Cell immobilization in composite agar layer microporous membrane structures: growth kinetics of gel-entrapped cultures and cell leakage limitation by a microporous membrane.

Agar discs containing different amounts of viable Escherichia coli cells (from 10 to 10(6) organisms.g-1 agar) were incubated in a nutrient medium and the growth of agar-entrapped bacteria and free (released) cells was monitored. The study was repeated with composite immobilized-cell structures obtained by placing a microporous membrane filter between the gel matrix and the incubation medium. In both cases, immobilized cells grew exponentially and reached a peak concentration an order of magnitude higher than that of free (suspended) cell cultures. The maximum specific growth rates of entrapped bacteria, ranging between 0.0115 min-1 and 0.0145 min-1, i.e., slightly higher than that of control free cultures (0.011 min-1), showed no clear dependence on the initial cell loading (ICL). The microporous filter proved efficient in limiting cell leakage since it noticeably lengthened the leakage time at a given ICL. This efficiency, however, decreased at high ICL and high growth rate of immobilized organisms.