Here we describe the construction and characterization of a biosensing reporter where luxCDABE genes from Photorhabdus luminescens, engineered for expression in Gram-positive organisms, are under the transcriptional control of QacR repressor from Staphylococcus aureus. In non-pathogenic S. aureus model system we analyzed the activity of the regulatory region acting as multidrug-resistance mediator in wild type strains. The use of full-length bacterial luciferase and the measurement of real-time light emission from intact, living cells make the present system suitable to follow the short term activity of different inducers. Among the tested molecules, tetracyclines showed a peculiar behavior by giving very high induction in a fashion seemingly more related to a general stress condition of the culture than to the direct binding and displacement of QacR from its operator. Temperature shocks confirmed that active transcription from qacA promoter is started in response to unspecific conditions where cell growth is strongly inhibited. (C) 2002 Elsevier Science (USA). All rights reserved.