Kinetics of permease catalyzed transport

Bacterial Growth on Distant Naphthalene Diffusing through Water, Air, and Water-Saturated and Nonsaturated Porous Media

The influence of substrate diffusion on bacterial growth was investigated. Crystalline naphthalene was supplied as the substrate at various distances in the range of centimeters from naphthalene-degrading organisms separated from the substrate by agar-solidified mineral medium. Within 2 weeks, the cells grew to final numbers which were negatively correlated with the distance from the substrate. A mathematical model that combined (i) Monod growth kinetics extended by a term for culture maintenance and (ii) substrate diffusion could explain the observed growth curves. The model could also predict growth on naphthalene that was separated from the bacteria by air. In addition, the bacteria were grown on distant naphthalene that had to diffuse to the cells through water-saturated and unsaturated porous media. The growth of the bacteria could be used to calculate the effective diffusivity of naphthalene in the three-phase system. Diffusion of naphthalene in the pore space containing 80% air was roughly 1 order of magnitude faster than in medium containing only 20% air because of the high Henry's law coefficient of naphthalene. It is proposed that the effective diffusivities of the substrates and the spatial distribution of substrates and bacteria are the main determinants of final cell numbers and, consequently, final degradation rates.

Bacterial choices for the consumption of multiple resources for current and future needs

Microorganisms differ in their effectiveness in uptake and selection of substances that they bring in from the environment. They also differ in how they balance the allocation of nutrients for immediate and for delayed use. Moreover, they may not take up resources as fast as they seemingly could, and they may extrude derivatives of substances just pumped in. A good deal of these apparent choices must reside in the uptake systems and the linkage of these with the cell's intermediate metabolism. An important feature is that a resource may vary in concentration from time to time, nutrient to nutrient, and habitat to habitat. This variation must have been critical to the evolution of regulatory processes. Some possibilities for the combined uptake and consumption are considered for substrates serving the same (homologous) and different (heterologous) roles for the bacterium. From the membrane transport processes diagrammed in Fig. 1c and Fig. 2 and corresponding computer program given in Appendix A, the combined effect of uptake processes and cell growth can be studied. The model can be modified for various alternate models to study the possible control of cellular uptake and metabolism for the range of ecological roles of the bacterium.

Microbial physiology and ecology of slow growth

The uptake capabilities of the cell have evolved to permit growth at very low external nutrient concentrations. How are these capabilities controlled when the substrate concentrations are not extremely low and the uptake systems could import substrate much more rapidly than the metabolic capabilities of the cell might be able to handle? To answer this question, earlier theories for the kinetics of uptake through the cell envelope and steady-state systems of metabolic enzymes are discussed and a computer simulation is presented. The problems to the cell of fluctuating levels of nutrient and too much substrate during continuous culture are discussed. Too much substrate can lead to oligotrophy, substrate-accelerated death, entry into the viable but not culturable state, and lactose killing. The relationship between uptake and growth is considered. Finally, too little substrate may lead to catastrophic attempts at mounting molecular syntheses that cannot be completed.

What size should a bacterium be? A question of scale

There are living prokaryotes (Bacteria and Archaea) that have cell sizes that range from 0.02-400 microns3. Over this tremendous range, various abilities to cope with the environment are needed. This review attempts to formulate some of the problems and some of the solutions. The smallest size for a free-living organism is suggested to be largely set by the catalytic efficiency of enzymes and protein synthetic machinery. Because of fluctuations in the environment, cells must maintain machinery to cope with various catastrophes; these mechanisms increase the minimum size of the cell. On the other hand, the largest cell is reasonably assumed to be limited by the ability of diffusion to bring nutrients to the appropriate part of the cell and to dispose of waste products. To explore the limitation imposed by diffusion, analysis is developed of diffusion processes through stirred and unstirred media, diffusion through media that contains obstacles, and the effect of size and shape.

