Antibiotic selective pressure in microcosms: Pollution influences the persistence of multidrug resistant Shigella flexneri 2a YSH6000 strain in polluted river water samples

Some urban rivers reach dangerous concentrations of residual antibiotics imposing a certain level of selective pressure on microorganisms to develop various antibiotic resistance mechanisms. In the current work, we have measured the persistence and growth of a multidrug resistant strain of Shigella flexneri 2a YSH6000 under a mock release of lethal concentration of oxytetracycline in microcosms of River Thames water. The water was sampled from upstream (lower levels of pollution) and downstream (higher levels of pollution) of London city centre. In our in-vitro microcosms, in the presence of 160 µg/mL of oxytetracycline, growth of S. flexneri in the downstream sector was up to 2 log(cfu/mL) higher relative to the upstream sector. This difference in growth is a sum of undefined interactions of different chemicals with the antibiotic. We extrapolated the contribution of two abundant pollutants in downstream sector: iron and phenanthrene. In the presence of selection pressure, iron at a concentration of 6.49 mg/L was found to foster the growth of resistant bacteria while phenanthrene at concentration of 160 µg/L reduced the growth of the resistant strain. In addition, label free proteomics analysis showed that there are 64 proteins that were differentially expressed by the bacteria exposed to the upstream section versus the downstream sector. In the presence of oxytetracycline, at concentration of 160 µg/mL, the differences reduced to only a few proteins, demonstrating that environmental stress impacts protein synthesis. Such mock studies contribute to our knowledge of chemicals that reduce growth of resistant strains and aids in the identification of selective biomarkers.