Incidence and risk factors for venous reflux in the general population: Edinburgh Vein Study.

OBJECTIVE/BACKGROUND: Chronic venous disease (CVD) is common, but the incidence of venous reflux, a precursor to this condition, is unknown. This study measured the incidence of venous reflux and associated risk factors, and examined the association between venous reflux and the incidence of CVD. METHODS: In the Edinburgh Vein Study, a random sample of 1566 men and women aged 18-64 years were examined at baseline. Eight hundred and eighty of these patients were followed up 13 years and underwent an examination comprising clinical classification of CVD and duplex scanning of the deep and superficial systems to measure venous reflux ≥0.5 s. RESULTS: The 13-year incidence of reflux was 12.7% (95% confidence interval [CI] 9.2-17.2), equivalent to an annual incidence of 0.9% (95% CI 0.7-1.3). The 13-year incidence of isolated superficial, isolated deep, and combined deep and superficial reflux was 8.8% (95% CI 5.6-12.0), 2.6% (95% CI 1.2-5.0), and 1.3% (95% CI 0.4-3.2), respectively. The highest incidence was in the great saphenous vein in the lower thigh (8.1%, 95% CI 5.4-11.8). There were no age or sex differences (p > .050). The risk of developing reflux was associated with being overweight (odds ratio [OR] 2.1, 95% CI 1.0-4.4) and with history of deep vein thrombosis (OR 11.3, 95% CI 1.0-132.3). Venous reflux at baseline was associated with new varicose veins at follow up (p < .001): the age- and sex-adjusted OR was 4.4 (95% CI 1.8-10.8) in those with isolated superficial reflux and 7.3 (95% CI 2.6-22.5) in those with combined deep and superficial reflux. CONCLUSION: For every year of follow-up, around 1% of this adult population developed venous reflux. In two thirds of cases, the superficial system was affected. Venous reflux increased the risk of developing varicose veins, especially when combined deep and superficial reflux was present.

Identification of quantitative trait Loci for resistance to southern leaf blight and days to anthesis in a maize recombinant inbred line population.

ABSTRACT A recombinant inbred line population derived from a cross between the maize lines NC300 (resistant) and B104 (susceptible) was evaluated for resistance to southern leaf blight (SLB) disease caused by Cochliobolus heterostrophus race O and for days to anthesis in four environments (Clayton, NC, and Tifton, GA, in both 2004 and 2005). Entry mean and average genetic correlations between disease ratings in different environments were high (0.78 to 0.89 and 0.9, respectively) and the overall entry mean heritability for SLB resistance was 0.89. When weighted mean disease ratings were fitted to a model using multiple interval mapping, seven potential quantitative trait loci (QTL) were identified, the two strongest being on chromosomes 3 (bin 3.04) and 9 (bin 9.03-9.04). These QTL explained a combined 80% of the phenotypic variation for SLB resistance. Some time-point-specific SLB resistance QTL were also identified. There was no significant correlation between disease resistance and days to anthesis. Six putative QTL for time to anthesis were identified, none of which coincided with any SLB resistance QTL.

A Lagrangian model of the evolution of the particulate size distribution of vehicular emissions.

The emission inventory for London indicates that nearly 80% of the particulate emissions derive from vehicular sources. Most of this mass is in the form of ultrafine submicrometer particles which are of concern because of their influence on lung function. The prediction of their dispersion in the atmosphere coupled to the physical and chemical transformations which affect their size distribution and concentration are of great importance. This paper reports the first results from a new meso-scale Lagrangian model which follows the particulate emissions and the evolution of their size distribution across the city. The vehicular emissions are based on the published inventory, corrected to time of day, while other emissions are assumed steady. The initial size distributions of background and emitted particles are represented by the sum of three lognormal distributions. Meteorological data are derived from Meteorological Office reports and are preprocessed to obtain the hourly values of boundary layer depth, Monin-Obukov (MO) length, friction velocity, etc., needed for the computation of the vertical dispersion process via eddy diffusivities and the aerodynamic component of the dry deposition process. In the vertical direction, three layers are assumed-surface layer (typically 50 m), canopy layer and one further layer up to the prevailing boundary layer depth. Currently, the model includes wet and dry deposition and coagulation but not chemical reaction, nucleation or deliquescence. Trajectories are evolved for several hours across the city and the number size distributions and mass concentrations (PM10, PM2.5, PM1 and PM0.1) output at each step. This enables the vehicular contributions over and above the background concentration in each size range to be studied in detail. Data from the model have been compared with experimental data for one of the London background sites where particle number size distribution up to 450 nm (SMPS), plus PM10 and PM2.5 (TEOM) data are available.

