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A G Fredrickson

Publications and source records attributed to A G Fredrickson.

At least 19 recordsLinked to original sources
PubMed1992

A statistical analysis of flow cytometric determinations of phagocytosis rates.

Uptake of particles by phagocytosing cells is a process that exhibits variability of its rate. This variability is inherent in the mechanism of particle uptake and in the mechanisms that determine the distribution of physiological states within a population of phagocytosing cells. When numbers of particles ingested by cells are determined flow cytometrically an additional measurement variability is superimposed on and interacts with the aforementioned biological variability. In one method of determining population phagocytosis parameters, which involves fitting theoretical equations to experimental time course data on the fractions of cells which have ingested 0, 1, 2, ... particles, the effects of measurement variability are circumvented, although this usually has the cost of not using all the sample data obtained. However, in a second, simpler, method which is based on determining the time course of the number of particles ingested by an average cell, measurement variability is not circumvented and its effects must be considered. An analysis of the combined effects of biological and measurement variability on the results obtained with the simpler method is presented in this paper. Experimental results for phagocytosis of latex microspheres of uniform size and fluorochrome content by populations of the ciliate Tetrahymena pyriformis show that, for this system, measurement variability is entirely negligible in comparison with biological variability. This conclusion might not apply to other systems, however, and situations which might make measurement variability of some significance are mentioned in the paper. The equations given can be used for the analysis of such situations.

Analysis of Variance↗
PubMed1990

Flow cytometric measurement of rates of particle uptake from dilute suspensions by a ciliated protozoan.

Flow cytometry is used to measure rates of ingestion of particles from dilute monodisperse suspensions by the ciliate Tetrahymena pyriformis. The particles used are polystyrene microspheres containing a fluorescent dye. Measurements were made directly, that is, by determining the fluorescence intensities from microspheres ingested by cells in samples collected from the experimental feeding apparatus. The fact that fluorescence intensities from individual cells can be grouped into discrete classes based on the numbers of fluorescent particles associated with the cells makes it possible to calibrate the flow cytometer and convert fluorescence measurements into numbers of particles ingested by average cells. At low particle concentration or high ciliate concentration, ingestion data must be corrected for depletion of particles during the assay, and a method for doing this is described. Experiments at various ciliate concentrations show that ingestion rates are not affected by this concentration. The methods developed should allow measurements of rates of ingestion of particles from concentrated and polydisperse suspensions. For such measurements, nonfluorescent particles together with a fraction of fluorescent tracer particles would be used.

Animals↗
PubMed1990

A discrete, stochastic model for microbial filter feeding: a model for feeding of ciliated protists on spatially uniform, nondepletable suspensions.

Suspension-feeding ciliates, either bacteriovorous or planktonic, are adapted to feed on particulate food matter of size much smaller than their own size. These microorganisms collect their prey by generating water currents that draw prey toward their capture surfaces. Under such conditions food particles are treated in bulk, and captures of individual food particles from a suspension by individual single-celled organisms are discrete events that occur at random intervals of time. Each such event is followed by a sequence of additional events that also occur at random intervals of time. This sequence culminates in the incorporation of the digestible portion of the food particle into the cell's cytoplasm and the expulsion of the indigestible portion from the cell. In theory, the rate of the overall ingestion-digestion process can be limited by the passage of particles through any stage of this sequence of events. In this paper, we assume that only the initial events in the sequence, those that occur in the oral region of the cell, limit the rate of the ingestion-digestion process, and we develop a discrete, stochastic model of filter feeding based on that assumption. We use the model to show how advanced instrumentation, such as flow cytometry, can be used to measure parameters of the model and also to answer a number of important questions about the mechanism of filter feeding. We show also how the model can be applied to nonhomogeneous cell populations for which parameters of the model are distributed.

Animals↗
PubMed1981

Microbial competition.

Populations of microorganisms inhabiting a common environment complete for nutrients and other resources of the environment. In some cases, the populations even excrete into the environment chemicals that are toxic or inhibitory to their competitors. Competition between two populations tends to eliminate one of the populations from their common habitat, especially when competition is focused on a single resource and when the populations do not otherwise interact. However, a number of factors mitigate the severity of competition and thus competitors often coexist.

Bacterial Physiological Phenomena↗
PubMed1981

Kinetics of growth of the ciliate Tetrahymena pyriformis on Escherichia coli.

The growth of the ciliate Tetrahymena pyriformis on non-growing Escherichia coli has been studied by following the time courses of population densities and protozoan mean cell volume in batch cultures. Viable, non-encysted protozoa always stopped feeding before the bacterial density was reduced to zero and non-feeding ciliates tended to swim faster than feeding ciliates. In addition, the number of bacteria and other particles of bacterial size consumed in the formation of one new ciliate, when averaged over the lag and reproductive phases of a culture, declined toward a limiting value of about 1.6 x 10(4) particles per ciliate as the initial density of such particles was increased.

