共查询到20条相似文献,搜索用时 48 毫秒
1.
Ice crystallization by Pseudomonas syringae 总被引:4,自引:0,他引:4
Several bacterial species can serve as biological ice nuclei. The best characterized of these is Pseudomonas syringae, a widely distributed bacterial epiphyte of plants. These biological ice nuclei find various applications in different fields,
but an optimized production method was required in order to obtain the highly active cells which may be exploited as ice nucleators.
The results presented here show that P. syringae cells reduce supercooling of liquid or solid media and enhance ice crystal formation at sub-zero temperatures, thus leading
to a remarkable control of the crystallization phenomenon and a potential for energy savings. Our discussion focuses on recent
and future applications of these ice nucleators in freezing operations, spray-ice technology and biotechnological processes.
Received: 21 December 1999 / Received revision: 29 February 2000 / Accepted: 6 March 2000 相似文献
2.
The diffusion gradient chamber (DGC) is a novel device developed to study the response of chemotactic bacteria to combinations
of nutrients and attractants [7]. Its purpose is to characterize genetic variants that occur in many biological experiments.
In this paper, a mathematical model which describes the spatial distribution of a bacterial population within the DGC is developed.
Mathematical analysis of the model concerning positivity and boundedness of the solutions are given. An ADI (Alternating Direction
Implicit) method is constructed for finding numerical solutions of the model and carrying out computer simulations. The numerical
results of the model successfully reproduced the patterns that were observed in the experiments using the DGC.
Received: 3 June 1997 / Revised version: 15 August 2000 / Published online: 20 December 2000 相似文献
3.
Periodic solutions in a model of competition between plasmid-bearing and plasmid-free organisms in a chemostat with an inhibitor 总被引:3,自引:0,他引:3
Ai S 《Journal of mathematical biology》2001,42(1):71-94
We obtain necessary and sufficient conditions on the existence of a unique positive equilibrium point and a set of sufficient
conditions on the existence of periodic solutions for a 3-dimensional system which arises from a model of competition between
plasmid-bearing and plasmid-free organisms in a chemostat with an inhibitor. Our results improve the corresponding results
obtained by Hsu, Luo, and Waltman [1].
Received: 20 November 1997 / Revised version: 12 February 1999 / Published online: 20 December 2000 相似文献
4.
Anaerobic sequencing batch reactors for wastewater treatment: a developing technology 总被引:4,自引:0,他引:4
Zaiat M Rodrigues JA Ratusznei SM de Camargo EF Borzani W 《Applied microbiology and biotechnology》2001,55(1):29-35
This paper describes and discusses the main problems related to anaerobic batch and fed-batch processes for wastewater treatment.
A critical analysis of the literature evaluated the industrial application viability and proposed alternatives to improve
operation and control of this system. Two approaches were presented in order to make this anaerobic discontinuous process
feasible for industrial application: (1) optimization of the operating procedures in reactors containing self-immobilized
sludge as granules, and (2) design of bioreactors with inert support media for biomass immobilization.
Received: 22 May 2000 / Received revision: 20 July 2000 / Accepted: 21 July 2000 相似文献
5.
Biological phosphate removal processes 总被引:9,自引:0,他引:9
M. C. M. van Loosdrecht C. M. Hooijmans D. Brdjanovic J. J. Heijnen 《Applied microbiology and biotechnology》1997,48(3):289-296
Biological phosphate removal has become a reliable and well-understood process for wastewater treatment. This review describes
the historical development of the process and the most important microbiological and process-engineering aspects. From a microbiological
point of view, the role of␣poly(hydroxyalkanoates) as storage material in a dynamic process and the use of polyphosphate as
an energy reserve are the most important findings. From a process-engineering point of view, the study of biological phosphate
removal has shown that highly complex biological processes can be designed and controlled, provided that the importance of
the prevailing microbiological ecological processes is recognised.
Received: 3 April 1997 / Received revision: 9 June 1997 / Accepted: 14 June 1997 相似文献
6.
Bioaugmentation of activated sludge systems with specialised bacterial strains could be a powerful tool to improve several
aspects in wastewater treatment processes, such as improved flocculation and degradation of recalcitrant compounds. This review
focuses on the addition of strains to activated sludge to enhance the biodegradation of recalcitrant compounds, either through
the activity of the inoculated strain or after transfer of degradative plasmids to activated sludge bacteria. Different factors
that improve the aggregation of the sludge flocs and their influence on biodegradation are described. This review further
deals with the role of bacterial plasmids in natural genetic exchange between inoculated and indigenous sludge bacteria, and
in the construction of new genetically modified organisms. The few successful cases of bioaugmentation described in this review,
together with future research, must lead to a better understanding of sludge bioaugmentation.
