Biochemical fundamentals and improvement of the selectivity of microbial sensors — a minireview

Abstract

Knowledge of the physiological and biochemical background of microbial sensors is an essential prerequisite for the development of these sensors, especially for increasing their stability and improving their selectivity. The following main aspects will be discussed: physiological basis of microbial sensors, principle of measurement, and approaches for enhancing the selectivity and sensitivity. In this review it is shown that the stability of microbial sensors depends on szringent control and that the response of the sensor-microorganisms is connected with substrate uptake. Microbial sensors, which consist of microorganisms in intimate contact with transducer devices and convert a biochemical reaction into a quantifiable electrical response signal, open new possibilities for the analysis of biotechnological processes and for environmental monitoring, especidly by their variability and multireceptor capacity. The first microbial sensor was developed by Divies in 1975 [I] for the detcrmination of ethanol using cells of Acetobacter xyhum. The number of microbial sensors has grown rapidly since then. About fifty electrochemical sensors that utilize microorganisms have been developed for the determination of various organic compounds (sugars, alcohols, organic acids, vitamins, antibiotics, steroids, peptides), enzyme activities, inorganic molecules (ammonia, nitrate, nitrite, sulfide, phosphate) and especially for the determination of complex parameters such as BOD (biochemical oxygen demand), and mutagenicity [2--51. The use of microbial cells instead of isolated enzymes offers several advantages over enzyme electrodes;, such as (i) elimination of the enzyme extraction and purification steps, (ii) avoidance of the need for cofactors, (iii) an incrcascd stability. Furthermore, (iv) the biosensor activity can be regenerated by immersion into nutrient medium and (v) the cell may perform multistep transformations which, with single enzymes, would be difficult if not impossible to perform.