The effect of 2,4-dinitrophenol on the growth ofKlebsiella aerogenesNCTC 418 in aerobic chemostat cultures

Comparison of four chemical uncouplers for excess sludge reduction

A substantial part of the operating costs of wastewater treatment plants (WWTP) is associated with the management and treatment of the excess sludge generated during the treatment process. Different strategies have been applied for excess sludge reduction, such as the oxic-settling-anaerobic process, the high dissolved oxygen process, the uncoupler-containing activated sludge process, the ozonation-combined activated sludge process, control of sludge retention time and biodegradation of sludge in a membrane-assisted reactor. Chemical uncouplers have been shown to reduce excess sludge production, disassociating the energy coupling between catabolism and anabolism. These metabolic uncouplers may be organic compounds, such as 2,4-dinitrophenol (2,4-DNP) or 3,3',4',5-tetrachlorosalicylanilide (TCS), or heavy metals. In this paper, four different chemicals (2,4-DNP, TCS, copper (Cu) and zinc (Zn)) were chosen for short-term tests for studying their ability to reduce sludge yield (Y(x/s)) and, consequently, their potential for reducing excess sludge production. According to the results obtained, only TCS seems to be very effective in reducing sludge production from the activated sludge process. Compared with the control test, Y(x/s) can be reduced by over 30% at 0.8 mg/l TCS. It was also found that the substrate removal capability was not adversely affected by the presence of TCS. Furthermore, an increase in the microbial activity of the system was observed.

Effects of limited aeration and of the ArcAB system on intermediary pyruvate catabolism in Escherichia coli

The capacity of Escherichia coli to adapt its catabolism to prevailing redox conditions resides mainly in three catabolic branch points involving (i) pyruvate formate-lyase (PFL) and the pyruvate dehydrogenase complex (PDHc), (ii) the exclusively fermentative enzymes and those of the Krebs cycle, and (iii) the alternative terminal cytochrome bd and cytochrome bo oxidases. A quantitative analysis of the relative catabolic fluxes through these pathways is presented for steady-state glucose-limited chemostat cultures with controlled oxygen availability ranging from full aerobiosis to complete anaerobiosis. Remarkably, PFL contributed significantly to the catabolic flux under microaerobic conditions and was found to be active simultaneously with PDHc and cytochrome bd oxidase-dependent respiration. The synthesis of PFL and cytochrome bd oxidase was found to be maximal in the lower microaerobic range but not in a delta ArcA mutant, and we conclude that the Arc system is more active with respect to regulation of these two positively regulated operons during microaerobiosis than during anaerobiosis.

Response of the central metabolism in Corynebacterium glutamicum to the use of an NADH-dependent glutamate dehydrogenase

The extensive use of 13C enrichments in precursor metabolites for flux quantification does not rely on NADPH stoichiometries and can therefore be used to quantify reducing power fluxes. As an application of this concept, the NADPH fluxes were quantified in an L-lysine producer of Corynebacterium glutamicum grown into metabolic and isotopic steady state with [1-13C]glucose. In this case, where the organism's NADPH-dependent glutamate dehydrogenase consumes reducing power, the NADPH flux generated is 210% (molar flux relative to glucose uptake rate) with its major part (72% of the total) generated via the pentose phosphate pathway activity. An isogenic strain in which the glutamate dehydrogenase of C. glutamicum was replaced by the NADH-dependent glutamate dehydrogenase of Peptostreptococcus asaccharolyticus was made and the metabolite fluxes were again estimated. The major response to this local perturbation is a drastically reduced NADPH generation of only 139%. Most of the NADPH (62% of the total) is now generated via the tricarboxylic acid cycle activity. This shows the extraordinary flexibility of the central metabolism and provides a picture of the global regulatory properties of the central metabolism. Furthermore, a detailed analysis of the fluxes and exchange fluxes within the anaplerotic reactions is given. It is hypothesized that these reactions might also serve to balance the total reducing power budget as well as the energy budget within the cell.

