Genetically constructed microbial biosensors for measuring organic pollutants are mostly applied in aqueous samples. Unfortunately, the detection limit of most biosensors is insufficient to detect pollutants at low but environmentally relevant concentrations. However, organic pollutants with low levels of water solubility often have significant gas-water partitioning coefficients, which in principle makes it possible to measure such compounds in the gas rather than the aqueous phase. Here we describe the first use of a microbial biosensor for measuring organic pollutants directly in the gas phase. For this purpose, we reconstructed a bioluminescent Pseudomonas putida naphthalene biosensor strain to carry the NAH7 plasmid and a chromosomally inserted gene fusion between the sal promoter and the luxAB genes. Specific calibration studies were performed with suspended and filter-immobilized biosensor cells, in aqueous solution and in the gas phase. Gas phase measurements with filter-immobilized biosensor cells in closed flasks, with a naphthalene-contaminated aqueous phase, showed that the biosensor cells can measure naphthalene effectively. The biosensor cells on the filter responded with increasing light output proportional to the naphthalene concentration added to the water phase, even though only a small proportion of the naphthalene was present in the gas phase. In fact, the biosensor cells could concentrate a larger proportion of naphthalene through the gas phase than in the aqueous suspension, probably due to faster transport of naphthalene to the cells in the gas phase. This led to a 10-fold lower detectable aqueous naphthalene concentration (50 nM instead of 0.5 M). Thus, the use of bacterial biosensors for measuring organic pollutants in the gas phase is a valid method for increasing the sensitivity of these valuable biological devices.Increasingly, genetically modified bacteria are being developed and tested for use as measuring devices for pollutants in the environment (reviewed recently in references 5, 8, and 19). Such bacterial biosensors either constitutively express a signal, such as bioluminescence, with any signal decrease related to exposure of the cells to pollutants (28), or induce expression of an analytically useful signal specifically and only in the presence of one or a limited set of structurally related compounds (1,14,20,26,32). For a number of reasons, it is useful to have microorganisms themselves monitor the immediate environment rather than using analytical chemistry. First, bacterial biosensors can be used for quick screening purposes, either for detecting the presence of toxic compounds in aqueous samples or for detecting a predefined set of target compounds for which the biosensors had been developed. Second, having microorganisms themselves "measure" the concentrations of pollutants is thought to be more representative for estimation of the bioavailable fraction and could solve some of the difficulties of extrapolating bioavailable concentrations from differential chemical extrac...