Multiple assembly signals in gamma-aminobutyric acid (type A) receptor subunits combine to drive receptor construction and composition.

Mammalian gamma-aminobutyric acid type A (GABAA) receptors are constructed from a large repertoire of subunits (alpha1-alpha6, beta1-beta3, gamma1-gamma3, delta, epsilon, theta and pi) into a pentameric ion channel. GABA(A) receptor assembly occurs within the endoplasmic reticulum (ER) and involves interactions with chaperone molecules. Only specific subunit combinations can produce functional surface receptors (with a fixed stoichiometry); other subunit combinations are retained within the ER and degraded. Thus, receptor assembly occurs by defined pathways to limit the diversity of GABAA receptors. The key to understanding how receptor diversity is achieved and controlled is the identification of assembly signals capable of distinguishing between other subunit partners. Analysis of an assembly box in alpha1 (residues 57-68) has revealed an absolute requirement for this region in the assembly of alphabeta receptors. Furthermore, a selective requirement for a single amino acid (R66) is observed for the assembly of alpha1beta2, but not alpha1beta1 or alpha1beta3, receptors. In addition, we have characterized an assembly signal in the beta3 subunit that is capable of driving the assembly of beta3, gamma2beta3 and alpha1beta3 receptors. Interestingly, this signal does not appear to utilize the alpha1 assembly box, suggesting the presence of alternative assembly signals within the alpha1 subunit. Although this beta3 signal is sufficient to permit the formation of betagamma receptors it is not necessary, suggesting that alternative assembly signals also exist within the beta3 subunit. These findings support the belief that GABAA receptor assembly occurs via multiple defined pathways that may be determined by subunit availability.

Thioalkalimicrobium aerophilum gen. nov., sp. nov. and Thioalkalimicrobium sibericum sp. nov., and Thioalkalivibrio versutus gen. nov., sp. nov., Thioalkalivibrio nitratis sp.nov., novel and Thioalkalivibrio denitrificancs sp. nov., novel obligately alkaliphilic and obligately chemolithoautotrophic sulfur-oxidizing bacteria from soda lakes.

Forty-three strains of obligately chemolithoautotrophic sulfur-oxidizing bacteria were isolated from highly alkaline soda lakes in south-east Siberia (Russia) and in Kenya using a specific enrichment procedure at pH 10. The main difference between the novel isolates and known sulfur bacteria was their potential to grow and oxidize sulfur compounds at pH 10 and higher. The isolates fell into two groups that were substantially different from each other physiologically and genetically. Most of the Siberian isolates belonged to the group with a low DNA G+C content (48.0-51.2 mol%). They were characterized by a high growth rate, a low growth yield, a high cytochrome content, and high rates of oxidation of sulfide and thiosulfate. This group included 18 isolates with a DNA homology of more than 40%, and it is described here as a new genus, Thioalkalimicrobium, with two species Thioalkalimicrobium aerophilum (type species) and Thioalkalimicrobium sibericum. The other isolates, mainly from Kenyan soda lakes, fell into a group with a high DNA G+C content (61.0-65.6 mol%). In general, this group was characterized by a low growth rate, a high molar growth yield and low, but relatively equal, rates of oxidation of thiosulfate, sulfide, elemental sulfur and polythionates. The group included 25 isolates with a DNA homology of more than 30%. It was less compact than Thioalkalimicrobium, containing haloalkalophilic, carotenoid-producing, nitrate-reducing and facultatively anaerobic denitrifying strains. These bacteria are proposed to be assigned to a new genus, Thioalkalivibrio, with three species Thioalkalivibrio versutus (type species), Thioalkalivibrio denitrificans and Thioalkalivibrio nitratis. Phylogenetic analysis revealed that both groups belong to the gamma-Proteobacteria. The Thioalkalimicrobium species were closely affiliated with the neutrophilic chemolithoautotrophic sulfur bacteria of the genus Thiomicrospira, forming a new alkaliphilic lineage in this cluster. In contrast, Thioalkalivibrio was not related to any known chemolithoautotrophic taxa, but was distantly associated with anaerobic purple sulfur bacteria of the genus Ectothiorhodospira.