[Mechanisms of efflux of a substrate accumulated by the lactose permease of Escherichia coli: theoretical and experimental study]

At the steady-state of accumulation of intracellular lactose by the beta-galactoside permease of Escherichia coli, the rate of efflux of the substrate is equal to its rate of influx. An original experimental method and a mathematical processing of the experimental data are proposed to evaluate the relative involvements of the permease-mediated pathway and of the diffusion component in this efflux. The method consists of inducing the lac operon of the bacteria, and then of removing the inducer and allowing the cells to grow further. The permease content and the membrane surface of diffusion are thus varying independently in such a "de-induction" experiment, along which lactose uptake has been monitored at different times. The analysis of the experimental data show that, under conditions of maximal induction, over 95% of the efflux passes through the energized permease. The relevant parameters of the efflux of lactose have been computed and their values allow the prediction of most classical observations, as well as the prediction, never checked, that under physiological conditions, the higher the external substrate concentration, the higher the permease-mediated efflux, according to a saturation kinetics.

Interaction of maltose transport with the transport of glucose and galactosides

In cells of Escherichia coli possessing both maltose and galactoside permease, fluxes via one permease are independent of the substrate for the other permease. However, both fluxes are partially inhibited by glucose or alpha-methyl glucoside at low concentrations in cells grown on glucose. Neither maltose nor galactosides have an inhibitory effect on glucose permease function. These observations are consistent with the hypothesis that the number of glucose permease systems on the cell surface of such cells is much larger than the number for maltose or galactosides.

Control of mixed-substrate utilization in continuous cultures of Escherichia coli

The chemostat culture technique was used to study the control mechanisms which operate during utilization of mixtures of glucose and lactose and glucose and l-aspartic acid by populations of Escherichia coli B6. Constitutive mutants were rapidly selected during continuous culture on a mixture of glucose and lactose, and the beta-galactosidase level of the culture increased greatly. After mutant selection, the specific beta-galactosidase level of the culture was a decreasing function of growth rate. In cultures of both the inducible wild type and the constitutive mutant, glucose and lactose were simultaneously utilized at moderate growth rates, whereas only glucose was used in the inducible cultures at high growth rates. Catabolite repression was shown to be the primary mechanism of control of beta-galactosidase level and lactose utilization in continuous culture on mixed substrates. In batch culture, as in the chemostat, catabolite repression acting by itself on the lac enzymes was insufficient to prevent lactose utilization or cause diauxie. Interference with induction of the lac operon, as well as catabolite repression, was necessary to produce diauxic growth. Continuous cultures fed mixtures of glucose and l-aspartic acid utilized both substrates at moderate growth rates, even though the catabolic enzyme aspartase was linearly repressed with increasing growth rate. Although the repression of aspartase paralleled the catabolite repression of beta-galactosidase, l-aspartic acid could be utilized even at very low levels of the catabolic enzyme because of direct anabolic incorporation into protein.

Auxin movement in corn coleoptiles

Movement of radioactive auxins was analysed in corn coleoptile sections. The results support the idea that processes involved in the transport of indoleacetic acid (IAA) are specific for growth-promoting auxins.Inhibition of IAA transport by triiodobenzoic acid is caused by a reversible block of the exit; the auxin held back remains in the transport pool. The observed increase in immobilization may be a secondary effect caused by the increased concentration of free IAA in the tissue.Auxin molecules are most likely transported by anon-covalent mechanism. IAA and naphthaleneacetic acid (NAA) move through the cell and exit as free molecules. A search for a transient auxin complex, chaseable as required for any transport carrier intermediate, yielded negative results. No(18)O was lost from NAA labeled with(18)O in the carboxyl group during transport of the auxin through coleoptile tissue.After application of IAA to auxin-depleted tissue, the transport rate undergoes oscillations with a period length of ca. 25 min.The movement of the auxin 2.4-dichlorophenoxyacetic acid which is usually sluggish, increased several times if some IAA was added. Auxin, thus, stimulates its own transport.A model is discussed in which auxin-binding to the plasma membrane and reversible changes of membrane conformation may provide a basis for active secretion and for the observed cooperativity.

Galactose transport in Saccharomyces cerevisiae. I. Nonmetabolized sugars as substrates and inducers of the galactose transport system

The inducible galactose transport system in bakers' yeast carries out the facilitated diffusion of the nonmetabolized galactose analogues d-fucose and l-arabinose. This capacity depends on the activity of the Ga 2 gene. In some strains, d-fucose and l-arabinose are also gratuitous inducers. Mutants in which the inducibility of the galactose pathway enzymes is altered show a parallel alteration of the inducibility of the galactose transport system.