A toxicity monitoring system based on the metabolic properties of sulfur-oxidizing bacteria (SOB) in continuous and fed-batch modes has been applied for the detection of nitrite (NO2--N). In this study, the effects of different concentrations of NO2--N (0.1 to 5 mg/L) on the SOB bioreactors were tested. We found that 5 mg/L NO2--N was very toxic to the SOB bioreactors in both continuous (R1) and fed-batch (R2) modes, showing complete inhibition of SOB activity within 2 h of operation. R1 and R2 were operated in different ways; however, the EC inhibition and recovery patterns were very similar. The EC rate increased with an increasing NO2--N concentration in both continuous and fed-batch modes. The addition of 5 mg/LNO2--N in continuous mode decreased the average EC rate by 14.38 +/- 2.1 mu S/cm/min; while in fed-batch mode, the EC rate decreased by 23 mu S/cm/min. Although the toxicity monitoring system could detect 0.5-5 mg/L NO2--N, it could not detect 0.1 mg/L NO2--N in either continuous or fed-batch operation. Thus, the SOB biosensor method presented is useful to detect toxic agents such as NO2--N within a few minutes or hours.