Isolation and characterization of alkaliphilic, chemolithoautotrophic, sulphur-oxidizing bacteria.

Alkaliphilic sulphur-oxidizing bacteria were isolated from samples from alkaline environments including soda soil and soda lakes. Two isolates, currently known as strains AL 2 and AL 3, were characterized. They grew over a pH range 8.0-10.4 with an optimum at 9.5-9.8. Both strains could oxidize thiosulphate, sulphide, polysulphide, elemental sulphur and tetrathionate. Strain AL 3 more actively oxidized thiosulphate and sulphide, while isolate AL 2 had higher activity with elemental sulphur and tetrathionate. Isolate AL 2 was also able to oxidize trithionate. The pH optimum for thiosulphate and sulphide oxidation was between 9-10. Some activity remained at pH 11, but was negligible at pH 7. Metabolism of tetrathionate by isolate AL 2 involved initial anaerobic hydrolysis to form sulphur, thiosulphate and sulphate in a sequence similar to that in other colourless sulphur-oxidizing bacteria. Sulphate was produced by both strains. During batch growth on thiosulphate, elemental sulphur and sulphite transiently accumulated in cultures of isolates AL 2 and AL 3, respectively. At lower pH values, both strains accumulated sulphur during sulphide and thiosulphate oxidation. Both strains contained ribulose bisphosphate carboxylase. Thiosulphate oxidation in isolate AL 3 appeared to be sodium ion-dependent. Isolate AL 2 differed from AL 3 by its high GC mol % value (65.5 and 49.5, respectively), sulphur deposition in its periplasm, the absence of carboxysomes, lower sulphur-oxidizing capacity, growth kinetics (lower growth rate and higher growth yield) and cytochrome composition.

Purification and characterization of sulfide dehydrogenase from alkaliphilic chemolithoautotrophic sulfur-oxidizing bacteria.

Extracts of the alkaliphilic sulfur-oxidizing autotroph strain AL3 contained sulfide:cytochrome c oxidoreductase. This was active above pH 8, and was associated with the cell membranes. Although up to 60% of the initial activity was lost during Triton X-100 extraction, further purification resulted in an enzyme that catalyzed sulfide oxidation with horse heart cytochrome c. This enzyme was a 41 kDa protein containing heme c554. The optimum pH of the membrane bound enzyme was 9.0, but after extraction this fell to 8.0. The enzyme catalyzed a single electron oxidation of HS-. Hydrosulfide radical is therefore the most probable product.

Thiobacillus sp. W5, the dominant autotroph oxidizing sulfide to sulfur in a reactor for aerobic treatment of sulfidic wastes.

The floating filter technique was successfully adapted for the isolation of the dominant, chemolithoautotrophic, sulfide-oxidizing bacterium from a sulfur-producing reactor after conventional isolation techniques had failed. The inoculated polycarbonate filters, floating on mineral medium, were incubated under gaseous hydrogen sulfide at non-toxic levels. This technique gave 200-fold higher recoveries than conventional isolation techniques. Viable counts on the filters, making up 15% of the total count, appeared to be all of the same species. Chemostat cultures of the new isolate had a very high sulfur-forming capacity, converting almost all hydrogen sulfide in the medium to elemental sulfur under high sulfide loads (27.5 mmol l-1 h-1) and fully aerobic conditions. This behaviour closely resembled that of the microbial community in the sulfur-producing reactor. Moreover, similar protein patterns were obtained by electrophoresis of cell-free extracts from the isolate and the mixed culture. It has therefore been concluded that this isolate represents the dominant sulfide-oxidizing population in the reactor. The isolate has been shown to be a new Thiobacillus species, related to Thiobacillus neapolitanus. In view of the general confusion currently surrounding the taxonomy of the thiobacilli, a new species has not been formally created. Instead, the isolate has been given the working name Thiobacillus sp. W5.

