A possible alternative explanation of fast dilution rates is that organisms attach to surfaces and persist in the bioreactor. This would give an apparent fast growth rate while, in fact, the organisms were merely extracting nutrients as the media flowed through rapidly. Lin and Bungay (1993) sampled a pH-auxostat and counted the cells by direct microscopic examination of a Petroff-Hauser chamber and have made viable cell counts. One count was made on a fresh sample and the others on samples held for a brief time interval. The difference in numbers confirms fast growth that corresponds to doubling times of ten minutes or less for several independent runs. This happens with either a wild inoculum or a pure culture. Some typical results are shown in Figure 3 in which the times for each line denote when a sample was taken.. There is little chance for mistakes because the samples were simply placed in the chamber, and counts were reported as the ratio of numbers. Viable counts, direct counting under the microscope, and dilution rate in continuous culture all confirm that cells can double in about 10 minutes or less.
Attached cells retard the selection of
rapidly-growing suspended cells because of competition for
nutrients. It stands to reason that nutrient concentrations are
lower when both attached cells and suspended cells are competing.
When the attached cells are eliminated or reduced in number, the
nutrient concentrations will be greater. Since specific growth rate
coefficient is a function of nutrient concentration, the continuous
culture will grow faster thus developing more intense competition
that will select even faster growing organisms.
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