Evaluation of heat resistance of microorganisms by an amperometric measurement of the microbial substrate-oxidizing activity using a whole-cell modified electrode

A mediated bioelectrocatalysis using a bacterial cell-modified electrode wa s applied to the evaluation of heat resistance of Escherichia coli and Lact obacillus plantarum. The first derivative of the time course of the bioelec trocatalysis current for the bacterial D-glucose oxidation had a maximum d I/dt(max) at about 10 to 60 s after the addition of D-glucose to the soluti on. The dI/dt(max) value decreased in different manners depending on the co nditions of heat treatment of the bacterial cells prior to their immobiliza tion on electrode. The decreasing tendency in the dI/dt(max) value was simi lar to that in the colony formation unit measured by a conventional colony counting method. The dI/dt(max) method which takes only 2 h, is much more t ime saving compared with the conventional method taking 24 h incubation for E. coli and 72 h incubation for L. plantarum. In some cases, however, ther e was a difference in the magnitude of relative decrease between the two me thods, which could be attributed to the appearance of injured cells that we re active in the dI/dt(max) measurement but not in the colony formation.