Sulfur production by obligately chemolithoautotrophic thiobacillus species.

Transient-state experiments with the obligately autotrophic Thiobacillus sp. strain W5 revealed that sulfide oxidation proceeds in two physiological phases, (i) the sulfate-producing phase and (ii) the sulfur- and sulfate-producing phase, after which sulfide toxicity occurs. Specific sulfur-producing characteristics were independent of the growth rate. Sulfur formation was shown to occur when the maximum oxidative capacity of the culture was approached. In order to be able to oxidize increasing amounts of sulfide, the organism has to convert part of the sulfide to sulfur (HS(sup-)(symbl)S(sup0) + H(sup+) + 2e(sup-)) instead of sulfate (HS(sup-) + 4H(inf2)O(symbl)SO(inf4)(sup2-) + 9 H(sup+) + 8e(sup-)), thereby keeping the electron flux constant. Measurements of the in vivo degree of reduction of the cytochrome pool as a function of increasing sulfide supply suggested a redox-related down-regulation of the sulfur oxidation rate. Comparison of the sulfur-producing properties of Thiobacillus sp. strain W5 and Thiobacillus neapolitanus showed that the former has twice the maximum specific sulfide-oxidizing capacity of the latter (3.6 versus 1.9 (mu)mol/mg of protein/min). Their maximum specific oxygen uptake rates were very similar. Significant mechanistic differences in sulfur production between the high-sulfur-producing Thiobacillus sp. strain W5 and the moderate-sulfur-producing species T. neapolitanus were not observed. The limited sulfide-oxidizing capacity of T. neapolitanus appears to be the reason that it can convert only 50% of the incoming sulfide to elemental sulfur.

Novel principles in the microbial conversion of nitrogen compounds.

Some aspects of inorganic nitrogen conversion by microorganisms like N2O emission and hydroxylamine metabolism studied by Beijerinck and Kluyver, founders of the Delft School of Microbiology, are still actual today. In the Kluyver Laboratory for Biotechnology, microbial conversion of nitrogen compounds is still a central research theme. In recent years a range of new microbial processes and process technological applications have been studied. This paper gives a review of these developments including, aerobic denitrification, anaerobic ammonium oxidation, heterotrophic nitrification, and formation of intermediates (NO2-, NO, N2O), as well as the way these processes are controlled at the genetic and enzyme level.

Purification and characterization of a periplasmic Thiosulfate dehydrogenase from the obligately autotrophic Thiobacillus sp. W5.

A periplasmic thiosulfate dehydrogenase (EC 1.8.2.2) was purified to homogeneity from the neutrophilic, obligately chemolithoautotrophic Thiobacillus sp. W5. A five-step procedure resulted in an approximately 2,300-fold purification. The purified protein had a molecular mass of 120 +/- 3 kDa, as determined by gel filtration. It is probably a tetramer containing two different subunits with molecular masses of 33 +/- 1 kDa and 27 +/- 0.5 kDa, as determined by SDS-PAGE. UV/visible spectroscopy revealed that the enzyme contained haem c; haem staining showed that both subunits contained haem c. A haem c content of 4 mol per mol of enzyme was calculated using the pyridine haemochrome test. The pH optimum of the enzyme was 5.5. At pH 7.5, the Km and Vmax were 120 +/- 10 microM and 1,160 +/- 30 U mg-1, respectively. The absence of 2-heptyl-4-hydroquinoline-N-oxide (HQNO) inhibition for the oxidation of thiosulfate by whole cells suggested that the electrons enter the respiratory chain at the level of cytochrome c. Comparison with thiosulfate dehydrogenases from other Thiobacillus species showed that the enzyme was structurally similar to the thiosulfate dehydrogenase of the acidophilic, facultatively chemolithoautotrophic Thiobacillus acidophilus, but not to the thiosulfate dehydrogenases published for the obligately chemolithoautotrophic Thiobacillus tepidarius and Thiobacillus thioparus.

