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

On how the power supply shapes microbial survival

Understanding how environmental factors affect microbial survival is an important open problem in microbial ecology. Patterns of microbial community structure have been characterized across a wide range of different environmental settings, but the mechanisms generating these patterns remain poorly understood. Here, we use mathematical modelling to investigate fundamental connections between chemical power supply to a system and patterns of microbial survival. We reveal a complex set of interdependences between power supply and distributions of survival probability across microbial habitats, in a case without interspecific resource competition. We also find that different properties determining power supply, such as substrate fluxes and Gibbs energies of reactions, affect microbial survival in fundamentally different ways. Moreover, we show how simple connections between power supply and growth can give rise to complex patterns of microbial survival across physicochemical gradients, such as pH gradients. Our findings show the importance of taking energy fluxes into account in order to reveal fundamental connections between microbial survival and environmental conditions, and to obtain a better understanding of microbial population dynamics in natural environments.

Changes in Microbial Energy Metabolism Measured by Nanocalorimetry during Growth Phase Transitions

Calorimetric measurements of the change in heat due to microbial metabolic activity convey information about the kinetics, as well as the thermodynamics, of all chemical reactions taking place in a cell. Calorimetric measurements of heat production made on bacterial cultures have recorded the energy yields of all co-occurring microbial metabolic reactions, but this is a complex, composite signal that is difficult to interpret. Here we show that nanocalorimetry can be used in combination with enumeration of viable cell counts, oxygen consumption rates, cellular protein content, and thermodynamic calculations to assess catabolic rates of an isolate of Shewanella oneidensis MR-1 and infer what fraction of the chemical energy is assimilated by the culture into biomass and what fraction is dissipated in the form of heat under different limiting conditions. In particular, our results demonstrate that catabolic rates are not necessarily coupled to rates of cell division, but rather, to physiological rearrangements of S. oneidensis MR-1 upon growth phase transitions. In addition, we conclude that the heat released by growing microorganisms can be measured in order to understand the physiochemical nature of the energy transformation and dissipation associated with microbial metabolic activity in conditions approaching those found in natural systems.

Comparing the responses of rumen ciliate protozoa and bacteria to excess carbohydrate

When given excess carbohydrate, certain microbial species respond by storing energy (synthesizing reserve carbohydrate), but other species respond by dissipating the energy as heat (spilling energy). To determine the importance of these responses in the rumen microbial community, this study quantified the responses of mixed ciliate protozoa vs bacteria to glucose. We hypothesized that ciliates would direct more glucose to synthesis of reserve carbohydrate (and less to energy spilling) than would bacteria. Ciliates and bacteria were isolated from rumen fluid using filtration and centrifugation, resuspended in nitrogen-free buffer to limit growth, and dosed with 5 mM glucose. Compared with bacteria, ciliates consumed glucose >3-fold faster and synthesized reserve carbohydrate 4-fold faster. They incorporated 53% of glucose carbon into reserve carbohydrate-nearly double the value (27%) for bacteria. Energy spilling was not detected for ciliates, as all heat production (104%) was accounted by synthesis of reserve carbohydrate and endogenous metabolism. For bacteria, reserve carbohydrate and endogenous metabolism accounted for only 68% of heat production, and spilling was detected within 11 min of dosing glucose. These results suggest that ciliates alter the course of ruminal carbohydrate metabolism by outcompeting bacteria for excess carbohydrate, maximizing reserve carbohydrate synthesis, and minimizing energy spilling.

Maximizing efficiency of rumen microbial protein production

Rumen microbes produce cellular protein inefficiently partly because they do not direct all ATP toward growth. They direct some ATP toward maintenance functions, as long-recognized, but they also direct ATP toward reserve carbohydrate synthesis and energy spilling (futile cycles that dissipate heat). Rumen microbes expend ATP by vacillating between (1) accumulation of reserve carbohydrate after feeding (during carbohydrate excess) and (2) mobilization of that carbohydrate thereafter (during carbohydrate limitation). Protozoa account for most accumulation of reserve carbohydrate, and in competition experiments, protozoa accumulated nearly 35-fold more reserve carbohydrate than bacteria. Some pure cultures of bacteria spill energy, but only recently have mixed rumen communities been recognized as capable of the same. When these communities were dosed glucose in vitro, energy spilling could account for nearly 40% of heat production. We suspect that cycling of glycogen (a major reserve carbohydrate) is a major mechanism of spilling; such cycling has already been observed in single-species cultures of protozoa and bacteria. Interconversions of short-chain fatty acids (SCFA) may also expend ATP and depress efficiency of microbial protein production. These interconversions may involve extensive cycling of intermediates, such as cycling of acetate during butyrate production in certain butyrivibrios. We speculate this cycling may expend ATP directly or indirectly. By further quantifying the impact of reserve carbohydrate accumulation, energy spilling, and SCFA interconversions on growth efficiency, we can improve prediction of microbial protein production and guide efforts to improve efficiency of microbial protein production in the rumen.

