DIALYSIS FLASK FOR CONCENTRATED CULTURE OF MICROORGANISMS

Synthetic Microbial Ecology: Engineering Habitats for Modular Consortia

The metabolic diversity present in microbial communities enables cooperation toward accomplishing more complex tasks than possible by a single organism. Members of a consortium communicate by exchanging metabolites or signals that allow them to coordinate their activity through division of labor. In contrast with monocultures, evidence suggests that microbial consortia self-organize to form spatial patterns, such as observed in biofilms or in soil aggregates, that enable them to respond to gradient, to improve resource interception and to exchange metabolites more effectively. Current biotechnological applications of microorganisms remain rudimentary, often relying on genetically engineered monocultures (e.g., pharmaceuticals) or mixed-cultures of partially known composition (e.g., wastewater treatment), yet the vast potential of “microbial ecological power” observed in most natural environments, remains largely underused. In line with the Unified Microbiome Initiative (UMI) which aims to “discover and advance tools to understand and harness the capabilities of Earth's microbial ecosystems,” we propose in this concept paper to capitalize on ecological insights into the spatial and modular design of interlinked microbial consortia that would overcome limitations of natural systems and attempt to optimize the functionality of the members and the performance of the engineered consortium. The topology of the spatial connections linking the various members and the regulated fluxes of media between those modules, while representing a major engineering challenge, would allow the microbial species to interact. The modularity of such spatially linked microbial consortia (SLMC) could facilitate the design of scalable bioprocesses that can be incorporated as parts of a larger biochemical network. By reducing the need for a compatible growth environment for all species simultaneously, SLMC will dramatically expand the range of possible combinations of microorganisms and their potential applications. We briefly review existing tools to engineer such assemblies and optimize potential benefits resulting from the collective activity of their members. Prospective microbial consortia and proposed spatial configurations will be illustrated and preliminary calculations highlighting the advantages of SLMC over co-cultures will be presented, followed by a discussion of challenges and opportunities for moving forward with some designs.

Properties of single-strain cheese starter bacteria grown in diffusion culture

The production of a range of single-strain cheese starter concentrates by diffusion culture and some of their subsequent properties are described. Five strains ofStreptococcus cremorisand 4 strains ofStr. lactiswere grown to concentrations of above 1.0 × 1011cells/ml, these concentrates all being of satisfactory cheese-making potential. Ammonium lactate accumulation in the system appeared to be the limiting factor to growth, although some strains were more tolerant than others. A continuous harvesting experiment demonstrated that the equipment was potentially capable of this mode of concentrate production. A cheese-making trial was carried out with concentrates produced by diffusion culture; a higher proportion of extra and upper selected cheese was obtained than from control cheese produced with conventionally propagated commercial mixed starters.

Dialysis Fermentor Systems for Concentrated Culture of Microorganisms

Cell-mass production of Serratia marcescens was studied in dialysis systems in which growth was managed in a fermentor remote from, but connected by conduits and pumps with, a nutrient reservoir. Dialysis was accomplished with membrane tubing in either the fermentor or the reservoir, or best with membrane sheet in a plate-and-frame dialyzer that was remote from but connected with both vessels. Growth trials with these systems demonstrated their ability to produce virtually unlimited population density in a liquid culture, viable counts in excess of 10 cells/ml and partial cell volume of 50% being attained. The system used for growth also may be used, after the growth cycle, to concentrate cells still further by osmotic dehydration with a hydrophilic colloid. The dialyzer-dialysis system that was evolved permits independent control of the component operations and is believed to be adaptable to any desired scale of size.

Growing E. coli to high cell density—A historical perspective on method development

E. coli is the major bacterial platform for expressing simple heterologous proteins. Growing E. coli to high densities has been the subject of numerous studies since the early 1970s, exploring the limits of bacterial culture density in order to achieve maximum productivity. Research strategies were focused on improving the cultivation techniques, manipulating the bacteria's physiology or both. As a result, batch, fed batch and dialysis fermentation techniques had been developed. These growth strategies, together with optimization of media composition and the application of molecular biology methods, made it possible to grow E. coli to cell densities of up to 190 g/l (dry weight), while avoiding media precipitation and preventing acetate accumulation. Additional research on the effects of heterologous protein biosynthesis on signal transduction, proteolysis and post transcription events in E. coli may improve its productivity.

Simplified method for preparation of concentrated exoproteins produced by Staphylococcus aureus grown on surface of cellophane bag containing liquid medium

A simplified method for preparation of concentrated exoproteins including protein A and alpha-toxin produced by Staphylococcus aureus was successfully devised. The concentrated proteins were obtained by cultivating S. aureus organisms on the surface of a liquid medium-containing cellophane bag enclosed in a sterilized glass flask. With the same amount of medium, the total amount of proteins obtained by the method presented here was identical with that obtained by conventional liquid culture. The concentration of proteins obtained by the method, however, was high enough to observe their distinct bands stained on polyacrylamide gel electrophoresis. This method was considered quite useful not only for large-scale cultivation for the purification of staphylococcal proteins but also for small-scale study using the proteins. The precise description of the method was presented and its possible usefulness was discussed.