Received: 5 January 1998 / Received revision: 20 April 1998 / Accepted: 20 April 1998 相似文献
7.
Watkins JC 《Journal of mathematical biology》2000,41(3):253-271
In this paper we prove a consistency theorem (law of large numbers) and a fluctuation theorem (central limit theorem) for
structured population processes. The basic assumptions for these theorems are that the individuals have no statistically distinguishing
features beyond their class and that the interaction between any two individuals is not too high. We apply these results to
density dependent models of Leslie type and to a model for flour beetle dynamics.
Received: 24 February 1999 / Revised version: 23 July 1999 / Published online: 14 September 2000 相似文献
8.
Towards optimization of cyanobacteria as biotechnologically relevant producers of molecular hydrogen, a clean and renewable energy source 总被引:15,自引:0,他引:15
In discussions about alternatives to our current fossil energy sources, basic and applied research leading to biological
production of molecular hydrogen utilizing cyanobacteria deserves serious attention. In these oxygenic phototrophic bacteria,
hydrogen can be produced by the activity of either nitrogenases or reversible/bidirectional hydrogenases. Knowledge of the
physiological and molecular basis of some of the processes involved in hydrogen metabolism in these peculiar microorganisms
has increased during the last decade. However, further efforts are required in basic as well as applied research in order
to obtain a clear impression of these processes and their regulation. This information might then constitute the basis for
optimizing the efficiency of hydrogen evolution by cyanobacteria. Progress might be achieved by screening more cyanobacterial
strains for their ability to produce and evolve hydrogen, by genetically manipulating specific strains as well as by improving
the conditions for cultivation in bioreactors.
Received: 17 February 1998 / Received revision: 24 April 1998 / Accepted: 27 April 1998 相似文献
9.
Daniel HJ Otto RT Binder M Reuss M Syldatk C 《Applied microbiology and biotechnology》1999,51(1):40-45
In order to produce sophorolipids from whey, thereby lowering the lactose content and biological oxygen demand, a two-step
batch cultivation process was developed including medium sterilization by filtration. In the first step, whey was sterilized
by a combination of crossflow and sterile filtration. Because the sophorolipid-producing yeast Candida bombicola ATCC 22214 was not able to use lactose as a carbon source directly, the oleaginous yeast Cryptococcus curvatus ATCC 20509 was grown on deproteinized whey concentrates (DWC). With 1: 1 diluted DWC-20, lactose was consumed as the carbon
source and biomass (24 g/l dry weight content) as well as single-cell oil (SCO, 10 g/l) were produced. The cultivation broth
was disrupted with a glass bead mill and it served as medium for growth (29 g cell dry mass/l) and sophorolipid production
(12 g/l) of the yeast C. bombicola.
Received: 29 July 1998 / Received revision: 5 October 1998 / Accepted: 11 October 1998 相似文献
10.
Shake-flask cultivation of T. lanuginosus strain SSBP on coarse corn cobs yielded β-xylanase levels of 56,500 nkat/ml at 50 °C, whereas other hemicellulases (β-xylosidase,
β-glucosidase, and α-l-arabinofuranosidase) were produced at levels less than 7 nkat/ml. Cultivation on d-xylose yielded much lower levels of xylanase (350 nkat/ml), although other hemicellulase levels were similar to those produced
on corn cobs. The influence of agitation rate and dissolved oxygen tension (DOT) on hemicellulase production was studied further
in a bioreactor. On xylose, xylanase activities of 4,330 nkat/ml and 4,900 nkat/ml were obtained at stirrer speeds up to 1,400 rpm
to control DOT. At a constant stirrer speed of 400 rpm, xylanase activities of 10,930 nkat/ml and 15,630 nkat/ml were obtained
when cultivated on xylose and beechwood xylan respectively, despite DOT levels below 5% for the duration of fermentation.
The results indicate that there is an interaction between agitation rate and DOT, impacting on xylanase and accessory enzyme
production. Higher agitation rates favoured the production of xylosidase, arabinofuranosidase and glucosidase by T. lanuginosus strain SSBP, whereas the lower agitation rates favoured xylanase production. Rheological difficulties precluded cultivation
on corn cobs in the bioreactor. Volumetric xylanase productivities of 1,060,000 nkat/l · h and 589,000 nkat/l · h obtained
on beechwood xylan and xylose indicate that T. lanuginosus strain SSBP is a hyper-xylanase producer with considerable industrial potential.
Received: 5 May 1999 / Received revision: 31 May 2000 / Accepted: 3 June 2000 相似文献
11.