Animals↗
PubMed1976

Protozoan feeding and bacterial wall growth.

Monod's model is often assumed to describe the kinetics of feeding of a protozoan population on a bacterial population in a chemostat. An earlier study (J. L. Jost et al., J. Bacteriol., 113, 84 (1973)) of the feeding of Tetrahymena pyriformis on either Escherichia coli or Azotobacter vinelandii found that this model correctly predicted the occurrence of sustained oscillations of population densities but made predictions of minimum bacterial population densities that were much smaller than those observed. The earlier study removed the discrepancy between the model and data by replacing Monod's model with a different model. It is shown in the present study that the discrepancy can be explained equally as well if Monod's model for the feed relation is retained and if, in addition, growth of bacteria on the chemostat walls is allowed for in the model equations.

Bacteria↗
PubMed1976

Dynamics of mixed cultures of Lactobacillus plantarum and Propionibacterium shermanii.

The mixed culture of Lactobacillus plantarum and Propionibacterium shermanii grown anaerobically in glucose minimal medium exhibits features typical of a commensal interaction even though a number of complicating factors, such as a large maintenance requirement of L. plantarum and inhibition of growth of P. shermanii at low pH, are present. A simple mathematical model of the system is presented and is shown to reproduce rather well some of the features of the continuous mixed culture system in both steady-state and transient situations.

Bacteriological Techniques↗
PubMed1976

Damped oscillations in continuous culture of Lactobacillus plantarum.

Lactobacillus plantarum exhibited long-period damped oscillations when grown aerobically with glucose as the rate-limiting substrate. Hydrogen peroxide accumulated in the cultures and its concentration also exhibited damped oscillations. It is suggested that inhibition of growth by hydrogen peroxide is involved in the production of the damped oscillations of microbiol population density.

Aerobiosis↗
PubMed1974

Diauxic growth of Propionibacterium shermanii.

Propionibacterium shermanii has been anaerobically propagated in batch and continuous culture with glucose and/or lactate as energy source. Specific growth rate on lactate was observed to be the same as that on glucose. In terms of cell density, the yield on glucose is higher than the yield on lactate. But the molar ratio of yield on glucose to that on lactate, 8.35, is in good agreement with the theoretical value of 8. In a mixture of glucose and lactate, P. shermanii showed diauxic growth. It used lactate before glucose utilization began. Neither temporary growth cessation nor two distinct growth phases were observed. A mathematical model is proposed to describe the diauxic growth.

Anaerobiosis↗
PubMed1973

Interactions of Tetrahymena pyriformis, Escherichia coli, Azotobacter vinelandii, and glucose in a minimal medium.

A study was made of the food web formed from a protozoon, two bacteria, and a glucose minimal medium in chemostat culture. The system was also divided into simpler parts, first by omitting the protozoon to obtain a competition system, and then by omitting one or the other of the bacteria to obtain two food chains. In the competition studies, one bacterium was displaced by the other at all holding times used. In the food chain studies, sustained oscillations of the population densities of predator and prey developed at short holding times, and then changed to damped oscillations at longer holding times. In addition, the level of residual glucose remained high at long holding times. A new model of microbial growth is necessary to explain these results. In the food web studies, predation of the protozoon on the two bacteria stabilized the competition between the latter and allowed their coexistence in the same habitat. Thus, Gause's principle was circumvented.

Antibiosis↗
PubMed1972

Batch- and continuous-culture transients for two substrate systems.

Batch growth of Escherichia coli in the presence of equal initial concentrations of glucose and a secondary substrate (xylose) is characterized by sequential utilization of the substrates, whereas continuous-culture systems with equal concentrations of the two substrates in the feed are characterized by complete utilization of both substrates at both high and low dilution rates. Such systems at steady state at a low dilution rate, when suddenly shifted to a higher dilution rate, experience a transient drop in population density accompanied by accumulation of the secondary substrate but virtually no accumulation of glucose. Systems at steady state with 200 mg of glucose per liter were found to undergo a transient population decrease and eventual recovery when switched to feed containing 200 mg of a secondary substrate per liter.

Bacteriological Techniques↗
PubMed1972

Predator-prey interactions of Dictyostelium discoideum and Escherichia coli in continuous culture.

Dictyostelium discoideum and Escherichia coli were aerobically propagated in mixed continuous culture in a predator-prey relationship, and the effects of temperature and holding times were examined. Oscillations developed in the concentration of glucose, the limiting substrate for E. coli, and in the densities of the two populations, but eventually steady-state populations were reached. The experimental data were analyzed according to the Lotka-Volterra model for prey-predator relationships and by the Monod model for saturation kinetics. A comparison of the adequacy of the two models in describing predation is given.

Aerobiosis↗