Accumulation of reserve carbohydrate by rumen protozoa and bacteria in competition for glucose

The aim of this study was to determine if rumen protozoa could form large amounts of reserve carbohydrate compared to the amounts formed by bacteria when competing for glucose in batch cultures. We separated large protozoa and small bacteria from rumen fluid by filtration and centrifugation, recombined equal protein masses of each group into one mixture, and subsequently harvested (reseparated) these groups at intervals after glucose dosing. This method allowed us to monitor reserve carbohydrate accumulation of protozoa and bacteria individually. When mixtures were dosed with a moderate concentration of glucose (4.62 or 5 mM) (n = 2 each), protozoa accumulated large amounts of reserve carbohydrate; 58.7% (standard error of the mean [SEM], 2.2%) glucose carbon was recovered from protozoal reserve carbohydrate at time of peak reserve carbohydrate concentrations. Only 1.7% (SEM, 2.2%) was recovered in bacterial reserve carbohydrate, which was less than that for protozoa (P < 0.001). When provided a high concentration of glucose (20 mM) (n = 4 each), 24.1% (SEM, 2.2%) of glucose carbon was recovered from protozoal reserve carbohydrate, which was still higher (P = 0.001) than the 5.0% (SEM, 2.2%) glucose carbon recovered from bacterial reserve carbohydrate. Our novel competition experiments directly demonstrate that mixed protozoa can sequester sugar away from bacteria by accumulating reserve carbohydrate, giving protozoa a competitive advantage and stabilizing fermentation in the rumen. Similar experiments could be used to investigate the importance of starch sequestration.

A Cronobacter turicensis O1 antigen-specific monoclonal antibody inhibits bacterial motility and entry into epithelial cells

Cronobacter turicensis is an opportunistic foodborne pathogen that can cause a rare but sometimes lethal infection in neonates. Little is known about the virulence mechanisms and intracellular lifestyle of this pathogen. In this study, we developed an IgG monoclonal antibody (MAb; MAb 2G4) that specifically recognizes the O1 antigen of C. turicensis cells. The antilipopolysaccharide antibody bound predominantly monovalently to the O antigen and reduced bacterial growth without causing cell agglutination. Furthermore, binding of the antibody to the O1 antigen of C. turicensis cells caused a significant reduction of the membrane potential which is required to energize flagellar rotation, accompanied by a decreased flagellum-based motility. These results indicate that binding of IgG to the O antigen of C. turicensis causes a direct antimicrobial effect. In addition, this feature of the antibody enabled new insight into the pathogenicity of C. turicensis. In a tissue culture infection model, pretreatment of C. turicensis with MAb 2G4 showed no difference in adhesion to human epithelial cells, whereas invasion of bacteria into Caco-2 cells was significantly inhibited.

The inefficient aerobic energetics of Zymomonas mobilis: identifying the bottleneck

To investigate the mechanisms of Zymomonas mobilis uncoupled aerobic metabolism, growth properties of the wild-type strain Zm6 were compared to those of its respiratory mutants cytB and cydB, and the effects of the ATPase inhibitor DCCD on growth and intracellular ATP concentration were studied. The effects of the ATPase inhibitor DCCD on growth and intracellular ATP concentration strongly indicated that the apparent lack of oxidative phosphorylation in aerobically growing Z. mobilis culture might be caused by the ATP hydrolyzing activity of the H(+) -dependent ATPase in all analyzed strains. Aerobic growth yields of the mutants, and their capacity of oxidative ATP synthesis with ethanol were closely similar, not supporting presence of one major, yet energetically inefficient electron transport branch causing the observed poor aerobic growth and lack of oxidative phosphorylation in Z. mobilis. We suggest that rapidly operating Entner-Doudoroff pathway generates too high phosphorylation potential for the weakly coupled respiratory system to shift the H(+) -dependent ATPase toward ATP synthesis.

Measurement of soil bacterial colony temperatures and isolation of a high heat-producing bacterium

BACKGROUND

The cellular temperatures of microorganisms are considered to be the same as those of their surroundings because the cellular volume is too small to maintain a cellular temperature that is different from the ambient temperature. However, by forming a colony or a biofilm, microorganisms may be able to maintain a cellular temperature that is different from the ambient temperature. In this study, we measured the temperatures of bacterial colonies isolated from soils using an infrared imager and investigated the thermogenesis by a bacterium that increases its colony temperature.

RESULTS

The temperatures of some colonies were higher or lower than that of the surrounding medium. A bacterial isolate with the highest colony temperature was identified as Pseudomonas putida. This bacterial isolate had an increased colony temperature when it grew at a temperature suboptimal for its growth. Measurements of heat production using a microcalorimeter showed that the temperature of this extraordinary, microcalorimetrically determined thermogenesis corresponded with the thermographically observed increase in bacterial colony temperature. When investigating the effects of the energy source on this thermal behavior, we found that heat production by this bacterium increased without additional biomass production at a temperature suboptimal for its growth.

CONCLUSIONS

We found that heat production by bacteria affected the bacterial colony temperature and that a bacterium identified as Pseudomonas putida could maintain a cellular temperature different from the ambient temperature, particularly at a sub-optimal growth temperature. The bacterial isolate P. putida KT1401 increased its colony temperature by an energy-spilling reaction when the incubation temperature limited its growth.

