Efflux Pump-Mediated Antibiotic Resistance in Dynamic Environments

Antibiotic resistance has become a major public health concern as bacteria produce strategies to evade drugs, leading to recurring infections and overuse of antibiotics. Understanding the complex strategies bacteria employ to resist treatment could provide insight into different methods of targeting these evasive cells. There are many methods that bacteria use to achieve antibiotic resistance including the production of antibiotic-inactivating enzymes, the transfer of genetic material encoding for resistance genes, and the use of efflux pumps. Here, we focus on efflux pumps, which are membrane transporters that can export undesired compounds such as drugs out of the cell. Multidrug resistance pumps, such as AcrAB-TolC from Escherichia coli, are especially well known for their ability to export a wide variety of antibiotics. Although these cell-membrane proteins provide antibiotic resistance for cells, there are potential trade-offs, as there is a substantial cost to cells when pumps are expressed. An excess of pumps can alter the fluidity of the cell membrane and slow growth. We hypothesize that the ideal pump expression level involves an environmentally-dependent trade-off between the benefit of pumps and the cost of their expression. For example, if antibiotics appear frequently then high pump expression levels are required. In addition, the frequency and duration of antibiotic appearance will affect this trade-off. In this work, we evaluate the benefit of the AcrAB-TolC pump under a range of induction conditions and under variable antibiotic stresses. We assess the performance of cells with and without pumps when the antibiotics are presented in different forms – step, ramp, pulse – and when the pumps are induced with respect to the antibiotic levels. Our overall goal is to quantitatively determine the trade-off between resistance and hindered growth for cells containing these pumps. To achieve this we are co-culturing cells with and without the efflux pumps. The relative fraction of cells with and without the pumps can change with time under variable antibiotic and inducer conditions. This work will provide insight into how bacteria optimize the use of efflux pumps as strategic mechanisms for surviving in stressful conditions.