A continuous culture study of an obligately psychrophilic Pseudomonas species

The nutritional composition and cell size of microbial biomass for food applications are defined by the growth conditions

BACKGROUND

It is increasingly recognized that conventional food production systems are not able to meet the globally increasing protein needs, resulting in overexploitation and depletion of resources, and environmental degradation. In this context, microbial biomass has emerged as a promising sustainable protein alternative. Nevertheless, often no consideration is given on the fact that the cultivation conditions affect the composition of microbial cells, and hence their quality and nutritional value. Apart from the properties and nutritional quality of the produced microbial food (ingredient), this can also impact its sustainability. To qualitatively assess these aspects, here, we investigated the link between substrate availability, growth rate, cell composition and size of Cupriavidus necator and Komagataella phaffii.

RESULTS

Biomass with decreased nucleic acid and increased protein content was produced at low growth rates. Conversely, high rates resulted in larger cells, which could enable more efficient biomass harvesting. The proteome allocation varied across the different growth rates, with more ribosomal proteins at higher rates, which could potentially affect the techno-functional properties of the biomass. Considering the distinct amino acid profiles established for the different cellular components, variations in their abundance impacts the product quality leading to higher cysteine and phenylalanine content at low growth rates. Therefore, we hint that costly external amino acid supplementations that are often required to meet the nutritional needs could be avoided by carefully applying conditions that enable targeted growth rates.

CONCLUSION

In summary, we demonstrate tradeoffs between nutritional quality and production rate, and we discuss the microbial biomass properties that vary according to the growth conditions.

Long-Term Changes in Chemostat Cultures of Cytophaga johnsonae

Long-term studies with a gliding, heterotrophic bacterium, Cytophaga johnsonae, were conducted in a glucose-limited chemostat at a high and a low dilution rate. To test the stability of the steady state during long-term experiments the following parameters were monitored: optical density, glucose concentration, glucose uptake potential, ATP content of the cells, and plate counts on two different agar media. Biomass remained relatively constant, although the observed changes could have been possible in both directions. During all steady states, glucose uptake showed a stepwise increase and the glucose concentration showed a corresponding decrease. Glucose uptake potential and glucose concentration in the chemostat were inversely proportional. The ATP content of the cells varied up to 33% during the steady state, but did not show a general trend. After long cultivation in all chemostats, plate counts on both agars dropped to values less than 20% of the original steady-state level. These decreases were due to an inability of the cells to grow on agar plates, not to a lack of vitality of the cells in the chemostat. This study showed that even during shorter chemostat runs, e.g., 1 week, changes in important parameters with the steady state must be expected, especially in the uptake potential and the concentration of the limiting substrate.

Transient increase of ATP as a response to temperature up-shift in Escherichia coli

SUMMARY: BACKGROUND: Escherichia coli induces the heat shock response to a temperature up-shift which is connected to the synthesis of a characteristic set of proteins, including ATP dependent chaperones and proteases. Therefore the balance of the nucleotide pool is important for the adaptation and continuous function of the cell. Whereas it has been observed in eukaryotic cells, that the ATP level immediately decreased after the temperature shift, no data are available for E. coli about the adenosine nucleotide levels during the narrow time range of minutes after a temperature up-shift. RESULTS: The current study shows that a temperature up-shift is followed by a very fast significant transient increase of the cellular ATP concentration within the first minutes. This increase is connected to a longer lasting elevation of the cellular respiration and glucose uptake. Also the mRNA level of typical heat shock genes increases within only one minute after the heat-shock. CONCLUSION: The presented data prove the very fast response of E. coli to a heat-shock and that the initial response includes the increase of the ATP pool which is important to fulfil the need of the cell for new syntheses, as well as for the function of chaperones and proteases.

