Improved detection of antibiotic compounds by bacterial reporter strains achieved by manipulations of membrane permeability and efflux capacity

The occurrence of pharmaceuticals, including antibacterial compounds, in the environment has been acknowledged as an emerging and troubling issue in environmental safety; their usage is constantly on the rise, and their effects on the environment are only partially understood. Such compounds can accumulate, contaminate the ecosystem, and contribute to the spreading of antibiotic resistance among bacteria, hindering human health. Bioluminescent Escherichia coli reporter strains, engineered to detect antibiotic compounds by fusing the promoter of the global regulator soxS to the Photorhabdus luminescens luxCDABE cassette, were further modified by altering their membrane permeability and efflux capabilities. This was accomplished by introducing several mutations in the efflux system (ΔemrE, ΔacrB, and ΔtolC) and by overexpressing OmpF, a porin located in the outer membrane that allows passive diffusion of molecules. Combinations of these alterations had a cumulative effect in lowering the detection threshold of several antibiotics, in some of the cases to concentrations reported from pharmaceutical-polluted environments.