Growth Rates In Chemostats Research Articles

Stoichiometric flexibility in diverse aquatic heterotrophic bacteria is coupled to differences in cellular phosphorus quotas.

It is frequently presumed that heterotrophic bacteria from aquatic environments have low carbon (C) content, high phosphorus (P) content, and maintain homeostasis at low C:P in their biomass. Dissolved and particulate organic matter from primary producers in terrestrial and aquatic environments typically has high C:P ratios, suggesting that heterotrophic bacteria consuming this resource experience stoichiometric imbalance in C and P. The strength of elemental homeostasis is important for understanding how heterotrophic bacteria couple C and P cycles in response to environmental change, yet these generalizations are based upon data from only a few species that might not represent the physiology of bacteria in freshwaters. However, recent research has indicated that some strains of bacteria isolated from freshwaters have flexible C:P stoichiometry and can acclimate to changes in resource C:P. Although it is apparent that strains differ in their biomass C:P and flexibility, the basis for these characteristics has not been explained. We evaluated biomass C:P homeostasis in 24 strains of bacteria isolated from temperate lakes using a uniform relative growth rate in chemostats. Overall, the strains exhibited a range of homeostatic regulation from strong homeostasis to highly flexible biomass stoichiometry, but strains that were isolated using P-rich media formulations were more homeostatic than strains isolated using P-poor media. Strains exhibiting homeostatic biomass C:P had high cellular C and P content and showed little morphological change between C and P limitation. In contrast, stoichiometrically flexible strains had low P quotas and increased their C quotas and cell size under P limitation. Because stoichiometric flexibility is closely coupled to absolute P content in bacteria, anthropogenic inputs of P could lead to prevalence of more homeostatic bacteria, reducing the ability of natural assemblages to buffer changes in the availability of P and organic C.

Subpopulation-proteomics reveal growth rate, but not cell cycling, as a major impact on protein composition in Pseudomonas putida KT2440.

Population heterogeneity occurring in industrial microbial bioprocesses is regarded as a putative effector causing performance loss in large scale. While the existence of subpopulations is a commonly accepted fact, their appearance and impact on process performance still remains rather unclear. During cell cycling, distinct subpopulations differing in cell division state and DNA content appear which contribute individually to the efficiency of the bioprocess. To identify stressed or impaired subpopulations, we analyzed the interplay of growth rate, cell cycle and phenotypic profile of subpopulations by using flow cytometry and cell sorting in conjunction with mass spectrometry based global proteomics. Adjusting distinct growth rates in chemostats with the model strain Pseudomonas putida KT2440, cells were differentiated by DNA content reflecting different cell cycle stages. The proteome of separated subpopulations at given growth rates was found to be highly similar, while different growth rates caused major changes of the protein inventory with respect to e.g. carbon storage, motility, lipid metabolism and the translational machinery.In conclusion, cells in various cell cycle stages at the same growth rate were found to have similar to identical proteome profiles showing no significant population heterogeneity on the proteome level. In contrast, the growth rate clearly determines the protein composition and therefore the metabolic strategy of the cells.

Slow Growth Induces Heat-Shock Resistance in Normal and Respiratory-deficient Yeast

Yeast cells respond to a variety of environmental stresses, including heat shock and growth limitation. There is considerable overlap in these responses both from the point of view of gene expression patterns and cross-protection for survival. We performed experiments in which cells growing at different steady-state growth rates in chemostats were subjected to a short heat pulse. Gene expression patterns allowed us to partition genes whose expression responds to heat shock into subsets of genes that also respond to slow growth rate and those that do not. We found also that the degree of induction and repression of genes that respond to stress is generally weaker in respiratory deficient mutants, suggesting a role for increased respiratory activity in the apparent stress response to slow growth. Consistent with our gene expression results in wild-type cells, we found that cells growing more slowly are cross-protected for heat shock, i.e., better able to survive a lethal heat challenge. Surprisingly, however, we found no difference in cross-protection between respiratory-deficient and wild-type cells, suggesting induction of heat resistance at low growth rates is independent of respiratory activity, even though many of the changes in gene expression are not.

Genes for two multicopper proteins required for Fe(III) oxide reduction in Geobacter sulfurreducens have different expression patterns both in the subsurface and on energy-harvesting electrodes

Previous studies have shown that Geobacter sulfurreducens requires the outer-membrane, multicopper protein OmpB for Fe(III) oxide reduction. A homologue of OmpB, designated OmpC, which is 36 % similar to OmpB, has been discovered in the G. sulfurreducens genome. Deletion of ompC inhibited reduction of insoluble, but not soluble Fe(III). Analysis of multiple Geobacter and Pelobacter genomes, as well as in situ Geobacter, indicated that genes encoding multicopper proteins are conserved in Geobacter species but are not found in Pelobacter species. Levels of ompB transcripts were similar in G. sulfurreducens at different growth rates in chemostats and during growth on a microbial fuel cell anode. In contrast, ompC transcript levels increased at higher growth rates in chemostats and with increasing current production in fuel cells. Constant levels of Geobacter ompB transcripts were detected in groundwater during a field experiment in which acetate was added to the subsurface to promote in situ uranium bioremediation. In contrast, ompC transcript levels increased during the rapid phase of growth of Geobacter species following addition of acetate to the groundwater and then rapidly declined. These results demonstrate that more than one multicopper protein is required for optimal Fe(III) oxide reduction in G. sulfurreducens and suggest that, in environmental studies, quantifying OmpB/OmpC-related genes could help alleviate the problem that Pelobacter genes may be inadvertently quantified via quantitative analysis of 16S rRNA genes. Furthermore, comparison of differential expression of ompB and ompC may provide insight into the in situ metabolic state of Geobacter species in environments of interest.

