Microbial electrode sensor for vitamin B12

A microbial sensor consisting of immobilized Escherichia coli 215 and an oxygen electrode is described for the determination of vitamin B₁₂. When the sample solution is injected into the microbial electrode system, the increased consumption of oxygen by the micro-organisms causes a decrease in the dissolved oxygen around the porous membrane of the oxygen electrode and the current decreases gradually with time until a steady state is reached. The response time for a rate measurement is 2 h. When 0.5 mg of Escherichia coli 215 is immobilized, a linear relationship is obtained between the current decrease and the vitamin B₁₂ concentration between 5 × 10^?9 and 25 × 10^?9 g ml^?1.