Molecular biology of somatostatin receptor subtypes.

Somatostatin (SRIF) receptors (ssts) comprise a family of heptahelical membrane proteins encoded by five related genes that map to separate chromosomes and which, with the exception of sst1, are intronless. The ssts1-4 display weak selectivity for SRIF-14 binding, whereas sst5 is SRIF-28-selective. Based on structural similarity and reactivity for octapeptide and hexapeptide sst analogs, ssts2,3 and sst5 belong to a similar sst subclass; ssts1-4 react poorly with these analogs and belong to a separate subclass. All five ssts are functionally coupled to inhibition of adenylyl cyclase via pertussis toxin-sensitive guanosine triphosphate (GTP)-binding proteins. mRNA for ssts1-5 is widely expressed in brain and peripheral organs and displays an overlapping but characteristic pattern that is subtype-selective and tissue- and species-specific. All pituitary cell subsets express sst2 and sst5, with sst5 being more abundant. Individual pituitary cells coexpress multiple sst subtypes. The binding pocket for SRIF-14 ligand lies deep within the membrane in transmembrane domains (TMDs) 3 to 7. Except for extracellular loop 2, it does not involve the other exofacial structures. Human (h)sst2A and hsst5 undergo agonist-mediated desensitization, associated with receptor internalization. The C-tail segment of hsst5 displays positive molecular internalization signals. The ssts inhibit the growth of tumor cells directly, through blockade of mitogenic signaling leading to growth arrest and through induction of apoptosis. This process is associated with translocation of phosphotyrosine phosphatase (PTP) 1C from the cytosol to the membrane.

Nitrous oxide production by Alcaligenes faecalis under transient and dynamic aerobic and anaerobic conditions.

Nitrous oxide can be a harmful by-product in nitrogen removal from wastewater. Since wastewater treatment systems operate under different aeration regimens, the influence of different oxygen concentrations and oxygen fluctuations on denitrification was studied. Continuous cultures of Alcaligenes faecalis TUD produced N2O under anaerobic as well as aerobic conditions. Below a dissolved oxygen concentration of 5% air saturation, the relatively highest N2O production was observed. Under these conditions, significant activities of nitrite reductase could be measured. After transition from aerobic to anaerobic conditions, there was insufficient nitrite reductase present to sustain growth and the culture began to wash out. After 20 h, nitrite reductase became detectable and the culture started to recover. Nitrous oxide reductase became measurable only after 27 h, suggesting sequential induction of the denitrification reductases, causing the transient accumulation of N2O. After transition from anaerobic conditions to aerobic conditions, nitrite reduction continued (at a lower rate) for several hours. N2O reduction appeared to stop immediately after the switch, indicating inhibition of nitrous oxide reductase, resulting in high N2O emissions (maximum, 1.4 mmol liter-1 h-1). The nitrite reductase was not inactivated by oxygen, but its synthesis was repressed. A half-life of 16 to 22 h for nitrite reductase under these conditions was calculated. In a dynamic aerobic-anaerobic culture of A. faecalis, a semisteady state in which most of the N2O production took place after the transition from anaerobic to aerobic conditions was obtained. The nitrite consumption rate in this culture was equal to that in an anaerobic culture (0.95 and 0.92 mmol liter-1 h-1, respectively), but the production of N2O was higher in the dynamic culture (28 and 26% of nitrite consumption, respectively).

Cloning of the gene encoding human somatostatin receptor 2: sequence analysis of the 5'-flanking promoter region.