Temperature Affects Stoichiometry and Biochemical Composition of Escherichia coli

Temperature is a master variable controlling biochemical processes in organisms, and its effects are manifested on many organizational levels in organisms and ecosystems. We examined the effects of temperature on the biochemical composition and stoichiometry of a model heterotrophic bacterium, Escherichia coli K-12, held at constant growth rate in chemostats. Increasing temperature led to increased cellular organic carbon (C) and organic nitrogen (N) with decreased phosphorus (P) content, leading to increased C/P and N/P biomass ratios. P content was related to cellular RNA, which is P-rich (9-10% by weight) and nonnucleic acid P (presumably composed of mostly phospholipids, intracellular phosphate, and polyphosphate). These results indicate that E. coli allocates an increased proportion of its P cell quota toward assembly (ribosomes) at low temperatures and an increasing proportion toward resource acquisition machinery (membranes) at higher temperatures. If these results are relevant to the behavior of prokaryotic heterotrophs in natural settings (the gut, soils, lakes, oceans, etc.), it suggests greater nutrient regeneration and less microbial nutrient retention as temperatures increase.

Survival of Plasmid-Containing Bacillus subtilis Released into Mushroom Compost

The survival of a plasmid-containing Bacillus subtilis released into mushroom compost was investigated. The indigenous Bacillus population of mushroom compost exhibited an antibiotic-resistance profile that was distinguished by almost complete absence of chloramphenicol resistance. Bacillus subtilis containing the chloramphenicol-resistance plasmid pC194 was released into mushroom compost microcosms and populations were monitored at different incubation temperatures. The organism colonized both sterile and untreated compost at 37°C, and to a lesser extent at 50°C, but was eliminated after 30 d at 65°C. Although sporulation of the B. subtilis population occurred within compost, the population was maintained for up to 13 weeks at 50°C, largely as vegetative cells. Experiments in which the B. subtilis host strain, without plasmid, was released demonstrated that plasmid carriage had no effect on the ability of the bacterium to colonize and survive in compost. Furthermore, the size and composition of the indigenous bacterial population was unaffected by the presence of the introduced B. subtilis strain. Virtually no loss of plasmid pC194 from the B. subtilis population in compost was observed, and experiments at low growth rates in chemostats confirmed the stability of this host/vector system in the absence of positive selection pressure.

Energy balance calculations as a tool to determine maintenance energy requirements under stress conditions

Abstract

Potential for hitchhiking in the eda-edd-zwf gene cluster of Escherichia coli.

SUMMARYThe locieda, eddandzwfform a tightly linked cluster inE. colithat functions in the metabolism of galacturonate, gluconate and glucose. This cluster has been transferred from six natural isolates into the genetic background ofE. coliK12 and examined with regard to effects on growth rate in chemostats. Although the naturally occurringedaandzwfalleles are selectively neutral, theeddalleles are not. Theeddalleles fall into three functional classes distinguished by their effects on growth rate in gluconate medium, the most common classes differing in fitness by approximately 1% per hour. This extensive non-neutral genetic variation ofeddis discussed in light of the evident rarity of gluconate as a natural substrate. We propose that gluconate selection is intermittent in space or time, providing the population an opportunity to accumulate non-neutral genetic variants during periods of relaxed selection. Such genetic variants will eventually be sorted out by the intermittent periods of gluconate selection, and during these periods the linkededaandzwfalleles will experience pronounced hitchhiking effects.

Measurement of the maximum specific growth rate in chemostat of Pseudomonas spp. with different abilities for biofilm formation

Attempts to measure the maximum specific growth rate (μmax) using the method of wash-out were made on two Pseudomonas spp. with different abilities for biofilm formation. The apparent “μmax” of the strain with the highest biofilm activity (P. putida ATCC 11172) was governed by the dilution rate of the steady-state preceding the wash-out experiment. The apparent “μmax” was increased from 0.4 to 3.8 h−1 by changing the dilution rate of the preceding steady-state from 0.3 to 2.5 h−1. The μmax determined from a single batch culture was 0.59 h−1.

The cell size and macromolecular composition of Aerobacter aerogenes in various systems of continuous culture

1. 1. The cell size (σ) and the macromolecular composition of Aerobacter aerogenes has been studied over a wide range of growth rates in chemostats and at the optimum rate in the turbidostat. A minimal medium was used and in the chemostats the growth rate was controlled by limiting in turn the concentration of glucose, (NH 4) 2SO 4(N), K +, Mg 2+, SO 4 2− and PO 4 3−. 2. 2. α was higher in the turbidostat than in any chemostat, where it varied little in any given system at dilution rates between 0.1 and 0.75 h −1. σ was significantly higher when K + was limited and slightly higher in Mg 2+-limitation than in the other chemostats. 3. 3. In the C-limited system, in contrast to other reports, the cell mass did not decrease as the growth rate was decreased; rather there was a slight increase. In conditions of N-limitation the increase was more marked and in the other systems it was considerable. 4. 4. The composition of the cells is discussed in terms of the role of the various nutrients and a molar proportionality of C:N:PO 4 3−:K +:SO 4 2−:Mg 2+ = 1433:183: 8.3:4.5:1.5:1 claimed. Saturation constants for Mg 2+, K + and SO 4 2− are reported. 5. 5. Turbidimetric and dry weight determinations of cell mass are critically assessed.