Diffusion in immobilized-cell agar layers: influence of microbial burden and cell morphology on the diffusion coefficients ofl-malic acid and glucose

Summary The diffusivities ofl-malic acid and glucose in an agar membrane entrapping small amounts ofEscherichia coli orRhodospirillum rubrum whole cells were measured using time lag (TL) and steady state (SS) methods. Diffusivities were overestimated by the SS method. For concentrations of immobilizedR. rubrum cells ranging between 10⁴ and 10⁹ organisms cm^–3 agar (20 ng-2 mg dry weight cm^–3 agar), the diffusion coefficient ofl-malic acid, determined by both methods, was related to the logarithm of the membrane cell content by a decreasing linear relationship. The diffusion coefficient of glucose obtained by TL analysis was not significantly affected by the presence in the membrane of 3 ng-0.3 mg dry wt.E. coli cm^–3 agar. However, values arising from the SS method decreased linearly as a function of the amount of immobilized organisms. Membranes containingR. rubrum cells offered higher diffusional resistance tol-malic acid and glucose than those loaded with the same amount ofE. coli cells.