Transcriptional fusions with gfp driven by the promoter region of mbla (NE2571) in pPRO/mbla4 and clpB (NE2402) in pPRO/clpb7 were used to transform the ammonia-oxidizing bacterium Nitrosomonas europaea (ATCC 19718). The two genes were chosen because their transcript levels were found at much higher levels in N. europaea in response to oxidation of chloroform and chloromethane. In N. europaea transformed with pPRO/mbla4, green fluorescent protein (GFP)-dependent fluorescence increased from 3- to 18-fold above control levels in response to increasing chloroform concentrations (7 to 28 microM), and from 8- to 10-fold in response to increasing hydrogen peroxide concentrations (2.5-7.5 mM). The GFP-dependent fluorescence of N. europaea transformed with pPRO/clpb7 also showed an increase of 6- to 10-fold in response to chloroform (28-100 microM) but did not respond to H(2)O(2). Our data provide proof of concept that biosensors can be fabricated in ammonia-oxidizing bacteria using "sentinel" genes that up-regulate in response to stress caused either by co-oxidation of chlorinated solvents or by the presence of H(2)O(2). The fabricated biosensors had a consistent concentration-dependent response to chloroform; however, these did not respond to other chlorinated compounds that cause similar cellular stress.