The holding time in pure and mixed culture fermentations.

Continuous mixed culture fermentations have been studied in the continuous-stirred tank reactor. The residence or holding time, theta, is important in determining which of two mixed organisms shall dominate in numbers. Continuous ethanol and acetic-acid fermentations are known to the brewing industry. The continuous production of ethanol and acetic acid are contingent upon the cells of Saccharomyces and Acetobacter being alive and growing. These are known as growth-associated products. On the other hand, alpha-amylase and glucamylase, or fungal amylase, are known as nongrowth-associated products or secondary metabolites. The organisms that produce the secondary metabolites, for example, penicillin, cephalosporin, streptomycin, and aureomycin, undergo differentiation and growth. These are the higher microbial forms and are produced by batchwise fermentation of molds and actinomycetes. It is submitted that these higher microbial forms can be grown continuously and produce secondary metabolites in amounts so as to make the processes economically viable. It is possible to grow the organisms continuously in plug flow reactors so that the secondary metabolites that have hitherto not been considered for continuous production be so considered.

Kinetics of the removal of iron pyrite from coal by microbial catalysis.

Different strains of Thiobacillus ferrooxidans and Thiobacillus thiooxidans were used to catalyze the oxidative dissolution of iron pyrite, FeS(2), in nine different coal samples. Kinetic variables and parametric factors that were determined to have a pronounced effect on the rate and extent of oxidative dissolution at a fixed Po(2) were: the bacterial strain, the nitrogen/phosphorus molar ratio, the partial pressure of CO(2), the coal source, and the total reactive surface area of FeS(2). The overall rate of leaching, which exhibited a first-order dependence on the total surface area of FeS(2), was analyzed mathematically in terms of the sum of a biochemical rate, nu(1), and a chemical rate, nu(2). Results of this study show that bacterial desulfurization (90 to 98%) of coal samples which are relatively high in pyritic sulfur can be achieved within a time-frame of 8 to 12 days when pulp densities are

Nitrogen requirement of iron-oxidizing thiobacilli for acidic ferric sulfate regeneration.

Ammonium was shown to be a limiting nutrient for iron oxidation in cultures of Thiobacillus ferrooxidans. In addition, one strain was also able to assimilate nitrate, but not nitrite, for growth and coupled iron oxidation. Some amino acids (0.5 mM) were tested as a source of nitrogen; none clearly stimulated bacterial activity and inhibition was commonly encountered. Complex nitrogenous compounds were inhibitory at high concentrations (0.1 to 0.5%, wt/vol) and, at low concentrations, some clearly stimulated the bacterial iron oxidation in ammonium-limited cultures. Enhancement of iron oxidation by these compounds was also observed in ammonium-unlimited cultures, suggesting their possible role in providing trace nutrients and possibly carbon for the bacteria.

Growth kinetics of Colpoda steinii on Escherichia coli.

Colpoda steinii was grown in two-stage continuous cultures with Escherichia coli as prey species. The concentration of prey and the ciliate mean cell volume, dry weight, and number per milliliter were determined at known growth rates. Steady states were reached in the second-stage continuous cultures at all growth rates. Although changes occurred in mean cell size of the ciliates and in the number per milliliter at various growth rates, the yield of protozoan biomass per unit of prey consumed was constant at all growth rates. The data were compared with several equations proposed to describe the kinetics of protozoan growth as a function of prey density.

Predation of Escherichia coli by Colpoda steinii.

A study of single-stage chemostat cultures of Colpoda steinii. Escherichia coli, and glucose is reported here. Two levels of glucose were fed as the limiting nutrient to the chemostat cultures. The cultures were studied at three holding times. Oscillations developed at short holding time and damped oscillations developed a long-residence times that approached steady-state conditions of populations of C. steinii and E. coli and concentrations of glucose. The experimental data are fitted to and compared with Jost's model.

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.

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.

Cell size as an indicator of changes in intracellular composition of Azotobacter vinelandii.

Cell size, measured electronically, was correlated to changes in cellular composition, number, and morphology of Azotobacter vinelandii OP during batch growth. The effect of a changing abiotic environment on these features of the cell is discussed. For this organism exponential growth was unbalanced growth and cell-size change was a sensitive indicator of this growth pattern. Cell-size measurements have the potential to give a rapid assessment of intracellular compositional changes.

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.

Differential counting in mixed cultures with coulter counters.

A critical comparison of Coulter, viable, and microscope counts for several mixed cultures of microorganisms has been made. This investigation shows that Coulter counting can provide reliable estimates of microbial numbers in mixed cultures. Precautions and limitations of Coulter counting in mixed cultures are discussed.

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.

Hydrodynamic focusing and electronic cell-sizing techniques.

The technique of hydrodynamic focusing, used to improve the resolution of the Coulter counter for the sizing of bacteria, was examined. Latex particles of 0.26 mum to 6.7 mum volume were used to examine the characteristics of the system with and without hydrodynamic focusing. The system then was evaluated for sizing mixed bacterial populations as well as single populations. Possible applications are also discussed.

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.

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.