Using a cDNA probe, genomic clones were obtained for the 5' flanking promoter sequence of the human somatostatin receptor 2-encoding gene (SSTR2). A 3.8-kb sequence directly upstream from the start codon was analyzed. This sequence shares a number of characteristics with the promoters of other G-protein-coupled receptor (GPCR)-encoding genes including a number of G+C-rich regions, binding sites for several transcriptional factors, and the absence of coupled TATAA and CAAT sequences.

Anaerobic oxidation of ammonium is a biologically mediated process.

A newly discovered process by which ammonium is converted to dinitrogen gas under anaerobic conditions (the Anammox process) has now been examined in detail. In order to confirm the biological nature of this process, anaerobic batch culture experiments were used. All of the ammonium provided in the medium was oxidized within 9 days. In control experiments with autoclaved or raw wastewater, without added sludge or with added sterilized (either autoclaved or gamma irradiated) sludge, no changes in the ammonium and nitrate concentrations were observed. Chemical reactions could therefore not be responsible for the ammonium conversion. The addition of chloramphenicol, ampicillin, 2,4-dinitrophenol, carbonyl cyanide m-chlorophenyl-hydrazone (CCCP), and mercuric chloride (HgIICl2) completely inhibited the activity of the ammonium-oxidizing sludge. Furthermore, the rate of ammonium oxidation was proportional to the initial amount of sludge used. It was therefore concluded that anaerobic ammonium oxidation was a microbiological process. As the experiments were carried out in an oxygen-free atmosphere, the conversion of ammonium to dinitrogen gas did not even require a trace of O2. That the end product of the reaction was nitrogen gas has been confirmed by using 15NH4+ and 14NO3-. The dominant product was 14-15N2. Only 1.7% of the total labelled nitrogen gas produced was 15-15N2. It is therefore proposed that the N2 produced by the Anammox process is formed from equimolar amounts of NH4+ and NO3-.

Sequence analysis of the 5'-flanking promoter region of the human somatostatin receptor 5.

We have determined the sequence of 2.2 kb of 5' flanking promoter region of the human somatostatin receptor 5 (hsstr5) gene. A number of widely distributed promoter elements were identified including AP1, AP2, AP3, E2A, GCF, and SP1 consensus sequences. hsstr5/CAT gene fusions showed that the 0.9 kb of DNA immediately upstream of the ATG functions as a promoter in rat pituitary GH3 but not in CHO ovary cells. Insertion of this hsstr5 fragment in the anti-sense orientation led to a four fold reduction in CAT activity. Dibutyryl cAMP produced a three fold induction of CAT activity whereas estradiol and retinoic acid had no significant effect. These results indicate that we have identified a DNA fragment at the 5' end of the hsstr5 gene which contains both tissue-specific and regulated elements. The absence of CRE consensus sequence suggests that the cAMP effect is mediated by the multiple AP1 and AP2 sites.

Heterotrophic nitrification and aerobic denitrification in Alcaligenes faecalis strain TUD.

Heterotrophic nitrification and aerobic and anaerobic denitrification by Alcaligenes faecalis strain TUD were studied in continuous cultures under various environmental conditions. Both nitrification and denitrification activities increased with the dilution rate. At dissolved oxygen concentrations above 46% air saturation, hydroxylamine, nitrite and nitrate accumulated, indicating that both the nitrification and denitrification were less efficient. The overall nitrification activity was, however, essentially unaffected by the oxygen concentration. The nitrification rate increased with increasing ammonia concentration, but was lower in the presence of nitrate or nitrite. When present, hydroxylamine, was nitrified preferentially. Relatively low concentrations of acetate caused substrate inhibition (KI = 109 microM acetate). Denitrifying or assimilatory nitrate reductase were not detected, and the copper nitrite reductase, rather than cytochrome cd, was present. Thiosulphate (a potential inhibitor of heterotrophic nitrification) was oxidized by A. faecalis strain TUD, with a maximum oxygen uptake rate of 140-170 nmol O2.min-1.mg prot-1. Comparison of the behaviour of A. faecalis TUD with that of other bacteria capable of heterotrophic nitrification and aerobic denitrification established that the response of these organisms to environmental parameters is not uniform. Similarities were found in their responses to dissolved oxygen concentrations, growth rate and ammonia concentration. However, they differed in their responses to externally supplied nitrite and nitrate.