Function and regulation of temperature-inducible bacterial proteins on the cellular metabolism

Temperature is an important environmental factor which, when altered, requires adaptive responses from bacterial cells. While a sudden increase in the growth temperature induces a heat shock response, a decrease results in a cold shock response. Both responses involve a transient increase in a set of genes called heat and cold shock genes, respectively, and the transient enhanced synthesis of their proteins allows the stressed cells to adapt to the new situation. A sudden increase in the growth temperature results in the unfolding of proteins, and hydrophobic amino acid residues normally buried within the interior of the proteins become exposed on their surface. Via these hydrophobic residues which often form hydrophobic surfaces proteins can interact and form aggregates which may become life-threatening. Here, molecular chaperones bind to these exposed hydrophobic surfaces to prevent the formation of protein aggregates. Some chaperones, the foldases, allow refolding of these denatured proteins into their native conformation, while ATP-dependent proteases degrade these non-native proteins which fail to fold. Most chaperones and energy-dependent proteases are heat shock proteins, and their genes are either regulated by alternate sigma factors or by repressors. The cold shock response evokes two major threats to the cells, namely a drastic reduction in membrane fluidity and a transient complete stop of translation at least in E. coli. Membrane fluidity is restored by increasing the amount of unsaturated fatty acids and translation resumes after adaptation of the ribosomes to cold. Neither an alternative sigma factor nor a repressor seems to be involved in the regulation of the cold shock genes in E. coli, the only species studied so far in this respect.

Effect of temperature on sulphate reduction, growth rate and growth yield in five psychrophilic sulphate-reducing bacteria from Arctic sediments

Five psychrophilic sulphate-reducing bacteria (strains ASv26, LSv21, PSv29, LSv54 and LSv514) isolated from Arctic sediments were examined for their adaptation to permanently low temperatures. All strains grew at -1.8 degrees C, the freezing point of sea water, but their optimum temperature for growth (T(opt)) were 7 degrees C (PSv29), 10 degrees C (ASv26, LSv54) and 18 degrees C (LSv21, LSv514). Although T(opt) was considerably above the in situ temperatures of their habitats (-1.7 degrees C and 2.6 degrees C), relative growth rates were still high at 0 degrees C, accounting for 25-41% of those at T(opt). Short-term incubations of exponentially growing cultures showed that the highest sulphate reduction rates occurred 2-9 degrees C above T(opt). In contrast to growth and sulphate reduction rates, growth yields of strains ASv26, LSv54 and PSv29 were almost constant between -1.8 degrees C and T(opt). For strains LSv21 and LSv514, however, growth yields were highest at the lowest temperatures, around 0 degrees C. The results indicate that psychrophilic sulphate-reducing bacteria are specially adapted to permanently low temperatures by high relative growth rates and high growth yields at in situ conditions.

Evidence for two domains of growth temperature for the psychrotrophic bacterium Pseudomonas fluorescens MF0

The variations in the maximal specific growth rate of the psychrotrophic bacterium Pseudomonas fluorescens MF0 with respect to temperature were studied between 0 and 30 degrees C (optimal for growth). The Arrhenius plot showed a drastic change in slope at the intermediate temperature of 17 degrees C. Over the cold domain from 0 to 17 degrees C, the temperature characteristic was twofold higher than over the suboptimal domain from 17 to 30 degrees C. The macromolecular composition of exponentially growing cells was invariant over the entire range from 0 to 30 degrees C. Variations of temperature and growth rate were independently investigated through chemostat experiments in order to characterize their respective effects on cell macromolecular composition and size. The effect of growth rate in this psychrotrophic strain is identical to that of all other bacteria assayed so far. In contrast, an original biphasic variation of total protein concentration was demonstrated in strain MF0 with respect to temperature, with a maximum at 17 to 20 degrees C. Indeed, increasing the temperature in the chemostat resulted in a biphasic decrease in the net protein production rate: a very slight decrease below 17 degrees C and a much larger decrease from 17 to 28 degrees C. These results could signify an increase in the cellular protein degradation rate with increasing temperature, especially above 17 degrees C.