The ability of Escherichia coli O157:H7 to decrease its intracellular pH and resist the toxicity of acetic acid

Batch cultures of Escherichia coli K-12 grew well in an anaerobic glucose medium at pH 5.9, but even small amounts of acetate (20 mM) inhibited growth and fermentation. E. coli O157:H7 was at least fourfold more resistant to acetate than K-12. Continuous cultures of E. coli K-12 (pH 5.9, dilution rate 0.085 h-1) did not wash out until the sodium acetate concentration in the input medium was 80 mM, whereas E. coli O157:H7 persisted until the sodium acetate concentration was 160 mM. E. coli K-12 cell accumulated as much as 500 mM acetate, but the intracellular acetate concentration of O157:H7 was never greater than 300 mM. Differences in acetate accumulation could be explained by intracellular pH and the transmembrane pH gradient (delta pH). E. coli K-12 maintained a more or less constant delta pH (intracellular pH 6.8), but E. coli O157:H7 let its delta pH decrease from 0.9 to 0.2 units as sodium acetate was added to the medium. Sodium acetate increased the rate of glucose consumption, but there was little evidence to support the idea that acetate was creating a futile cycle of protons. Increases in glucose consumption rate could be explained by increases in D-lactate production and decreases in ATP production. Intracellular acetate was initially lower than the amount predicted by delta pH, but intracellular acetate and delta pH were in equilibrium when the external acetate concentrations were high. Based on these results, the acetate tolerance of O157:H7 can be explained by fundamental differences in metabolism and intracellular pH regulation. By decreasing the intracellular pH and producing large amounts of D-lactate, O157:H7 is able to decrease delta pH and prevent toxic accumulations of intracellular acetate anion.

Gluconate metabolism of Klebsiella pneumoniae NCTC 418 grown in chemostat culture

The metabolism of gluconate by Klebsiella pneumoniae NCTC 418 was studied in continuous culture. Under all gluconate-excess conditions at low culture pH values (pH 4.5-5.5) the majority (70-90%) of the gluconate metabolized was converted to 2-oxogluconate via gluconate dehydrogenase (GADH), although specific 2-oxogluconate production rates under potassium-limited conditions were significantly lower than under other gluconate-excess conditions. At high culture pH values, metabolism shifted towards production of acetate. Levels of GADH were highest at low culture pH values and synthesis was stimulated by the presence of (high concentrations of) gluconate. An increase in activity of the tricarboxylic acid cycle was accompanied by a decrease in GADH activity in vivo and in vitro, suggesting that the GADH serves a role as an alternative energy-generating system. Anaerobic 2-oxogluconate production was found to be possible in the presence of nitrate as electron acceptor. Levels of gluconate kinase were highest when K. pneumoniae was grown under gluconate-limited conditions. Under carbon-excess conditions, levels of this enzyme correlated with the intracellular catabolic flux.

Metabolite production and growth efficiency

The capacity to sustain the large fluxes of carbon and energy required for rapid metabolite production appears to be inversely related to the growth efficiency of micro-organisms. From an overall energetic point of view three main classes of metabolite may be distinguished. These are not discrete categories, as the energetics of biosynthesis will depend on the precise biochemical pathways used and the nature of the starting feed stock(s). (1) For metabolites like exopolysaccharides both the oxidation state and the specific rate of production appear to be inversely related to the growth efficiency of the producing organism. Maximum rates of production are favored when carbon and energy flux are integrated, and alteration of this balance may negatively effect production rates. (2) The production of metabolites like organic acids and some secondary metabolites results in the net production of reducing equivalents and/or ATP. It is thought that the capacity of the organism to dissipate this product-associated energy limits its capacity for rapid production. (3) For metabolites like biosurfactants and certain secondary metabolites that are composed of moieties of significantly different oxidation states production from a single carbon source is unfavorable and considerable improvements in specific production rate and final broth concentration may be achieved if mixed carbon sources are used. By careful selection of production organism and starting feedstock(s) it may be possible to tailor the production, such that the adverse physiological consequences of metabolite overproduction on the production organism are minimized.