For biofilm studies, artificial models can be very helpful in studying processes in hydrogels of defined composition and
structure. Two different types of artificial biofilm models were developed. Homogeneous agarose beads (50–500 μm diameter)
and porous beads (260 μm mean diameter) containing pores with diameters from 10 to 80 μm (28 μm on average) allowed the embedding
of cells, particles and typical biofilm matrix components such as proteins and polysaccharides. The characterisation of the
matrix structures and of the distribution of microorganisms was performed by confocal laser scanning microscopy. The physiological
condition of the embedded bacteria was examined by redox activity (CTC-assay) and membrane integrity (Molecular Probes LIVE/DEAD-Kit).
Approximately 35% of the immobilised cells (Pseudomonas aeruginosa SG81) were damaged due to the elevated temperature required for the embedding process. It was shown that the surviving cells
were able to multiply when provided with nutrients. In the case of homogeneous agarose beads, cell growth only occurred near
the bead surface, while substrate limitation prevented growth of more deeply embedded cells. In the porous hydrogel, cell
division was observed across the entire matrix due to better mass transport. It could be shown that embedding in the artificial
gel matrix provided protection of immobilized cells against toxic substances such as sodium hypochlorite (0.5 mg/l, 30 min)
in comparison to suspended cells, as observed in other immobilized systems. Thus, the model is suited to simulate important
biofilm matrix properties.
Received: 21 December 1999 / Received revision: 7 March 2000 / Accepted: 10 March 2000 相似文献
12.
Studies of microbial purification of a model waste water containing 4-nitrophenol were carried out in a continuously working
aerobic solid-bed reactor. The main emphasis was on the dynamic behaviour of the system after a sudden change in cultivation
conditions and on the steady-state performance of the reactor as a function of the pollution load. A change from ammonium-free
to ammonium-containing medium hardly influenced the nitrophenol degradation. The reactor responded differently to an increase
in pollutant load, which was brought about by increasing either the 4-nitrophenol content or the flow of the waste water.
Up to a load of 270 mg l−1 h−1 the pollutant was stably and almost completely degraded. At a higher load, only a partial 4-nitrophenol degradation took
place. A mathematical model was derived to describe the processes that occurred in the reactor. By segregation into two compartments
– the aqueous phase and the biofilm – account was taken of the fact that the pollutant is carried into the biofilm by diffusion
and is degraded there. The observed relations between the pollutant load, the pollutant concentration in the outlet of the
reactor and the reactor performance agreed with the simulated process behaviour. As the model simulation showed, the incomplete
pollutant degradation at a higher reactor load was caused by oxygen limitation.
Received: 5 August 1998 / Received revision: 22 October 1998 / Accepted: 24 October 1998 相似文献
13.
The effect of biomass concentration on the formation of Aspergillus oryzaeα-amylase during submerged cultivation with A. oryzae and recombinant A. nidulans strains has been investigated. It was found that the specific rate of α-amylase formation in chemostats decreased significantly
with increasing biomass concentration in the range of approx. 2–12 g dry weight kg−1. When using a recombinant A. nidulans strain in which the gene responsible for carbon catabolite repression of the A. oryzaeα-amylase gene (creA) was deleted, no significant decrease in the specific rate of α-amylase formation was observed. On the basis of the experimental
results, it is suggested that the low value of the specific α-amylase productivity observed at high biomass concentration
is caused by slow mixing of the concentrated feed solution in the viscous fermentation medium.
Received: 13 January 2000 / Received revision: 30 June 2000 / Accepted: 1 July 2000 相似文献
14.
Chemically defined media for commercial fermentations 总被引:13,自引:0,他引:13
The use of chemically defined media is gaining popularity in some commercial fermentations, particularly for the preparation
of biological products. Although these media are still not frequently developed for industrial processes, they do exhibit
favorable characteristics at large scale that are not observed with traditional complex media. This review focuses on the
application, development, and practical considerations, especially process economics, of fermentations in chemically defined
media in an industrial environment.
Received: 3 August 1998 / Received revision: 16 November 1998 / Accepted: 21 November 1998 相似文献
15.
Kinetic analysis of arachidonic acid (AA)-oil biosynthesis by Mortierella alpina 1S-4 growing under lipid-accumulating (LN medium) and non-lipid-accumulating (HN medium) conditions was investigated and
compared with industrial AA fermentation. Various kinetic parameters of these cultivation processes demonstrate a characteristic
pattern of the lipogenesis in this fungus, where growth phase, phase of oil accumulation and phase of AA synthesis are distinct
from each other. The fungus utilizing LN medium synthesized 32.3 g fatty acid 100 g−1 glucose on the 4th day of cultivation and reached the maximum daily fatty acid accumulation (expressed as differential specific
rate q
D(FA/B)) of 9.5%. Our results also indicate that a q
D(FA/B) value of about 2.5% might be critical for lipid overproduction in M. alpina. AA was rapidly incorporated into triacylglycerols (90% of total AA) at the later cultivation phase and overall AA yield was
directly related to the total yield of fatty acid.