Microbial life under extreme energy limitation

A great number of the bacteria and archaea on Earth are found in subsurface environments in a physiological state that is poorly represented or explained by laboratory cultures. Microbial cells in these very stable and oligotrophic settings catabolize 10⁴- to 10⁶-fold more slowly than model organisms in nutrient-rich cultures, turn over biomass on timescales of centuries to millennia rather than hours to days, and subsist with energy fluxes that are 1,000-fold lower than the typical culture-based estimates of maintenance requirements. To reconcile this disparate state of being with our knowledge of microbial physiology will require a revised understanding of microbial energy requirements, including identifying the factors that comprise true basal maintenance and the adaptations that might serve to minimize these factors.

Insights into the bovine rumen plasmidome

Plasmids are self-replicating genetic elements capable of mobilization between different hosts. Plasmids often serve as mediators of lateral gene transfer, a process considered to be a strong and sculpting evolutionary force in microbial environments. Our aim was to characterize the overall plasmid population in the environment of the bovine rumen, which houses a complex and dense microbiota that holds enormous significance for humans. We developed a procedure for the isolation of total rumen plasmid DNA, termed rumen plasmidome, and subjected it to deep sequencing using the Illumina paired-end protocol and analysis using public and custom-made bioinformatics tools. A large number of plasmidome contigs aligned with plasmids of rumen bacteria isolated from different locations and at various time points, suggesting that not only the bacterial taxa, but also their plasmids, are defined by the ecological niche. The bacterial phylum distribution of the plasmidome was different from that of the rumen bacterial taxa. Nevertheless, both shared a dominance of the phyla Firmicutes, Bacteroidetes, and Proteobacteria. Evidently, the rumen plasmidome is of a highly mosaic nature that can cross phyla. Interestingly, when we compared the functional profile of the rumen plasmidome to two plasmid databases and two recently published rumen metagenomes, it became apparent that the rumen plasmidome codes for functions, which are enriched in the rumen ecological niche and could confer advantages to their hosts, suggesting that the functional profiles of mobile genetic elements are associated with their environment, as has been previously implied for viruses.

Kinetics of Cellulose Digestion by Fibrobacter succinogenes S85

Growing cultures of Fibrobacter succinogenes S85 digested cellulose at a rapid rate, but nongrowing cells and cell extracts did not have detectable crystalline cellulase activity. Cells that had been growing exponentially on cellobiose initiated cellulose digestion and succinate production immediately, and cellulose-dependent succinate production could be used as an index of enzyme activity against crystalline cellulose. Cells incubated with cellulose never produced detectable cellobiose, and cells that were preincubated for a short time with thiocellobiose lost their ability to digest cellulose (competitive inhibition [K(infi)] of only 0.2 mg/ml or 0.56 mM). Based on these results, the crystalline cellulases of F. succinogenes were very sensitive to feedback inhibition. Different cellulose sources bound different amounts of Congo red, and the binding capacity was HCl-regenerated cellulose > ball-milled cellulose > Sigmacel > Avicel > filter paper. Congo red binding capacity was highly correlated with the maximum rates of metabolism of cellulose digestion and inversely related to K(infm). Congo red (250 (mu)g/ml) did not inhibit the growth of F. succinogenes S85 on cellobiose, but this concentration of Congo red inhibited the rate of ball-milled cellulose digestion. A Lineweaver-Burk plot of ball-milled cellulose digestion rate versus the amount of cellulose indicated that Congo red was a competitive inhibitor of cellulose digestion (K(infi) was 250 (mu)g/ml).

Bovicin HC5, a lantibiotic produced by Streptococcus bovis HC5, catalyzes the efflux of intracellular potassium but not ATP

Bovicin HC5, a broad-spectrum lantibiotic produced by Streptococcus bovis HC5, catalyzed the efflux of intracellular potassium from Streptococcus bovis JB1, a sensitive strain. The level of ATP also decreased, but this decline appeared to be caused by the activity of the F(1)F(0) ATPase rather than efflux per se.

The energy spilling reactions of bacteria and other organisms

For many years it was assumed that living organisms always utilized ATP in a highly efficient manner, but simple growth studies with bacteria indicated that the efficiency of biomass production was often at least 3-fold lower than the amount that would be predicted from standard biosynthetic pathways. The utilization of energy for maintenance could only explain a small portion of this discrepancy particularly when the growth rate was high. These ideas and thermodynamic arguments indicated that cells might have another avenue of energy utilization. This phenomenon has also been called 'uncoupling', 'spillage' and 'overflow metabolism', but 'energy spilling' is probably the most descriptive term. It appears that many bacteria spill energy, and the few that do not can be killed (large and often rapid decrease in viability), if the growth medium is nitrogen-limited and the energy source is in 'excess'. The lactic acid bacterium, Streptococcus bovis, is an ideal bacterium for the study of energy spilling. Because it only uses substrate level phosphorylation to generate ATP, ATP generation can be calculated with a high degree of certainty. It does not store glucose as glycogen, and its cell membrane can be easily accessed. Comparative analysis of heat production, membrane voltage, ATP production and Ohm's law indicated that the energy spilling reaction of S. bovis is mediated by a futile cycle of protons through the cell membrane. Less is known about Escherichia coli, but in this bacterium energy spilling could be mediated by a futile cycle of potassium or ammonium ions. Energy spilling is not restricted to prokaryotes and appears to occur in yeasts and in higher organisms. In man, energy spilling may be related to cancer, ageing, ischemia and cardiac failure.

