EFFECTS OF THE BACTERIA ESCHERICHIA COLI STIFFNESS IN RESPONSE TO AMPICILIN

The prevalence of multidrug-resistant bacteria is increasing the risk of infection worldwide. One mechanism of resistance is physical changes to the cells. Prior studies have measured the instantaneous response of Escherichia coli (E. coli) cells to an antibiotic; they immediately became stiffer when exposed to ampicillin. In this study, we tracked the long-term effects of exposure to ampicillin on the biophysical properties of E. coli. The question addressed by this study was: Are antibiotic-resistant bacterial cells more rigid? Our hypothesis was that E. coli cells that are more rigid (higher elastic modulus) would be more resistant to ampicillin. We tested this hypothesis by analyzing the physical properties of E. coli cells using the Atomic Force Microscope (AFM), comparing native E. coli to an enriched population upon culturing in the presence of ampicillin. E. coli was initially grown in nutrient-rich broth, to which ampicillin was added in increasing concentrations. Force-distance curves were measured for each cell in order to determine any changes in adhesion or elasticity. In addition to the AFM technique, we measured the rheological properties of the E. coli with and without ampicillin in a motility buffer. We hypothesized that changes in stiffness at the cellular level would manifest as collective behavior changing the shear stress, viscosity, and shear modulus. Our results from AFM show an increase in stiffness as well as an increase in viscosity compared to native cells. Our study demonstrated the effectiveness of the AFM and rheometer in measuring the biophysical properties of gram-negative cells and indicated a trend that could support our hypothesis