The environmental bacterium Bacillus licheniformis was transformed with two different shuttle-vectors (pCSS810 and pGFPratiometric) containing insect luciferase and green fluorescent protein genes, respectively. The cells were treated with various antimicrobial agents and the emitted bioluminescence and fluorescence were measured. Plasmid harboring the green fluorescent protein gene was totally segregated without selective pressure, and fluorescent B. licheniformis showed a slower growth rate than the wild-type strain; those cells were bright green as visualized by epifluorescent microscopy. However, fluorescence was not correlated to the growth state of cells or affected by the antibiotic treatments. To the contrary, luminescent transformant was found to be stable without antibiotic selection and showed analogous growth behavior compared to non-plasmid-bearing cells. The luminescent strain functioned as a biosensor for the antibiotics employed. Bioluminescence measurements allowed one to determine the viability of the recombinant cells and the kinetics of the antibacterial action could be followed. Thus, the light emission was found to be a reliable, sensitive, and real-time indicator of the "well-being" of cells, whereas fluorescence allowed one to visualize both metabolically active and inactive cells.