Physiology of Zymomonas mobilis: some unanswered questions

The ethanol-producing bacterium Zymomonas mobilis can serve as a model organism for the study of rapid catabolism and inefficient energy conversion in bacteria. Some basic aspects of its physiology still remain poorly understood. Here, the energy-spilling pathways during uncoupled growth, the structure and function of electron transport chain, and the possible reasons for the inefficient oxidative phosphorylation are analysed. Also, the interaction between ethanol synthesis and respiration is considered. The search for mechanisms of futile transmembrane proton cycling, as well as identification of respiratory electron transport complexes, like the energy-coupling NAD(P)H:quinone oxidoreductase and the cyanide-sensitive terminal oxidase(s), are outlined as the key problems for further research of Z. mobilis energy metabolism.

The ability of acidic pH, growth inhibitors, and glucose to increase the proton motive force and energy spilling of amino acid-fermenting Clostridium sporogenes MD1 cultures

Clostridium sporogenes MD1 grew rapidly with peptides and amino acids as an energy source at pH 6.7. However, the proton motive force (Deltap) was only -25 mV, and protonophores did not inhibit growth. When extracellular pH was decreased with HCl, the chemical gradient of protons (ZDeltapH) and the electrical membrane potential (DeltaPsi) increased. The Deltap was -125 mV at pH 4.7, even though growth was not observed. At pH 6.7, glucose addition did not cause an increase in growth rate, but DeltaPsi increased to -70 mV. Protein synthesis inhibitors also significantly increased DeltaPsi. Non-growing, arginine-energized cells had a DeltaPsi of -80 mV at pH 6.7 or pH 4.7, but DeltaPsi was not detected if the F1F0 ATPase was inhibited. Arginine-energized cells initiated growth if other amino acids were added at pH 6.7, and DeltaPsi and ATP declined. At pH 4.7, ATP production remained high. However, growth could not be initiated, and neither DeltaPsi nor the intracellular ATP concentration declined. Based on these results, it appears that C. sporogenes MD1 does not need a large Deltap to grow, and Deltap appears to serve as a mechanism of ATP dissipation or energy spilling.

The glucomannokinase of Prevotella bryantii B(1)4 and its potential role in regulating beta-glucanase expression

Prevotella bryantii B(1)4 has a transport system for glucose and mannose, but beta-glucanase expression is only catabolite-repressed by glucose. P bryantii B(1)4 cell extracts had ATP-dependent gluco- and mannokinase activities, and significant phosphoenolpyruvate- or GTP-dependent hexose phosphorylation was not observed. Mannose inhibited glucose phosphorylation (and vice versa), and activity gels indicated that a single protein was responsible for both activities. Glucose was phosphorylated at a faster rate than was mannose [V(max) 280 nmol hexose (mg protein)(-1) min(-1) versus 60 nmol hexose (mg protein)(-1) min(-1), respectively] and glucose was a better substrate for the kinase (K(m) 0.12 mM versus 1.2 mM, respectively). The purified glucomannokinase (1250-fold) had a molecular mass of 68 kDa, but SDS-PAGE gels indicated that it was a dimer (monomer 34.5 kDa). The N-terminus (25 residues) had an 8 amino acid segment that was homologous to other bacterial glucokinases. The glucomannokinase was competitively inhibited by the nonmetabolizable glucose analogue 2-deoxyglucose (2DG), and cells grown with glucose and 2DG had lower rates of glucose consumption than did cells given only glucose. When the ratio of 2DG to glucose was increased, the glucose consumption rate decreased and the beta-glucanase activity increased. The glucose consumption rate and the glucomannokinase activity of cells treated with 2DG were highly correlated (r(2)=0.98). This result suggested that glucomannokinase activity was either directly or indirectly regulating beta-glucanase expression.

Protonmotive force regulates the membrane conductance of Streptococcus bovis in a non-ohmic fashion

Because the DCCD (dicyclohexylcarbodiimide)-sensitive, F-ATPase-mediated, futile ATP hydrolysis of non-growing Streptococcus bovis JB1 cells was not affected by sodium or potassium, ATP hydrolysis appeared to be dependent only upon the rate of proton flux across the cell membrane. However, available estimates of bacterial proton conductance were too low to account for the rate of ATP turnover observed in S. bovis. When de-energized cells were subjected to large pH gradients (2.75 units, or -170 mV), internal pH declined at a rate of 0.15 pH units s(-1). Based on an estimated cellular buffering capacity of 200 nmol H+ (mg protein)(-1) per pH unit, H+ flux across the cell membrane (at -170 mV) was 108 mmol (g protein)(-1) h(-1). When potassium-loaded cells were treated with valinomycin in low-potassium buffers, initial K+ efflux generated membrane potentials in close agreement with values predicted by the Nernst equation. These artificial membrane potentials drove H+ uptake, and H+ influx was counterbalanced by a further loss of cellular K+. Flame photometry indicated that the rate of K+ loss was 215 (+/-26) mmol K+ (g protein)(-1) h(-1) at -170 mV, but the potassium-sensitive fluorescent compound CD222 indicated that this rate was only 110 (+/-44) mmol K+ (g protein)(-1) h(-1). As pH gradients or membrane potentials were reduced, the rate of H+ flux declined in a non-ohmic fashion, and all rates were <25 mmol (g protein)(-1) h(-1) at a driving force of -80 mV. Previous estimates of bacterial proton flux were based on low and unphysiological protonmotive forces, and the assumption that H+ influx rate would be ohmic. Rates of H+ influx into S. bovis cells [as high as 9x10(-11) mol H+ (cm membrane)(-2) s(-1)] were similar to rates reported for respiring mitochondria, but were at least 20-fold greater than any rate previously reported in lactic acid bacteria.