Cold shock and cold acclimation proteins in the psychrotrophic bacterium Arthrobacter globiformis SI55

The psychrotrophic bacterium Arthrobacter globiformis SI55 was grown at 4 and 25 degrees C, and the cell protein contents were analyzed by two-dimensional electrophoresis. Cells subjected to cold shocks of increasing magnitude were also analyzed. Correspondence analysis of protein appearance distinguished four groups of physiological significance. Group I contained cold shock proteins (Csps) overexpressed only after a large temperature downshift. Group II contained Csps with optimal expression after mild shocks. Group III contained proteins overexpressed after all cold shocks. These last proteins were also overexpressed in cells growing at 4 degrees C and were considered to be early cold acclimation proteins (Caps). Group IV contained proteins which were present at high concentrations only in 4 degrees C steady-state cells and appeared to be late Caps. A portion of a gene very similar to the Escherichia coli cspA gene (encoding protein CS7.4) was identified. A synthetic peptide was used to produce an antibody which detected a CS7.4-like protein (A9) by immunoblotting two-dimensional electrophoresis gels of A. globiformis SI55 total proteins. Unlike mesophilic microorganisms, this CS7.4-like protein was still produced during prolonged growth at low temperature, and it might have a particular adaptive function needed for balanced growth under harsh conditions. However, A9 was induced at high temperature by chloramphenicol, suggesting that CS7.4-like proteins have a more general role than their sole implication in cold acclimation processes.

Physiology of yeasts in relation to biomass yields

The stoichiometric limit to the biomass yield (maximal assimilation of the carbon source) is determined by the amount of CO2 lost in anabolism and the amount of carbon source required for generation of NADPH. This stoichiometric limit may be reached when yeasts utilize formate as an additional energy source. Factors affecting the biomass yield on single substrates are discussed under the following headings: Energy requirement for biomass formation (YATP). YATP depends strongly on the nature of the carbon source. Cell composition. The macroscopic composition of the biomass, and in particular the protein content, has a considerable effect on the ATP requirement for biomass formation. Hence, determination of for instance the protein content of biomass is relevant in studies on bioenergetics. Transport of the carbon source. Active (i.e. energy-requiring) transport, which occurs for a number of sugars and polyols, may contribute significantly to the calculated theoretical ATP requirement for biomass formation. P/O-ratio. The efficiency of mitochondrial energy generation has a strong effect on the cell yield. The P/O-ratio is determined to a major extent by the number of proton-translocating sites in the mitochondrial respiratory chain. Maintenance and environmental factors. Factors such as osmotic stress, heavy metals, oxygen and carbon dioxide pressures, temperature and pH affect the yield of yeasts. Various mechanisms may be involved, often affecting the maintenance energy requirement. Metabolites such as ethanol and weak acids. Ethanol increases the permeability of the plasma membrane, whereas weak acids can act as proton conductors. Energy content of the growth substrate. It has often been attempted in the literature to predict the biomass yield by correlating the energy content of the carbon source (represented by the degree of reduction) to the biomass yield or the percentage assimilation of the carbon source. An analysis of biomass yields of Candida utilis on a large number of carbon sources indicates that the biomass yield is mainly determined by the biochemical pathways leading to biomass formation, rather than by the energy content of the substrate.

Psychrophilic and psychrotrophic microorganisms

Psychrophilic and psychrotrophic microorganisms have the ability to grow at 0 degree C. Psychrotrophic microorganisms have a maximum temperature for growth above 20 degrees C and are widespread in natural environments and in foods. Psychrophilic microorganisms have a maximum temperature for growth at 20 degrees C or below and are restricted to permanently cold habitats. This ability to grow at low temperature may be correlated with a lower temperature characteristic than that of the mesophiles, an increasing proportion of unsaturated fatty acids in the lipid phase of the cell membrane, which makes it more fluid, and a protein conformation functional at low temperature. The relatively low maximum temperature of growth for these microorganisms is often considered to be due to the thermolability of one or more essential cellular components, particularly enzymes, while some degradative activities are enhanced, resulting in an exhaustion of cell energy, a leakage of intracellular substances or complete lysis. Psychrotrophic microorganisms are well-known for their degradative activities in foods. Some are pathogenic or toxinogenic for man, animals or plants. However in natural microbial ecosystems psychrotrophic and psychrophilic microorganisms can play a large role in the biodegradation of organic matter during cold seasons.

