The prevalence of multidrug-resistant bacteria is increasing the risk of infections 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 lost modulus 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 than cells that are susceptible to the antibiotic? Our hypothesis was that E. coli cells that are more rigid (higher elastic modulus) would be more resistant to ampicillin since this chemical attacks the cell membrane. We tested this hypothesis by analyzing the physical properties of E. coli cells using atomic force microscopy (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. This process was followed for four cycles. We used the AFM to measure the cell size (length and diameter) for each cycle. Force-distance curves were measured for individual cells in order to determine any changes in adhesion or elasticity. There was a decreasing trend in cell diameter from Cycle 2 through 4 that could indicate resistant cells are more dense and hence more rigid. An increase in rigidity was measured directly for cells from Cycle 4, with steeper force-distance curves compared with native cells. In future, more replicate measurements are needed in order to increase the statistical significance of these results. Our study demonstrated the effectiveness of the AFM in measuring the biophysical properties of gram-negative cells and indicated a trend that could support our hypothesis.

Patrick Kenney

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