Antimicrobial activity of a 14-residue peptide against Escherichia coli O157:H7

An amphiphilic, cationic peptide composed of eight leucines and six lysines was synthesized by solid phase peptide synthesis (SPPS). The synthetic peptide was bactericidal within 10 min at concentrations as low as 3 microg ml - 1 against mid-exponential Escherichia coli O157:H7 suspended in buffer. Concentrations of 25 microg ml - 1 caused up to 7 log10 cfu ml - 1 reductions. When tested against E. coli O157:H7 grown in TSB, the peptide was bactericidal and bacteriostatic at concentrations of 50 and 25 microg ml - 1, respectively. An inhibitory effect was also observed against stationary phase cells. The synthetic peptide caused the release of u.v.-absorbing materials from the E. coli O157:H7 as well as an increase in its O.D.600 nm. Intracellular K+ and ATP depletion were also observed. These results suggest that the peptide increased the cell membrane permeability but it did not lyse the cells.

Relationship between intracellular phosphate, proton motive force, and rate of nongrowth energy dissipation (energy spilling) in Streptococcus bovis JB1

When the rate of glucose addition to nongrowing Streptococcus bovis cell suspensions was increased, the fermentation was homolactic, fructose-1,6-diphosphate (FDP) increased, intracellular inorganic phosphate (P(i)) declined, and the energy-spilling rate increased. ATP and ADP were not significantly affected by glucose consumption rate, but the decrease in P(i) was sufficient to cause an increase in the free energy of ATP hydrolysis (delta G'p). The increase in delta G'p was correlated with an increase in proton motive force (delta p). S. bovis continuous cultures (dilution rate of 0.65 h-1) that were provided with ammonia as the sole nitrogen source also had high rates of lactate production and energy spilling. When Trypticase was added as a source of amino acids, lactate production decreased; a greater fraction of the glucose was converted to acetate, formate, and ethanol; and the energy-spilling rate decreased. Trypticase also caused a decrease in FDP, an increase in P(i), and a decrease in delta p. The change in delta p could be explained by P(i)-dependent changes in the delta G'p. When P(i) declined, delta G'p and delta p increased. The ratio of delta G'p to delta p (millivolt per millivolt) was always high (> 4) at low rates of energy spilling but declined when the energy-spilling rate increased. Based on these results, it appears that delta p and the energy-spilling rate are responsive to fluctuations in the intracellular P(i) concentration.

The effects of fermentation acids on bacterial growth

Anaerobic habitats often have low pH and high concentrations of fermentation acids, and these conditions can inhibit the growth of many bacteria. The toxicity of fermentation acids at low pH was traditionally explained by an uncoupling mechanism. Undissociated fermentation acids can pass across the cell membrane and dissociate in the more alkaline interior, but there is little evidence that they can act in a cyclic manner to dissipate protonmotive force. Fermentation acid dissociation in the more alkaline interior causes an accumulation of the anionic species, and this accumulation is dependent on the pH gradient (delta pH) across the membrane. Fermentation acid-resistant bacteria have low delta pH and are able to generate ATP and grow with a low intracellular pH. Escherichia coli O157:H7 is able to decrease its intracellular pH to 6.1 before growth ceases, but this modest decrease in delta pH can only partially counteract the toxic effect of fermentation anion accumulation. Fermentation acid-resistant bacteria are in most cases Gram-positive bacteria with a high intracellular potassium concentration, and even acid-sensitive bacteria like E. coli K-12 have increased potassium levels when fermentation acids are present. Intracellular potassium provides a counteraction for fermentation acid anions, and allows bacteria to tolerate even greater amounts of fermentation anions. The delta pH-mediated anion accumulation provides a mechanistic explanation for the effect of fermentation acids on microbial ecology and metabolism.

A Role for Fructose 1,6-Diphosphate in the ATPase-Mediated Energy-Spilling Reaction of Streptococcus bovis

The amount of ATP produced by Streptococcus bovis was larger than the amount that could be attributed to growth and maintenance, and even glucose-limited continuous cultures used ATP inefficiently (spilled ATP). Rapid-dilution-rate cultures always spilled more ATP than those growing at slow dilution rates, but rates of ATP spilling could also be enhanced by amino acid deprivation (with only ammonia as a nitrogen source). Energy spilling and intracellular ATP were not correlated, but energy spilling was always greatest when the rate of lactate production was high. The relationship between lactate production and energy spilling was supported by the observation that amino acid deprivation increased lactate production and ATP spilling. The lactate production rate of nongrowing (energy-spilling) S. bovis cells was fructose 1,6-diphosphate (FDP) dependent, and previous work showed that the lactate dehydrogenase of S. bovis was activated by FDP (M. J. Wolin, Science 146:775-777, 1964). The role of FDP in energy spilling was supported by the observation that the membrane-bound ATPase of S. bovis could be stimulated by FDP. FDP decreased the K(infm) for ATP by as much as fivefold. Other glycolytic intermediates could not stimulate the ATPase of washed membrane preparations, and FDP had no effect on soluble ATPase activity.

