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Background  Tetracyclines are among the most used antibiotics in livestock worldwide. Resistance is widely disseminated in  Escherichia coli,  where it is generally mediated by tetracycline efflux pumps, such as TetA. Expression of tetracycline efflux pumps is tightly controlled by the repressor TetR, which has been shown to be tetracycline-responsive at sub-MIC tetracycline concentrations. The objective of this study was to investigate the effects of increasing tetracycline concentrations on the growth of TetA-producing  E. coli , and to determine how expression of  tetA  and  tetR  related to each other in different growth phases in the presence of tetracycline.    Results  A tetracycline resistant  E. coli  strain containing  tetA  and  tetR  on the chromosome was constructed and cultured in the presence of increasing concentrations of tetracycline. Expression of  tetR  and  tetA  was measured at four time points in different growth phases by quantitative real-time PCR. The TetA-producing  E. coli  exhibited prolonged lag phase with increasing concentrations of tetracycline, while expression of  tetA  and  tetR  increased and decreased, respectively, with increasing tetracycline concentration. The levels of  tetA  and  tetR  mRNA varied depending on growth phase, resulting in a gradual decrease of the  tetA / tetR  ratio from approximately 4 in the lag phase to approximately 2 in the stationary phase.    Conclusion  This study shows that the expression of  tetR  and  tetA  is tetracycline concentration- and growth phase-dependent, contributing to improved understanding of the relationships between  E. coli  growth, tetracycline exposure and expression of tetracycline resistance.    Electronic supplementary material  The online version of this article (doi:10.1186/s12866-016-0649-z) contains supplementary material, which is available to authorized users.

Relation between tetR and tetA expression in tetracycline resistant Escherichia coli