Background: Multi-drug-resistant Escherichia coli  poses a great threat to human health, especially resistant to ampicillin (AMP), but the mechanism of drug resistance is not very clear.
Purpose: To understand the mechanism of resistance of E. coli  to beta-lactam antibiotics by inducing drug resistance of sensitive bacteria in laboratory.
Methods: Clinical sensitive E. coli  strain was induced into resistance strain by 1/2 minimum inhibitive concentration (MIC) induced trails of AMP. The drug resistance spectrum was measured by modified K-B susceptibility test. Whole-genome sequencing analysis was used to analyze primary sensitive strain, and resequencing was used to analyze induced strains. Protein tertiary structure encoded by the gene containing single nucleotide polymorphism (SNP) was analyzed by bioinformatics.
Results: After 315 hrs induced, the MIC value of E. coli 15743  reached to 256 μg/mL, 64 times higher than that of the sensitive bacteria. During the induction process, the bacterial resistance process is divided into two stages. The rate of drug resistance occurs rapidly before reaching the critical concentration of 32 μg/mL, and then the resistance rate slows down. Sequencing of the genome of resistant strain showed that E. coli 15743  drug-resistant strain with the MIC values of 32 and 256 μg/mL contained four and eight non-synonymous SNPs, respectively. These non-synonymous SNPs were distributed in the genes of frdD, ftsI, acrB, OmpD, marR, VgrG, and envZ.
Conclusion: These studies will improve our understanding of the molecular mechanism of AMP resistance of E. coli , and may provide the basis for prevention and control of multi-drug-resistant bacteria and generation of new antibiotics to treat E. coli  infection.
Keywords: Escherichia coli , ampicillin, drug resistance