Selection in chemostats.

Dykhuizen, Daniel E.; Hartl, Daniel L.

doi:10.1128/mmbr.47.2.150-168.1983

Cited by 255 publications

(120 citation statements)

References 55 publications

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“…An important approach to studying the evolution of bacterial populations has involved the use of the chemostat (historic early references in Kubitschek, 1970;Dykhuizen and Hartl, 1983). Prolonged culture of bacteria readily results in genetic adaptation to the selective conditions imposed by the culture conditions (Lenski, 1995).…”

Section: Introductionmentioning

confidence: 99%

Adaptive mgl‐regulatory mutations and genetic diversity evolving in glucose‐limited Escherichia coli populations

McRobb

Ferenci

1999

Environmental Microbiology

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The mutational adaptation of E. coli to low glucose concentrations was studied in chemostats over 280 generations of growth. All members of six independent populations acquired increased fitness through the acquisition of mutations at the mgl locus, increasing the binding protein-dependent transport of glucose. These mutations provided a strong fitness advantage (up to 10-fold increase in glucose affinity) and were present in most isolates after 140 generations. mgl constitutivity in some isolates was caused by base substitution, short duplication, small deletion and IS1 insertion in the 1041 bp gene encoding the repressor of the mgl system, mglD (galS). But an unexpectedly large proportion of mutations were located in the short mgl operator sequence (mglO), and the majority of mutations were in mglO after 280 generations of selection. The adaptive mglO substitutions in several independent populations were at exactly the positions conserved in the two 8 bp half-sites of the mgl operator, with the nature of the base changes also completely symmetrical. Either mutations were directed to the operator or the particular operator mutations had a selective advantage under glucose limitation. Indeed, isolates carrying mglO mutations showed greater rates of transport for glucose and galactose at low concentrations than those carrying mglD null mutations. mglO mutations avoid cross-talk by members of the GalR-Lacl repressor family, reducing transporter expression and providing a competitive advantage in a glucose-limited environment. Another interesting aspect of these results was that each adapted population acquired multiple mgl alleles, with several populations containing at least six different mgl-regulatory mutations co-existing after 200 generations. The diversity of mutations in the mglO/mglD region, generally in combination with mutations at other loci regulating glucose uptake (malT, mlc, ptsG), provided evidence for multiple clones in each population. Increased fitness was accompanied by the generation of genetic diversity and not the evolution of a single winner clone, as predicted by the periodic selection model of bacterial populations.

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Section: Introductionmentioning

confidence: 99%

Adaptive mgl‐regulatory mutations and genetic diversity evolving in glucose‐limited Escherichia coli populations

McRobb

Ferenci

1999

Environmental Microbiology

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“…However, the mechanism responsible for this phenomenon was not established. Growth and product formation in a chemostat can be considered an extreme test of the robustness of a host-plasmid construction due to the strong selective nature of the device (Dykhuizen and Hartl, 1983).…”

Section: Discussionmentioning

confidence: 99%

“…The mechanism for the change in the phenotype of the culture from the second peak of 8-lactamase activity to the end of the run is clearly different from the mechanism which displaced the original strain with the lac permease mutant. The gradual convergence of the mutant cell count and the selective count after the second peak in production implies that selection in a chemostat not displacement of dead cells is taking place in which a second mutation, ZacP, is favored (Dykhuizen and Hartl, 1983). The lac superrepressor mutation has been speculated to arise at alower frequency than that of the uptake mutation (Miller, 1978(Miller, , 1979.…”

Section: Discussionmentioning

confidence: 99%

Escherichia coli Host Cell Modifications in Continuous Culture Affecting Heterologous Protein Overproduction: A Population Dynamics Study

Togna

Shuler

et al. 1992

Biotechnology Progress

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There are many published studies of plasmid segregational instability in Escherichia coli in the literature. However, the formation of plasmid-free segregants can be controlled by the addition of selective chemical agents like antibiotics. This solution has become commonplace in both the laboratory and industry. On the other hand, host cell modifications, which result in low production of plasmid-encoded protein and lead to loss of culture productivity, have not been adequately addressed. Continuous culture of an inducible (ptac) Escherichia coli vector containing strain, RB791(pKN), was characterized by strong dynamic changes in the cell population and product (beta-lactamase) expression. Long-term cultivation resulted in the loss of high-level production of beta-lactamase. Loss of productivity was not due to the formation of plasmid-free cells or structural modifications to the plasmid; instead, continuous operation resulted in a culture dominated by irreversibly altered, low-producing cells. Two distinct classes of lac- mutants which inhibited induction were identified (Y- and I(s)).

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“…Therefore, the assumption that the specific death rate of the Xcells by an antibiotic is linear to its concentration is valid around the optimum points in this range of E values. For most cases when the coexistence steady state exists over two ranges of dilution rates the maximum value of the performance index occurs within the higher range (or low Da in the range [3][4]. and (c) k, = 0 Llglh, k, = 0 h-I.…”

Section: Optimizationmentioning

confidence: 99%

Static characteristics of a continuous flow bioreactor containing antibiotic‐resistant recombinant cells

Chang

Lim

1987

Biotech & Bioengineering

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The steady-state behavior of a continuous bioreactor containing antibiotic-resistant recombinant cells has been investigated. Only the plasmid-free cell is susceptible to and killed by antibiotics. A Monod form of specific death rate was found to simulate quite well the experimental death rates of various cells due to antibiotics. The stability characteristics, including bifurcation of the possible steady states, are examined. Appropriate numerical illustrations for the steady-state characteristics have been provided. Theoretically, two coexistence steady states (CO), three partial washout steady states (PW), and one total washout steady state (TW) are feasible, but only one CO, one PW, and one TW were realized. When antibiotic consumption is not extremely significant the CO can exist over one or two ranges of dilution rates depending upon the antibiotic concentration in the feed. The CO is globally stable. Whenever the PW and/or the TW exist(s) together with the CO they are unstable. Sensitivity analyses for several key kinetic parameters have been made. The rate at which the plasmid-bearing cells revert to the plasmid-free cells has the most significant effect on the antibiotic susceptibility of the system. Some simplified optimization calculations for maximum profit have been carried out.

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Selection in chemostats.

Cited by 255 publications

References 55 publications

Adaptive mgl‐regulatory mutations and genetic diversity evolving in glucose‐limited Escherichia coli populations

Adaptive mgl‐regulatory mutations and genetic diversity evolving in glucose‐limited Escherichia coli populations

Escherichia coli Host Cell Modifications in Continuous Culture Affecting Heterologous Protein Overproduction: A Population Dynamics Study

Static characteristics of a continuous flow bioreactor containing antibiotic‐resistant recombinant cells

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