Date of Award

Spring 5-12-2021

Document Type


Degree Name

Bachelor of Arts


Biochemistry & Molecular Biology

First Advisor

Dr. Joan Slonczewski


Multidrug efflux pumps allow bacterial cells to regulate their internal environment through the removal of toxic substances. AcrAB-TolC the most well-studied multidrug efflux pump in Escherichia coli, exports a wide range of substances and is upregulated by the plant hormone salicylate. Other efflux pumps, some with substantial homology to AcrAB-TolC, may have unique, unexplored functions that select for their persistence within the E. coli genome. Using flow cytometry, knock-out strains of multidrug efflux pumps were competed against an ancestral strain in a range of environmental conditions including bile acids, salicylate, and low-pH exposure to uncover their selection patterns. As expected, AcrAB-TolC is the major efflux mechanism of bile acids and underwent positive selection in the presence of bile acids and the proton motive force uncoupler salicylate. However, the MdtEF-TolC and EmrAB-TolC efflux pumps were not selected with bile acids. MdtEF-TolC was positively selected only with low-pH exposure combined with bile acids. Although AcrAB-TolC was also found to be selected with low pH-exposure in bile acids and salicylate, it was not found to have acid-dependent relative fitness like that of MdtEF-TolC. This finding indicates that MdtEF-TolC acts as an acid-dependent pump within E. coli’s Gad acid fitness island. The selection pattern for MdtEF-TolC differed from the selection pattern seen with the deletion of a majority of the gene island, suggesting that other genes within this region contribute to its selection in bile acids. Altogether, this research shows that even structurally similar efflux pumps can be selected under different environmental conditions.

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