The role of futile cycles in the energetics of bacterial growth

In this contribution we describe the occurrence of futile cycles in growing bacteria. These cycles are thought to be active when organisms contain two uptake systems for a particular nutrient (one with a high, the other with a low affinity for its substrate). The high-affinity system is responsible for uptake of the nutrient, some of which is subsequently lost to the medium again via leakage through the low-affinity-system. A special futile cycle is caused under some growth conditions by the extremely rapid diffusion of ammonia through bacterial membranes. When the ammonium ion is taken up via active transport, the couple NH3/NH4+ will act as an uncoupler. This is aggravated by the chemical similarity of the potassium and the ammonium ion, which leads to ammonium ion transport via the Kdp potassium transport system when the potassium concentration in the medium is low. Other examples of futile cycles, such as those caused by the production of fatty acids by fermentation, are briefly discussed.

ATPase-dependent energy spilling by the ruminal bacterium, Streptococcus bovis

When the ruminal bacterium Streptococcus bovis was grown in batch culture with glucose as the energy source, the doubling time was approximately 21 min and the rate of bacterial heat production was proportional to the optical density (1.72 microW/micrograms protein). If exponentially growing cultures were treated with chloramphenicol, there was a decline in heat production, but the rate was greater than 0.30 microW/micrograms protein even after growth ceased. Since there was no heat production after glucose depletion, this growth-independent energy dissipation (spilling) was not simply due to endogenous metabolism. Stationary cells which were washed and incubated in nitrogen-free medium containing an excess of glucose produced heat at a rate of 0.17 microW/micrograms protein. Monensin and tetrachlorosalicylanilide (TCS), compounds which facilitate an influx of protons, caused a more than 2-fold increase in heat production. Dicyclohexylcarbodiimide (DCCD) virtually eliminated growth-independent heat production regardless of the mode of growth inhibition. Because DCCD had little effect on the glucose phosphotransferase system, it appeared that the combined action of proton influx and the membrane bound F1F0 proton ATPase was responsible for energy spilling.

Physiological significance and bioenergetic aspects of glucose dehydrogenase

The regulation of the PQQ-linked glucose dehydrogenase in different organisms is reviewed. It is concluded that this enzyme functions as an auxiliary energy-generating mechanism, because it is maximally synthesized under conditions of energy stress. It is now definitively established that the oxidation of glucose to gluconate generates metabolically useful energy. The magnitude of the contribution of the oxidation of glucose to gluconate via this enzyme to the growth yield of organisms such as Acinetobacter calcoaceticus is not yet clear.

The functional significance of glucose dehydrogenase in Klebsiella aerogenes

In order to assess the functional significance of the quinoprotein glucose dehydrogenase recently found to be present in K+ -limited Klebsiella aerogenes, a broad study was made of the influence of specific environmental conditions on the cellular content of this enzyme. Whereas high activities were manifest in cells from glucose containing chemostat cultures that were either potassium- or phosphate-limited, only low activities were apparent in cells from similar cultures that were either glucose-, sulphate- or ammonia-limited. With these latter two cultures, a marked increase in glucose dehydrogenase activity was observed when 2,4-dinitrophenol (1 mM end concentration) was added to the growth medium. These results suggested that the synthesis of glucose dehydrogenase is not regulated by the level of glucose in the growth medium, but possibly by conditions that imposed an energetic stress upon the cells. This conclusion was further supported by a subsequent finding that K+ -limited cells that were growing on glycerol also synthesized substantial amounts of glucose dehydrogenase. The enzyme was found to be membrane associated, and preliminary evidence has been obtained that it is located on the periplasmic side of the cytoplasmic membrane and functionally linked to the respiratory chain. This structural and functional orientation is consistent with glucose dehydrogenase serving as a low impedance energy generating system.