The use of a metabolically structured model in the study of growth, nitrification, and denitrification by Thiosphaera pantotropha.

Thiosphaera pantotropha is capable of aerobic heterotrophic nitrification and both aerobic and anaerobic denitrification. These phenomena have been studied in acetate-limited aerobic and anaerobic continuous cultures supplied with ammonia and nitrate. The internal reaction rates were defined, based on biochemical knowledge. The observable external conversion rates are related through a linear equation on the basis of the specified internal reaction rates. The linear equation is a Pirt relation extended for microbial systems with multiple electron donors (acetate and ammonia) and electron acceptors (oxygen and nitrate). The coefficients in this equation were estimated from the continuous culture measurements, and are composed of parameters involved in ATP production and consumption by the microorganism. It is shown that with realistic values for these parameters, the metabolically structured model describes the aerobic as well as the anaerobic experiments.

Determination of growth and coupled nitrification/denitrification by immobilized Thiosphaera pantotropha using measurement and modeling of oxygen profiles.

An oxygen microsensor in combination with mathematical modeling was used to determine the behavior of immobilized Thiosphaera pantotropha. This organism can convert ammonia completely to nitrogen gas under aerobic conditions (coupled nitrification/denitrification) and denitrifies nitrate at highest rates under anaerobic conditions. Immobilization of T. pantotropha can result in aerobic and anaerobic zones inside the biocatalyst particle which will be advantageous for the conversion of ammonia and nitrate from wastewater. However, information of the effects of immobilization on the physiology of T. pantotropha is necessary for the development of such a system. This article gives the extension of a model developed to describe the behavior of chemostat cultures of T. pantotropha so that it can be used for immobilized cells. The original model was based on metabolic reaction equations. Kinetic and diffusion equations have now been added. Experimental verification was carried out using a stirred tank reactor and a Kluyver flask. After immobilization in agarose, the cells were grown in the particles under continuous culture conditions for 3 days. After 24 h the oxygen penetration depth showed a constant value of 100 microm, indicating that a steady state was reached. Scanning electron micrographs showed that large colonies of cells were present in this 100-microm aerobic layer.From the dynamics of the start-up phase, several parameters were determined from measurements of the oxygen concentration profiles made every few hours. The profiles simulated by the model were fitted to the measured data. The average value for the maximum specific growth rate was 0.52 h(-1), and the maximum oxygen conversion rate was 1.0 mol Cmol(-1) h(-1). The maximum specific acetate uptake rate was 2.0 mol Cmol(-1) h(-1), and the Monod constant for acetate was 2.9 x 10(-2) mol m(-3). The maximum specific nitrification rate was 0.58 x 10(-1) mol Cmol(-1) h(-1), and the amount of oxygen necessary for nitrification was 11% of the total oxygen uptake rate. Most of the kinetic parameters determined for the immobilized cells were in good agreement with those for the suspended cells. Only the maximum specific growth rate was significantly higher, and the maximum specific nitrification rate was some what lower than for suspended cells. The experimental results clearly show that an oxygen microsensor, in combination with mathematical modeling, can successfully be used to elucidate the kinetic behavior of immobilized, oxygen-consuming, cells.

Combined heterotrophic nitrification and aerobic denitrification in Thiosphaera pantotropha and other bacteria.

Reports of the simultaneous use of oxygen and denitrification by different species of bacteria have become more common over the past few years. Research with some strains (e.g. Thiosphaera pantotropha) has indicated that there might be a link between this 'aerobic denitrification' and a form of nitrification which requires rather than generates energy and is therefore known as heterotrophic nitrification. This paper reviews recent research into heterotrophic nitrification and aerobic denitrification, and presents a preliminary model which, if verified, will provide at least a partial explanation for the simultaneous occurrence of nitrification and denitrification in some bacteria.