Energetics of growth of Microbacterium thermosphactum at low temperatures

Microbacterium thermosphactum was grown at 5 degrees C in glucose-limited continuous cultures. The end products of glucose metabolism were L-lactate and ethanol, and these compounds accounted for 86--92% of the glucose utilized. With input glucose concentrations less than 3 mV, YgluMax was found to be 40--43, YATPMax 20--21 and ms 0.1--0.2. These values are almost identical to those found previously for cultures at 25 degrees C and show that this psychrotroph grows with a very high energetic efficiency over a wide range of temperatures. With a higher (but still limiting) input glucose concentration of 5.6 mM at 9 degrees C, cellular efficiency declined as there was a marked reduction in Yglu. This decrease was accounted for in mathematical terms by an increase in ms to 0.7, whilst YgluMax and gh energetic efficiency over a wide range of temperatures. With a higher (but still limiting) input glucose concentration of 5.6 mM at 9 degrees C, cellular efficiency declined as there was a marked reduction in Yglu. This decrease was accounted for in mathematical terms by an increase in ms to 0.7, whilst YgluMax and gh energetic efficiency over a wide range of temperatures. With a higher (but still limiting) input glucose concentration of 5.6 mM at 9 degrees C, cellular efficiency declined as there was a marked reduction in Yglu. This decrease was accounted for in mathematical terms by an increase in ms to 0.7, whilst YgluMax and YATPMax remained high at 38 and 19 respectively.

Phenotypic changes in the chemistry of Aspergillus nidulans: influence of culture conditions on mycelial composition

A quantitative study was made of macromolecular (nucleic acids, protein), carbohydrate and mineral (magnesium, potassium and phosphorus) components of Aspergillus nidulans in glucose limited chemostat cultures, under varying conditions of dilution rate, temperature, pH and NaCl concentration. The overall mineral content showed greatest variation in response to changes in culture salinity, which also affected the mycelial carbohydrate content. Concomitant and opposite changes in the content of cations and carbohydrates under conditions of increasing salinity may be interpreted in terms of mycelial osmoregulation. Slight variations in DNA content but gross fluctuations in the level of RNA were noted under the different cultural conditions examined. Co-ordinate changes in RNA and Mg2+ contents were evident only under certain conditions: dilution rate from 0.05--0.07 h-1 or temperature from 22--30 degrees C. The constant molar stoichiometry between RNA and Mg2+ characteristic of unicellular microorganisms was not a feature of fungal growth. The protein content was most affected by shifts of temperature and reached minimal values at 25 and 50 degrees C. The growth environment had a marked influence on the protein synthesising activity of RNA, which increased eightfold as the dilution rate was increased from 0.02--0.175 h-1, doubled within the temperature range 20--30 degrees C and fell by 50% between 40 and 50 degrees C. These observations are discussed in the context of the constant ribosomal efficiency in protein synthesis hypothesis.

Heterotrophic Uptake Experiments with 14 C-Labeled Histidine in a Histidine-Limited Chemostat

The histidine uptake by bacterial strain HIS 42 was determined with [ U - 14 C]histidine and through oxygen uptake experiments on samples taken from a histidine-limited chemostat. The uptake of [ U - 14 C]histidine was characterized by a saturation constant of 12.8 to 78.6 nM histidine. At higher growth rates, the measured maximum uptake rate of histidine was lower than the actual uptake rate in the culture. The percentage of respired substrate (76 to 93%) was about 30 to 40% higher than the comparable value for the culture. The uptake of histidine as analyzed through the measurement of oxygen uptake rates was characterized by a saturation constant of 1.7 to 10.5 μM histidine; the maximum uptake rate was always greater than the actual histidine uptake rate in the culture. By the application of the two cited methods, set up to determine the histidine uptake kinetics, two different uptake processes were analyzed. It appeared that the determination of the histidine uptake through measurement of the oxygen uptake rate showed a better reflection of the actual uptake process of histidine in the culture. With the available data it was impossible to assess a correlation between the uptake of histidine, as determined with [ U - 14 C]histidine, and the actual metabolism of the bacterial population.