Received: 10 December 1999 / Accepted: 13 February 2000 相似文献
16.
Molecular mechanisms of plant metal tolerance and homeostasis 总被引:68,自引:0,他引:68
Transition metals such as copper are essential for many physiological processes yet can be toxic at elevated levels. Other
metals (e.g. lead) are nonessential and potentially highly toxic. Plants – like all other organisms – possess homeostatic
mechanisms to maintain the correct concentrations of essential metal ions in different cellular compartments and to minimize
the damage from exposure to nonessential metal ions. A regulated network of metal transport, chelation, trafficking and sequestration
activities functions to provide the uptake, distribution and detoxification of metal ions. Some of the components of this
network have now been identified: a number of uptake transporters have been cloned as well as candidate transporters for the
vacuolar sequestration of metals. Chelators and chaperones are known, and evidence for intracellular metal trafficking is
emerging. This recent progress in the molecular understanding of plant metal homeostasis and tolerance is reviewed.
Received: 14 July 2000 / Accepted: 22 September 2000 相似文献
17.
We investigate the effect of spatial aggregation in the infection dynamics of nematode parasites in ruminants. We show that
a high degree of spatial aggregation is likely to lead to a dramatically enhanced rate of invasion by drug-resistant strains.
Received: 13 December 1999 / Revised version: 3 April 2000 / Published online: 4 October 2000 相似文献
18.
An efficient method for the exact numerical simulation of semi-Markov processes is used to study minimal models of the control
of eye movements in reading. When we read a text, typical sequences of fixations form a rather complicated trajectory – almost
like a random walk. Mathematical models of eye movement control can account for this behavior using stochastic transition
rules between few discrete internal states, which represent combinations of certain stages of lexical access and saccade programs.
We show that experimentally observed fixation durations can be explained by residence-time-dependent transition probabilities.
Stochastic processes with this property are known as semi-Markov processes. For our numerical simulations we use the minimal
process method (Gillespie algorithm), which is an exact and efficient simulation algorithm for this class of stochastic processes.
Within this mathematical framework, we study different forms of coupling between eye movements and shifts of covert attention
in reading. Our model lends support to the existence of autonomous saccades, i.e., the hypothesis that initiations of saccades
are not completely determined by lexical access processes.
Received: 21 March 2000 / Accepted in revised form: 10 January 2001 相似文献
19.
A two-phase membrane bioreactor was developed to continuously produce enantiopure epoxides using the epoxide hydrolase activity
of Rhodotorula glutinis. An aqueous/organic cascade, hydrophilic, hollow-fiber membrane bioreactor was used: (1) to carry out large-scale resolution
of epoxides, (2) to continuously extract residual enantiopure epoxides from the aqueous phase, and (3) to separate inhibitory
formed diol from the yeast cells contained in the aqueous phase. Dodecane was employed to dissolve-feed epoxide as well as
to extract residual epoxide. 1,2-Epoxyhexane was used as a model substrate. By use of this membrane bioreactor, enantiopure
(S)-1,2-epoxyhexane (>98% enantiomeric excess) was obtained with a volumetric productivity of 3.8 g l−1 h−1. The continuous-production system was operated for 12 days and resulted in 38 g enantiopure (S)-1,2-epoxyhexane.
Received: 14 February 2000 / Received revision: 15 June 2000 / Accepted: 18 June 2000 相似文献
20.
J. M. Obón J. R. Maiquez M. Cánovas H.-P. Kleber J. L. Iborra 《Applied microbiology and biotechnology》1999,51(6):760-764
The use of a biological procedure for l-carnitine production as an alternative to chemical methods must be accompanied by an efficient and highly productive reaction
system. Continuous l-carnitine production from crotonobetaine was studied in a cell-recycle reactor with Escherichia coli O44 K74 as biocatalyst. This bioreactor, running under the optimum medium composition (25 mM fumarate, 5 g/l peptone), was
able to reach a high cell density (26 g dry weight/l) and therefore to obtain high productivity values (6.2 g l-carnitine l−1 h−1). This process showed its feasibility for industrial l-carnitine production. In addition, resting cells maintained in continuous operation, with crotonobetaine as the only medium
component, kept their biocatalytic capacity for 4 days, but the biotransformation capacity decreased progressively when this
particular method of cultivation was used.
Received: 10 December 1998 / Received revision: 19 February 1999 / Accepted: 20 February 1999 相似文献