Nisin stimulates oxygen consumption by Staphylococcus aureus and Escherichia coli

Nisin stimulated oxygen consumption by nongrowing, glucose-metabolizing Staphylococcus aureus and Escherichia coli cells, indicating a protonophore mode of action. A similar stimulation in E. coli cells osmotically stressed to disrupt the outer cell membrane confirmed the cytoplasmic membrane as the site of nisin action and showed that nisin uptake was not prevented by the outer membrane.

The effect of growth and starvation on the lysis of the ruminal cellulolytic bacterium Fibrobacter succinogenes

Growing cultures of Fibrobacter succinogenes assimilated more ammonia than could be accounted for by cellular protein, RNA, or DNA and released large amounts of nonammonia nitrogen. The difference between net and true growth was most dramatic at low dilution rates, but mathematical derivations indicated that the lysis rate was a growth rate-independent function. The lysis rate was sevenfold greater than the true maintenance rate (0.07 h-1 versus 0.01 h-1). Because slowly growing cells had as much proton motive force and ATP as fast-growing cells, lysis was not a starvation response per se. Stationary-phase cells had a lysis rate that was 10-fold less than that of growing cells. Rapidly growing cells were not susceptible to phenylmethylsulfonyl fluoride, but phenylmethylsulfonyl fluoride increased the lysis rate of the cultures when they reached the stationary phase. This latter result indicated that autolysins of stationary-phase cells were being inactivated by a serine proteinase. When growing cells were treated with the glycolytic inhibitor iodoacetate, the proteinase-dependent transition to the stationary phase was circumvented, and the rate of lysis could be increased by as much as 50-fold.

Energetics of bacterial growth: balance of anabolic and catabolic reactions

Biomass formation represents one of the most basic aspects of bacterial metabolism. While there is an abundance of information concerning individual reactions that result in cell duplication, there has been surprisingly little information on the bioenergetics of growth. For many years, it was assumed that biomass production (anabolism) was proportional to the amount of ATP which could be derived from energy-yielding pathways (catabolism), but later work showed that the ATP yield (YATP) was not necessarily a constant. Continuous-culture experiments indicated that bacteria utilized ATP for metabolic reactions that were not directly related to growth (maintenance functions). Mathematical derivations showed that maintenance energy appeared to be a growth rate-independent function of the cell mass and time. Later work, however, showed that maintenance energy alone could not account for all the variations in yield. Because only some of the discrepancy could be explained by the secretion of metabolites (overflow metabolism) or the diversion of catabolism to metabolic pathways which produced less ATP, it appeared that energy-excess cultures had mechanisms of spilling energy. Bacteria have the potential to spill excess ATP in futile enzyme cycles, but there has been little proof that such cycles are significant. Recent work indicated that bacteria can also use futile cycles of potassium, ammonia, and protons through the cell membrane to dissipate ATP either directly or indirectly. The utility of energy spilling in bacteria has been a curiosity. The deprivation of energy from potential competitors is at best a teleological explanation that cannot be easily supported by standard theories of natural selection. The priming of intracellular intermediates for future growth or protection of cells from potentially toxic end products (e.g., methylglyoxal) seems a more plausible explanation.

Cloning, sequencing and characterization of the alkaline phosphatase gene (phoD) from Zymomonas mobilis

The phoD gene encoding the membrane-bound alkaline phosphatase (ALPI) from Zymomonas mobilis CP4 was cloned and sequenced. Both the translated sequence and the properties of the recombinant enzyme were unusual. Z. mobilis ALPI was monomeric (M(r) 62,926) and hydrolysed nucleotides more effectively than sugar phosphates. The translated sequence contained a single hydrophobic segment near the N-terminus which may serve as a membrane-anchor in Z. mobilis, although the recombinant enzyme was recovered in the cytoplasmic fraction of Escherichia coli. The predicted amino acid sequence for ALPI did not align well with other ALPs or other known genes. However, some similarity to E. coli ALP was noted in the metal-binding and phosphate-binding regions. Two other regions were identified with similarity to the active sites of pyruvate kinase and mammalian 5'-nucleotide phosphodiesterase (also membrane-bound), respectively. It is likely that Z. mobilis phoD represents a new class of alkaline phosphatase genes which has not been described previously.

Energy-spilling reactions of Streptococcus bovis and resistance of its membrane to proton conductance

Glucose-excess cultures of Streptococcus bovis consumed glucose faster than the amount that could be explained by growth or maintenance, and nongrowing chloramphenicol-treated cells had a rate of glucose consumption that was 10-fold greater than the maintenance rate. Because N,N-dicyclohexylcarbodiimide, an inhibitor of the membrane-bound F1F0 ATPase, eliminated the nongrowth energy dissipation (energy spilling) without a decrease in ATP and the rate of energy spilling could be increased by the protonophore 3,3',4',5-tetrachlorosalicylanilide, it appeared that a futile cycle of protons through the cell membrane was responsible for most of the energy spilling. When the rate of energy spilling was decreased gradually with iodoacetate, there was only a small decrease in the phosphorylation potential (delta G'p) and the theoretical estimate of H+ per ATP decreased from 4.2 to 3.6. On the bases of this ratio of H+ to ATP and the rate of ATP production, the flux of protons (amperage) across the cell membrane was directly proportional to the rate of energy spilling. Amperage values estimated from delta G'p were, however, nearly twice as great as values which were estimated from the heat production (delta H) of the cells [amperage = (0.38 x wattage)/delta p]. The last comparison indicated that only a fraction of the delta G of ATP hydrolysis was harvested by the F1F0 ATPase to pump protons. Both estimates of amperage indicated that the resistance of the cell membrane to proton conductance was inversely proportional to the log of the energy-spilling rate.