Effects of growth temperature on yield and maintenance during glucose-limited continuous culture of Escherichia coli

The effects of growth temperature on the aerobic growth yield with respect to oxygen consumption (Y0-grams [dry weight] per gram-atom of O) and the rate of maintenance respiration (m0-milligram-atoms of O/gram [dry weight] per hour) are reported for Escherichia coli B cultivated continuously in the presence of oxygen with limiting glucose. During anaerobic continuous culture, YATP(max) (grams [dry weight] per mole of ATP corrected for maintenance) increases from 10.3 to 12.7 as the growth temperature is lowered from 37 to 25 C. Over this same range, Y0(max) (Y0 corrected for maintenance respiration) rises from 12.5 to 28.8 and remains at the higher value down to 17.5 C. From 37 to 32 C, m0 increases from 0.9 to 4.4 but then falls to 1.5 as the temperature is lowered to 17.5 C. The value of m0 sharply rises some 13-fold as the temperature is raised to 42 C without a significant change in the value of Y0(max). Changes of Y0(max) are consistent with a temperature-sensitive doubling of the efficiency of oxidative phosphorylation, but the reasons for the changes of the rate of maintenance respiration are not known.

Psychrophilic bacteria

Images

Similarity in several properties of psychorophilic bacteria grown at low and moderate temperatures

Several properties of psychrophilic pseudomonads were studied with cells grown in batch culture in nutrient broth at 2 and 30 C. No differences were observed in the size, catalase activity, deoxyribonucleic acid, ribonucleic acid, or protein content of cells grown at either temperature. The importance of comparing physiologically similar cells is discussed.

Effects of temperature on composition and cell volume of Candida utilis

Candida utilis NCYC 321 was grown in steady-state culture in a chemostat under glucose limitation or NH(4) (+) limitation at temperatures of 30, 25, 20, and 15 C and at dilution rates (equal to growth rates) in the range of 0.35 to 0.05 hr(-1). Deoxyribonucleic acid contents of cells grown under the various conditions remained approximately constant, but the contents of several other cell components varied. Over the range of 30 to 15 C, the greatest differences were in the ribonucleic acid (RNA) and protein contents of cells grown under NH(4) (+) limitation, which increased as the temperature was decreased. The contents of other components, particularly adenosine triphosphate in cells grown under glucose limitation, varied more when the cells were grown at different rates at a fixed temperature. Cells grown at a fixed rate under NH(4) (+) limitation increased in volume as the temperature was decreased below 30 C. The increase in volume was closely correlated with increases in the proportions of RNA and protein in the dry weight of cells. Cells grown under glucose limitation showed much smaller increases in volume; these increases were poorly correlated with the increased RNA content and hardly at all with the increased protein content. Increases in volume with a decrease in growth temperature from 30 to 20 C were also demonstrated in cells grown under phosphate limitation and to a much smaller extent in cells grown under glycerol limitation. The increased RNA synthesized at low temperatures by cells grown under NH(4) (+) limitation was found almost exclusively in the 40,000 x g supernatant fluid, but only about 40% of it sedimented at 100,000 x g. Cells grown at a fixed rate under NH(4) (+) limitation synthesized less total carbohydrate when the temperature was decreased from 30 to 15 C. This decrease was mainly in the trichloroacetic acid-soluble fraction (probably trehalose) and in the intracellular hot alkali-soluble glucan (probably glycogen). Cells grown at a fixed rate under glucose limitation showed a small increase in carbohydrate content as the temperature was decreased from 30 to 15 C.