A Luminescent Escherichia coli Biosensor for the High Throughput Detection of β-Lactams

Abstract

A group-specific bioluminescent Escherichia coli strain for studying the action of β-lactam antibiotics is described. The strain contains a plasmid, pBlaLux1, in which the luciferase genes from Photorhabdus luminescent are inserted under the control of the β-lactam-responsive element ampR/ampC from Citrobacter freundii. In the presence of β-lactams, the bacterial cells are induced to express the luciferase enzyme and three additional enzymes generating the substrate for the luciferase reaction. This biosensor for β-lactams does not need any substrate or cofactor additions, and the bioluminescence can be measured very sensitively in real time by using a luminometer. Basic parameters affecting the light production and induction in the gram-negative model organism E. coli SNO301/pBlaLux1 by various β-lactams were studied. The dose-response curves were bell shaped, indicating toxic effects for the sensor strain at high concentrations of β-lactams. Various β-lactams had fairly different assay ranges: ampicillin, 0.05-1.0 μg/ml; piperacillin, 0.0025-25 μg/ml; imipenem, 0.0025-0.25 μg/ml; cephapirin, 0.025-2.5 μg/ml; cefoxitin, 0.0025-1.5 μg/ml; and oxacillin, 25-500 μg/ml. Also, the induction coefficients (signal over background noninduced control) varied considerably from 3 to 158 in a 2-hour assay. Different non-β-lactam antibiotics did not cause induction. Because the assay can be automated using microplate technologies, the approach may be suitable for higher throughput analysis of β-lactam action.