Abstract
Cell-cell interactions play an important role in bacterial antibiotic resistance. Here, we asked whether neighbor proximity is sufficient to generate single-cell variation in antibiotic resistance due to local differences in antibiotic concentrations. To test this, we focused on multidrug efflux pumps because recent studies have revealed that expression of pumps is heterogeneous across populations. Efflux pumps can export antibiotics, leading to elevated resistance relative to cells with low or no pump expression. In this study, we co-cultured cells with and without AcrAB-TolC pump expression and used single-cell time-lapse microscopy to quantify growth rate as a function of a cell’s neighbors. In inhibitory concentrations of chloramphenicol, we found that cells lacking functional efflux pumps (ΔacrB) grow more slowly when they are surrounded by cells with AcrAB-TolC pumps than when surrounded by ΔacrB cells. To help explain our experimental results, we developed an agent-based mathematical model, which demonstrates the impact of neighbors based on efflux efficiency. Our findings hold true for co-cultures of Escherichia coli with and without pump expression and also in co-cultures of E. coli and Salmonella typhumirium. These results show how drug export and local microenvironments play a key role in defining single-cell level antibiotic resistance.
Highlights
Despite intensive study, antibiotic resistance remains an essential problem, in part due to the myriad of mechanisms by which cells can evade drug treatment
Under chloramphenicol treatment just below the minimum inhibitory concentration (MIC) (1 μg/ml, Fig. S1), we found that the growth rate of ΔacrB cells with with green fluorescent protein (WT-GFP) neighbors was lower than those with ΔacrB-RFP neighbors (Fig. 1B), indicating that the influence of drug efflux by neighboring cells is important in local growth inhibition
Single cell level effects are important for bacterial growth and survival under antibiotic treatment
Summary
Antibiotic resistance remains an essential problem, in part due to the myriad of mechanisms by which cells can evade drug treatment. Classical tests, such as measurements of the minimum inhibitory concentration (MIC), are important for quantifying drug resistance, but can obscure single-cell level differences in resistance[1]. Certain cells within a community may exhibit altruistic behavior, such as those that release resistance proteins upon death to enable other cells to survive[10,12] These examples highlight the importance of cellular interactions and collective behavior in antibiotic resistance. Efflux pumps have been recognized to play a major role in clinical isolates in the emergence of resistant strains of E. coli, Salmonella enterica, and other pathogens, and have been identified as clinical targets[20,21]
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