Novel Method of Lactic Acid Production by Electrodialysis Fermentation

In lactic acid fermentation by Lactobacillus delbrueckii, the produced lactic acid affected the lactic acid productivity. Therefore, for the purpose of alleviating this inhibitory effect, an electrodialysis fermentation method which can continuously remove produced lactic acid from the fermentation broth was applied to this fermentation process. As a result, the continuation of fermentation activity was obtained, and the productivity was three times higher than in non-pH-controlled fermentation. In electrodialysis fermentation, the amount of produced lactic acid was 82.2 g/liter, which was about 5.5 times greater than that produced in non-pH-controlled fermentation. It was concluded that these good results were obtained on account of alleviating the lactic acid inhibitory effect by electrodialysis fermentation. However, the fouling of anion-exchange membranes by cells was observed in electrodialysis fermentation.

The suitability of a membrane diffusion growth chamber for studying bacterial interaction

Rates of diffusion of nutrients and metabolites through 0.1 micron pore diameter polycarbonate membranes are so low that the use of membrane-separated systems to study bacterial interaction seems to have little application. Effective nutrient availability throughout the system is very dependent on membrane area/medium volume ratio. An attempt to demonstrate a known interaction in the membrane diffusion chamber was not successful.

Dialysis Continuous Process for Ammonium-Lactate Fermentation of Whey: Mathematical Model and Computer Simulation

A mathematical model was developed to describe a dialysis process for the continuous fermentation of whey lactose to lactic acid, with neutralization to a constant pH by ammonia. In the process, whey of a relatively high concentration is fed into the fermentor circuit at a relatively low rate so that the residual concentration of lactose is low. The fermentor effluent contains ammonium lactate, bacterial cells, and residual whey solids and could be used as a nitrogen-enriched feedstuff for ruminant animals. Only water is fed into the dialysate circuit at a relatively high rate. The dialysate effluent contains purified ammonium lactate and could be converted to lactic acid and ammonium sulfate for industry. The fermentation was specifically modeled as a set of equations representing material balances and rate relationships in the two circuits. Dialysis continuous fermentations, in general, were modeled by combining these equations and by using dimensionless parameters. The generalized model was then solved for the steady state and used to simulate the specific fermentation on a digital computer. The results showed the effects of various material and operational and kinetic parameters on the process and predicted that it could be operated efficiently.

Dense autotrophic cultures of Alcaligenes eutrophus

Alcaligenes eutrophus was grown autotrophically in 23-liter batch cultures in a controlled H2-O2-CO2 atmosphere. It was demonstrated that the need for periodic supplements of individual nutrients could be anticipated before cell growth depleted these nutrients to the point of becoming growth rate limiting. As a result, exponential growth was extended to optical densities of 44, with doubling times maintained at 2 h. Cultures having an initial optical density of 0.040 to 0.70 reached the final optical density of 60 in about 25 h. The final viable count was 1.2 X 10(11) cells per ml, and the dry weight was 25 g/liter.

Quantitative requirements for exponential growth of Alcaligenes eutrophus

Quantitative nutrient requirements for unrestricted autotrophic growth of Alcaligenes eutrophus were determined. Minimum saturating concentrations of Mg2+, SO42-, PO43-, Fe3+, and Na2+ for an optical density increase of 2 were 10(-4) M 8 X10(-5) M, 5 X 10(-4) to 6 X 10(-4) M, 10(-5) M, and 10(-7) to 2 X 10(-7) M, respectively. Trace metal requirements for cobalt, chromium, and copper were also demonstrated, but minimum concentrations could not be determined because other reagents contributed a high background of these metals. Under certain conditions an apparent response to zinc was observed, although other experiments suggest the zinc salt contained another metal that was required for growth. Poly-beta-hydroxybutyrate biosynthesis was shown to be initiated by a magnesium or sulfate deficiency as well as by a nitrogen or phosphate deficiency.

Cycloheximide production by Streptomyces griseus: alleviation of end-product inhibition by dialysis-extraction fermentation

The use of dialysis fermentation with the continuous extraction of the dialysate has resulted in a twofold increase in the cycloheximide titer due to relief from product inhibition. Continuous extraction of the dialysate has eliminated the necessity for large reservoir volumes of fermentation medium normally used in dialysis fermentation. The apparatus used in this procedure is described.