The glutamine cyclotransferase reaction of Streptococcus bovis: a novel mechanism of deriving energy from non-oxidative and non-reductive deamination

Streptococcus bovis deaminated glutamine by a mechanism that did not involve glutaminase. Since pyroglutamate and ammonia were the only end-products, it appeared that glutamine deamination was catalyzed by a cyclotransferase reaction. Stationary S. bovis cells had essentially no intracellular ATP or membrane potential (delta psi), however, when they were provided with glutamine, intracellular ATP and delta psi increased to 0.52 mM and 158 mV, respectively. When glutamine-energized cells were treated with N,N-dicyclohexylcarbodiimide (DCCD, 150 microM), there was an even greater increase in intracellular ATP (> 5-fold) and the delta psi was dissipated. Because toluene-treated cells produced ATP from ADP and Pi, it did not appear that the cell membrane was directly involved in glutamine-dependent ATP generation. The rate of ammonia production was directly proportional to the glutamine concentration, but the stoichiometry of ATP to ammonia was always 1 to 1. Based on these results, it appeared that glutamine was deaminated by glutamine cyclotransferase which was coupled to ATP formation. The membrane bound ATPase then used the ATP to create a delta psi.

Effect of amino acids on the heat production and growth efficiency of Streptococcus bovis: balance of anabolic and catabolic rates

Streptococcus bovis JB1 grew nearly twice as fast (0.9 versus 1.6 h-1) and had a 40% greater growth yield (18 versus 12.5 mg of protein per mmol of glucose) when an ammonia-based medium was supplemented with amino acids, but the glucose consumption rate (88 mumol mg of protein-1 h-1) and specific rate of heat production (2.1 mW/mg of protein) were unaffected. Amino acid availability had little effect on the catabolic rate, but the specific heat decreased 40% (8.8 to 5.2 J/mg of protein). These growth rate-dependent changes in metabolic efficiency were fivefold greater than the maintenance energy. Chloramphenicol (100 mg/l), an inhibitor of protein synthesis, caused a gradual decrease in anabolic (growth) rate, but there was little change in the rate of glucose consumption and the specific heat increased. When growth was inhibited by iodoacetate, the catabolic and anabolic rates both declined and there was not increase in specific heat. On the basis of these results, the benefit of amino acid supplementation was largely explained by the balance of anabolic and catabolic rates. When amino acids were available, the anabolic and catabolic rates were more closely matched and less energy was spilled as heat.

Glucose toxicity and inability of Bacteroides ruminicola to regulate glucose transport and utilization

Ammonia-limited (3.5 mM ammonia) cultures of Bacteroides ruminicola B(1)4 had a high number of viable cells (greater than 10(9)/ml), but only when the concentration of glucose was not too high (10 mM or less). When the glucose concentration was increased from 10 to 50 mM, there was a marked decrease in viability (10(5)-fold or greater). Because there was little decline in pH and only a small increase in succinate and acetate as the glucose concentration was increased, it did not appear that end products were killing the cells. This conclusion was supported by the observation that reinoculated cultures grew in the spent medium which had been supplemented with ammonia. Unlabeled rhamnose did not inhibit [14C]-glucose uptake, and cultures which were selected with a low concentration of rhamnose tolerated high concentrations of glucose (50 mM). The glucose-resistant mutant transported glucose at a lower rate than the wild type, and the Vmax of glucose transport was fourfold lower. The wild type stored much more polysaccharide than the glucose-resistant mutant, but it is not clear if polysaccharide accumulation per se is responsible for the glucose toxicity. These results indicated that B. ruminicola B(1)4 is unable to regulate glucose transport and utilization when growth is limited by ammonia.

The effect of pH on the heat production and membrane resistance of Streptococcus bovis

Non-growing cultures of Streptococcus bovis JB1 which were incubated in 2-[N-moropholino] ethane-sulfonic acid (MES)-phosphate buffer (pH 6.8) and glucose (2 g/l) produced heat at a rate of 0.17 mW/mg protein, and this rate was proportional to the enthalpy change of the homolactic fermentation. Since the growth-independent heat production could be eliminated by dicyclohexylcarbodiimide (DCCD), an inhibitor of F1F0 ATPases, it appeared that virtually all of the energy was being used to counteract proton flux through the cell membrane. When the pH was decreased from 6.8 to 5.8, heat production and glucose consumption increased, the electrical potential (delta psi) declined, the chemical gradient of protons (Z delta pH) increased, and there was a small increase in total protonmotive force (delta p). Further decreases in pH (5.8 to 4.5) caused a marked decrease in heat production and glucose consumption even though there was only a small decline in membrane voltage. Based on the enthalpy of ATP (4 kcal or 16.8 kJ/mol), it appeared that 38% of the wattage was passing through the cell membrane. The relationship between membrane voltage and membrane wattage or glucose consumption was non-linear (non-ohmic), and it appeared that the resistance of the membrane to current flow was not constant. Based on the electrical formula, resistance = voltage2/wattage and resistance = voltage/amperage, there was a marked increase in membrane resistance when the pH was less than 6.0. The increase in membrane resistance at low pH allowed S. bovis to maintain its membrane potential and expend less energy when its ability to ferment glucose was impaired.