Thermodynamics of growth. Non-equilibrium thermodynamics of bacterial growth. The phenomenological and the mosaic approach

Microbial growth is analyzed in terms of mosaic and phenomenological non-equilibrium thermodynamics. It turns out that already existing parameters devised to measure bacterial growth, such as YATP, mu, and Q substrate, have as thermodynamic equivalents flow ratio, output flow and input flow. With this characterisation it becomes possible to apply much of the already existing knowledge of phenomenological non-equilibrium thermodynamics to bacterial growth. One of the conclusions is that the frequent observation that YATP is only 50% of its theoretical maximum does not mean that the microbe corresponds to a thermodynamic system that has been optimized for maximal output power, as has been suggested. Rather, at least in some cases, it corresponds to a system that has been optimized towards maximum growth rate. When the degree of reduction of the (single) carbon source is significantly smaller than that of the biomass produced, the efficiency of biomass synthesis has been kept as high (i.e., about 24%) as is consistent with maximization of the growth rate at optimal efficiency. Mosaic thermodynamics allows an analysis of processes which in microbial metabolism may be responsible for any particular growth behaviour. Equations are derived that predict the effect of uncoupling through leaks, futile cycling, or 'slip' on microbial growth. It turns out that uncoupling is expected to affect both the growth rate-independent and the growth rate-dependent 'maintenance coefficient'. The effect on the latter is different when catabolic substrate limits growth than when anabolic substrate limits growth. In the latter case, the growth rate-dependent maintenance coefficient is negative. It is concluded that mosaic non-equilibrium thermodynamics will be a powerful theoretical tool especially in future experimental analyses of the metabolic basis for microbial growth characteristics and growth regulation.

Maintenance energy: a general model for energy-limited and energy-sufficient growth

The new model proposed to account for the energy requirement for growth includes both a constant maintenance energy term (m) independent of the specific growth rate and a term (m') which decreases linearly with increase in specific growth rate and becomes zero at the maximum specific growth rate. The available data for testing the model do not deviate significantly from the relations predicted. Consistent values of the maximum growth yield (YG) can be derived, irrespective of whether the cultures are energy limited or energy sufficient. Attention is drawn to the possibility that the constant maintenance energy term may be estimated from the maximum specific growth rate.

Microbial ecology and activities in the rumen: part 1

This review describes the progress which has been made during the last 10 to 15 years in the field of rumen microbiology. It is basically an account of new discoveries in the bacteriology, protozoology, biochemistry, and ecology of the rumen microbial population. As such it covers a wide range of subjects including the isolation and properties of methanogenic bacteria, the role of rumen phycomycete fungi, anaerobic energy conservation, and general metabolic aspects of rumen microorganisms. It also attempts, however, to describe and develop new concepts in rumen microbiology. These consist principally of interactions of the microbemicrobe, microbe-food and microbe-host types, and represent the main areas of recent advance in our understanding of the rumen ecosystem. The development of experimental techniques such as chemostat culture and scanning electron microscopy are shown to have been instrumental in progress in these areas. The paper is concluded with an assessment of our present knowledge of the rumen fermentation, based on the degree of success of experiments with gnotobiotic ruminants inoculated with defined flora and in mathematical modeling of the fermentation. The efficacy of chemical manipulation of the fermentation in ruminant is also discussed in this light.

Influence of the glucose input concentration on the kinetics of metabolic production by Klebsiella aerogenes NCTC 418: growing in chemostat culture in potassium- or ammonia-limited environments

With chemostat cultures of Klebsiella aerogenes growing at a fixed dilution rate, initially under conditions of glucose-limitation, transition to either potassium-limitation or ammonia-limitation was found not to be a steep step function. A wide range of intermediate steady states could be established in which neither substrate was present in excess of the growth requirement. As the molar ratio of glucose: K+ in the feed medium was progressively increased, the additional glucose carbon was first converted solely to CO2. Thereafter, when the molar ratio exceeded 45, acetate, and then pyruvate and 2-ketogluconate were excreted at increasing rates. In contrast, transition to ammonia-limitation provoked an early excretion of 2-oxoglutarate and 2-ketogluconate, followed (at higher glucose input concentrations) by acetate and pyruvate. These patterns of product excretion are considered in relation to the specific nature of the growth-limitation, to probable changes in the energy charge and redox balance within the growing cells, and to the accompanying modulation of tricarboxylic acid-cycle activity.