Hemodialysis culture of Serratia marcescens in a goat-artificial kidney-fermentor system

Hemodialysis was employed to simulate in vivo conditions for growth in mammalian blood, but without phagocytosis, by using the goat and Serratia marcescens as a host-parasite model. The blood stream was shunted surgically via prosthetic tubing from a carotid artery through the hollow-fiber membranes in an artificial kidney hemodialyzer and back into a jugular vein. The dialysate solution concurrently was pumped from a modular fermentor through the hemodialyzer jacket outside of the membranes and back into the fermentor. Hemodialysis between the two circuits was maintained continuously. When equilibrium was attained, bacteria inoculated into the dialysate circuit multiplied first exponentially at the maximal rate and then arithmetically at a lesser rate equally well under aerobic or anaerobic conditions. When a population of about 10(9) viable bacteria/ml was exceeded, the goat reacted acutely with signs of general toxemia, pyrexia, and leukopenia, apparently because of dialyzable toxic material produced by the culture. The maximal molecular size of the toxic material was defined relative to a rigid globular protein of 15,000 in molecular weight and 1.9 nm in hydrodynamic radius or to a flexible fibrous polyglycol of 5,500 in molecular weight and 2.6 nm in hydrodynamic radius, based on determinations of the membrane porosity threshold for dialysis.

Dialysis culture of microorganisms: design, theory, and results

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Differential dialysis culture for separation and concentration of a macromolecular product

A differential dialysis flask, constructed with three chambers and two membranes of different porosity, was used to effect the separation and concentration of enterotoxin B produced extracellularly by a culture of Staphylococcus aureus. Variables were examined that affected the diffusion of glucose, as measured by half-equilibration time and permeability coefficient; the relative chamber volume, type of membrane, membrane masking, and mixing all exerted a substantial influence on diffusion rates. A number of membrane filters were tested for usefulness; one type, made with vinylidene fluoride, had desirable physical and diffusional properties, but neither it nor others consistently withheld the bacteria for more than a marginally useful period of about 50 hr. In ordinary two-chambered dialysis culture, the amount of enterotoxin reached 10 times that in control culture; in differential, three-chambered dialysis culture the comparable factor of increase was about 7, with about two-thirds of this amount being separated from cells in the product chamber of the flask.

Anaerobic stimulated mixed culture system

A compartmented, autoclavable culture vessel has been developed for the purpose of studying interactive associations of microorganisms which are essential to the anaerobic decomposition of sewage sludge. The unit employs sterile filter membranes to subdivide the interior culture space into individual compartments. Bacteria cultured in one compartment are denied access to adjacent compartments, even though rapid interchange of nutrients and metabolic waste products occurs throughout the unit. The obligate methane-forming anaerobe, Methanobacillus omelianskii has been successfully grown and concentrated in this system by use of a synthetic medium reduced with sodium sulfide. The feasibility of using this system to study microbial interactions was, in part, demonstrated by growing M. omelianskii in a thoroughly aerated medium which had been biologically reduced by Escherichia coli prior to inoculation with the anaerobe. Under this condition of simulated mixed culture growth, the cell yield of both microorganisms, as well as specific metabolic activities ascribed to each organism, was readily monitored.

Characteristics of the vegetative growth of Bacillus popilliae

Growth characteristics of the insect pathogen, Bacillus popilliae Dutky, were studied by propagation in shaken flasks and in 2-liter fermentors. Maximal populations between 5 x 10(8) and 2 x 10(9) viable cells per milliliter of culture medium routinely were obtained in incubation periods of 18 to 24 hr at 30 C in a medium composed of 1.5% yeast extract, 0.6% K(2)HPO(4), and 0.2% glucose or trehalose. The carbohydrate required for growth in liquid media was fermented with the formation of 2 meq of acid per mmole of carbohydrate utilized; acid products ordinarily were not subsequently metabolized. B. popilliae is an aerobe, and the amount of growth obtained varied with aeration to an optimum at oxygen absorption rates of about 0.5. Maximal populations persist in a culture for periods of only 1 to 4 hr; cessation of growth was followed immediately by rapid death of cultures, so that less than 1% of the cells remained viable after 48 hr, and viability often was lost entirely by the end of 72 hr of incubation. No cytological evidence for spore formation was observed under any growth condition. Death was not associated with lysis of the cells, although extensive granulation ultimately occurred. Continuous neutralizaiton, augmented buffering, various techniques of dialysis, or slow feeding of the carbohydrate did not markedly alleviate the characteristic death of the cultures.

Use of dialyzing culture technique for high yield of Mycoplasma

Pollock, Mary E. (University of Minnesota, Minneapolis). Use of dialyzing culture technique for high yield of Mycoplasma. J. Bacteriol. 90:1682-1685. 1965.-The saprophytic pleuropneumonia-like organism, Mycoplasma laidlawii, type A, was cultivated within a dialyzing membrane suspended in a reservoir of uninoculated medium. Generation time was not decreased by this method, but the period of active growth was prolonged, and the final yield was 10-fold that attained by the usual cultural methods. A defined mixture, which alone would not support growth of Mycoplasma, could replace the complex soy peptone-yeast extract medium in the inner membrane vessel of the dialyzing culture flask with no decrease in growth rate or yield.