Effect of pH and Monensin on Glucose Transport by Fibrobacter succinogenes , a Cellulolytic Ruminal Bacterium

Fibrobacter succinogenes S85, a cellulolytic ruminal bacterium, required sodium for growth and glucose uptake. Cells which were deenergized with iodoacetate (500 μM) could not take up [ 14 C]glucose. However, deenergized cells which were treated with valinomycin, loaded with potassium, and diluted into sodium or sodium plus potassium to create an artificial electrical gradient (ΔΨ) plus a chemical gradient of sodium (ΔpNa) or ΔpNa alone transported glucose at a rapid rate. Cells which were loaded with potassium plus sodium and diluted into sodium (ΔΨ with sodium, but no ΔpNa) also took up glucose at a rapid rate. Potassium-loaded cells that were diluted into buffers which did not contain sodium (ΔΨ without sodium) could not take up glucose. An artificial ZΔpH which was created by acetate diffusion could not drive glucose transport even if sodium was present. The maximum rate and affinity of glucose transport (pH 6.7) were 62.5 nmol/mg of protein per min and 0.51 mM, respectively. S85 was unable to grow at a pH of less than 5.5, and there was little glucose transport at this pH. When the extracellular pH was decreased, the glucose carrier was inhibited, intracellular pH declined, the cells were no longer able to metabolize glucose, and ΔΨ declined. Monensin (1 μM) or lasalocid (5 μM) decreased intracellular ATP and dissipated both the ΔΨ and ΔpNa. Since there was no driving force for transport, glucose transport was inhibited. These results indicated that F. succinogenes used a pH-sensitive sodium symport mechanism to take up glucose and that either a ΔΨ or a ΔpNa was required for glucose transport.

A re-assessment of bacterial growth efficiency: the heat production and membrane potential of Streptococcus bovis in batch and continuous culture

Glucose-limited, continuous cultures (dilution rate 0.1 h-1) of Streptococcus bovis JB1 fermented glucose at a rate of 3.9 mumol mg protein-1 h-1 and produced acetate, formate and ethanol. Based on a maximum ATP yield of 32 cells/mol ATP (Stouthamer 1973) and 3 ATP/glucose, the theoretical glucose consumption for growth would have been 2.1 mumol mg protein-1 h-1. Because the maintenance energy requirement was 1.7 mumol/mg protein/h (Russell and Baldwin 1979), virtually all of the glucose consumption could be explained by growth and maintenance and the YATP was 30. Glucose-limited, continuous cultures produced heat at a rate of 0.29 mW/mg protein, and this value was similar to the enthalpy change of the fermentation (0.32 mW/mg protein). Batch cultures (specific growth rate 2.0 h-1) fermented glucose at a rate of 81 mumol mg protein-1 h-1, and produced only lactate. The heat production was in close agreement with the theoretical enthalpy change (1.72 versus 1.70 mW/mg protein), but only 80% of the glucose consumption could be accounted by growth and maintenance. The YATP of the batch cultures was 25. Nitrogen-limited, glucose-excess, non-growing cultures fermented glucose at a rate of 6.9 mumol mg protein-1 h-1, and virtually all of the enthalpy for this homolactic fermentation could be accounted as heat (0.17 mW/mg protein). The nitrogen-limited cultures had a membrane potential of 150 mV, and nearly all of the heat production could be explained by a futile cycle of protons through the cell membrane (watts = amperes X voltage where H+/ATP was 3).(ABSTRACT TRUNCATED AT 250 WORDS)

Resistance of Streptococcus bovis to acetic acid at low pH: relationship between intracellular pH and anion accumulation

Streptococcus bovis JB1, an acid-tolerant ruminal bacterium, was able to grow at pHs from 6.7 to 4.5, and 100 mM acetate had little effect on growth rate or proton motive force across the cell membrane. When S. bovis was grown in glucose-limited chemostats at pH 5.2, the addition of sodium acetate (as much as 100 mM) had little effect on the production of bacterial protein. At higher concentrations of sodium acetate (100 to 360 mM), production of bacterial protein declined, but this decrease could largely be explained by a shift in fermentation products (acetate, formate, and ethanol production to lactate production) and a decline in ATP production (3 ATP per glucose versus 2 ATP per glucose). YATP (grams of cells per mole of ATP) was not decreased significantly even by high concentrations of acetate. Cultures supplemented with 100 mM sodium acetate took up [14C]acetate and [14C]benzoate in accordance with the Henderson-Hasselbalch equation and gave similar estimates of intracellular pH. As the extracellular pH declined, S. bovis allowed its intracellular pH to decrease and maintained a relatively constant pH gradient across the cell membrane (0.9 unit). The decrease in intracellular pH prevented S. bovis from accumulating large amounts of acetate anion. On the basis of these results it did not appear that acetate was acting as an uncoupler. The sensitivity of other bacteria to volatile fatty acids at low pH is explained most easily by a high transmembrane pH gradient and anion accumulation.