Susceptibility of agar-entrapped Escherichia coli cultures to cefotaxime as assessed by the potentiometric measurement of lipoic acid reduction.

We used an original potentiometric procedure to monitor the metabolic activity of biofilm-like structures and to assess their susceptibility to betalactam antibiotics. The reduction of exogeneous alpha-lipoic acid to dihydrolipoic acid by planktonic and agar-entrapped bacteria, metabolizing glucose, was followed by potential-time measurements using gold versus reference electrodes. Immobilized cultures displayed a lower susceptibility to cefotaxime than their free counterparts. The redox potential-time courses showed that the gel matrix was more rapidly oxygen depleted than the free-cell culture medium. The depletion of both glucose and dissolved O2 inside the gel induced a breakdown in the reducing activity of agar-entrapped organisms.