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Three sharply distinct phases were found in the growth of Escherichia coli in a continuous fermenter with 100% cell recycle. They were: an initial phase of batch-type growth reaching exponential rates that terminated abruptly when the glucose concentration in the fermenter fell below the cell's transport threshold; a second phase of linear growth whose rate was dependent on the rate of glucose provision to the fermenter, but whose length was fixed and independent of growth rate; and a third phase of slower linear growth, whose rate was also dependent on the glucose provision rate, characterized by a restriction in RNA accumulation, and indefinite in length. Guanosine 5'-diphosphate 3'-diphosphate (ppGpp) accumulation commenced at the start of phase 2 and rose throughout the phase. It reached a maximum at the start of phase 3, concurrent with the curbing of RNA accumulation; thereafter, the ppGpp concentration was constant unless the rate of glucose supply was increased, at which point it fell to a phase 2 level and the cells returned to a corresponding phase 2 growth rate. The length of growth phase 2 was changed in a precise manner by mutations in the relA, relX and spoT loci. Phase 2 was longer for reU and reZX mutants (decreased capability for ppGpp synthesis) and shorter for spoT mutants (decreased capability for ppGpp degradation). It is concluded that the continuously falling energy supply per cell in a recycling fermenter invokes ppGpp accumulation by at least three biochemical mechanisms and that the third, final growth phase in the fermenter is an indefinitely extended manifestation of strong regulation by PPGPP. I N T R O D U C T I O NIn a fermenter with 100% cell recycle the growing microbial population is completely retained in the fermentation vessel while fresh medium and spent broth are added to and removed from the vessel at equal rates. If the rate at which the carbon-energy substrate is supplied to the growing culture is kept constant, since the total cell mass in the fermenter is continuously increasing, then the rate at which energy is supplied per unit cell mass is continuously dwindling. For such circumstances, calculation of the effect of the maintenance energy requirement upon growth rate (Pirt, 1975;Schultz & Gerhardt, 1969) predicts a steady decrease toward zero for the growth rate, yielding a growth curve smoothly approaching an asymptote, whose value in turn should be predictable for any particular culture (Chesbro et al., 1979) from its maintenance coefficient. When Escherichia coli was grown in a recycling fermenter under carbon-energy limitation the observed pattern of growth differed significantly from the pattern predicted by application of the maintenance energy concept (Chesbro et al., 1979). Instead, a growth pattern with three phases that were sharply distinct both kinetically and biochemically was observed. The first phase was one of batch-type growth becoming exponential in rate. The exponential growth terminated abruptly when the glucose concentration in the fermentation ve...

SUMMARYGrowth systems appropriate for studying mass transfer in different bacterial environments are reviewed. Fed batch and recycling fermentors are suited to modelling nutrient limitation and slow growth. Use of these two growth systems reveals the existence of three growth rate regions, or domains, defined by maintenance energy demands, nucleotide regulation, metabolism, and physiological behavior. They are exemplified in Escherichia coil by domain-dependent synthesis of attachment antigens, heat-labile toxin, and inducible enzymes. Distribution of the bacterial population among cell cycle stages changes with growth rate domain because lengths of the stages differ in their dependence on growth rate. This produces subpopulafions whose ratios vary with growth rate and that are likely to differ in both molecular composition and stress resistance.

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