with the cost of bacterial adaptation was analyzed by resequencing variant loci and fluorescence quantitative PCR.【Result】Except that the growth of drug-resistant strain QL15 was lower in dilute LB medium(1/4,1/8 and 1/16)compared with the sensitive strain ATCC25922,the growth of drug-resistant strain QL15 was better than that of sensitive strain ATCC25922 in in vitro independent and mixed cultures under low carbon sources(0.1% and 0.5%),low nitrogen sources(0.01% and 0.05%),acid-base property(p H=5.0,pH=6.0 and pH=8.0),low and high temperature(20 ℃ and 42 ℃).Besides,the results of mixed culture were better than that of the independent culture. The genome sequence of drug-resistant strain QL15 was compared with that of sensitive strain ATCC25922,and a total of 509 InDel mutation sites were detected,including 233 delete mutation sites and 276 insert mutation sites;a total of 71 functional genes were annotated,most of which were mutations related to cell wall and cell membrane(10),accounting for 14.1%. Compared with the sensitive strain ATCC25922,in the drug-resistant strain QL15,genes related to efflux pump and outer membrane pore protein,namely emrB,acrB,tolC and ompC showed down-regulated expression,and ompF gene showed up-regulated expression under different environmental stress conditions.【Conclusion】The acquisition of drug resistance of multi-drug-resistant E. coli makes it more adaptive,while its acquisition of directly available nutrients is weaker than that of sensitive strains,that is,E. coli is at a competitive disadvantage. The multi-drug resistance of drug-resistant E. coli is mainly mediated by a variety of efflux pumps and outer membrane pore proteins. Among them,the ompF gene shows up-regulated expression under pH stress,allotrophic stress,high and low temperature stress,so it is presumed that the high expression of ompF gene is beneficial to the growth of E. coli under the above stress conditions.