Studies of the drug resistance response of sensitive and drug-resistant strains in a microfluidic systemElectronic supplementary information (ESI) available. See DOI: 10.1039/c3ib40164b

Since the discovery of bacterial drug resistance, its dynamics have been the focus in biophysics studies. In this paper, we used a new microfluidic system to monitor the responses of sensitive and drug-resistant strains of E. coli in different β-lactam ceftriaxone concentrations at the single cell level and traced each individual cell's states such as cell length, GFP protein expression and growth rate. The β-lactamase production of the drug-resistant strain is quantified by fluorescence intensity, as the GFP gene co-transcribes with the enzyme expression gene. Our results show that the drug-resistant strain can endure a much higher concentration of antibiotics than the sensitive strain and has an antibiotic concentration ratio from the cell death state to the cell elongation state that is much larger than that of the sensitive strain. The single cell data and simulation suggest that bacteria with slower growth rates have higher drug resistance both in the sensitive and drug-resistant strains. The drug-resistant strain shows adaptation behavior, but no adaptation is found in the sensitive strain after changing to a high antibiotic concentration. A mathematical model of cell growth can qualitatively explain the observed behavior. The quantitative measurement of single-cell phenotype changes and dynamic analysis presented in this study should shed light on the antibiotic process of different bacteria. Since the discovery of bacterial drug resistance, its dynamics have been the focus in biophysics studies.