Unlike PHB, which is accumulated intracellularly [91] , IBT is excreted from the cell (Sinskey laboratory, data not shown). Thus, continuous autotrophic IBT pro­duction is possible. A two stage approach could be used, in which the cells are first grown heterotrophically to a high cell density on the porous wall of hollow fibers. Then, the heterotrophic medium would be replaced by a nitrogen-deficient auto­trophic medium to induce the stringent response and trigger rapid IBT production. The gaseous substrates would then be added, with the H2 and O2 being maintained on opposite sides of the fiber wall. A schematic drawing of this hollow fiber reactor setup is shown in Fig. 7.

Since no growth will be supported by the production medium, the cells will need to be periodically regenerated by brief exposure to the heterotrophic growth medium. However, the intervals on growth medium can be short since there will be no require­ment to start the culture anew, thus maximizing the production time.