Bakterielles Wachstum

2008-06-11T19:44:54Z

Vojtech.dostal: cs iw

[[image:Bacterial_growth.png|250px|right|thumb|Growth is shown as ''L'' = log(numbers) where numbers is the number of colony forming units per ml, versus ''T'' (time.)]]
'''Bacterial growth''' is the [[Asexual reproduction|division]] of one [[bacterium]] into two idential daughter cells during a process called [[binary fission]]. Hence, '''local doubling''' of the bacterial population occurs. Both daughter cells from the division do not necessarily survive. However, if the number surviving exceeds unity on average, the bacterial population undergoes [[exponential growth]]. The measurement of an exponential bacterial growth curve in batch culture was traditionally a part of the training of all microbiologists; the basic means requires bacterial enumeration (cell counting) by direct and individual (microscopic, flow cytometry<ref name="pmid6341358">{{cite journal |author=Skarstad K, Steen HB, Boye E |title=Cell cycle parameters of slowly growing Escherichia coli B/r studied by flow cytometry |journal=[[J. Bacteriol.]] |volume=154 |issue=2 |pages=656–62 |year=1983 |pmid=6341358 |doi=}}</ref>), direct and bulk (biomass), indirect and individual (colony counting), or indirect and bulk (most probable number, turbidity, nutrient uptake) methods. Models reconcile theory with the measurements <ref >{{cite journal |journal=Applied and Environmental Microbiology |year=1990 |volume=56 |issue=6 |pages=1875-1881 |title=Modeling of the Bacterial Growth Curve |author=
Zwietering M H, Jongenburger I, Rombouts F M, van 'T Riet K}}</ref>.

In [[population ecology|autecological]] studies, '''bacterial growth''' in batch culture can be modeled with four different phases: lag phase (A), exponential or log phase (B), stationary phase (C), and death phase (D).

# During ''lag phase'', [[bacterium|bacteria]] adapt themselves to growth conditions. It is the period where the individual [[bacterium|bacteria]] are maturing and not yet able to divide. During the lag phase of the bacterial growth cycle, synthesis of RNA, enzymes and other molecules occurs.
# ''Exponential phase'' (sometimes called the log phase)is a period characterised by cell doubling.<ref name="Bacanova2008">"http://www.ifr.ac.uk/bacanova/project_backg.html". Retrieved on [[May 7]], [[2008]]</ref> The number of new bacteria appearing per unit time is proportional to the present population. If growth is not limited, doubling will continue at a constant rate so both the number of cells and the rate of population increase doubles with each consecutive time period. For this type of exponential growth, plotting the natural logarithm of cell number against time producing a straight line. The slope of this line is the specific growth rate of the organism, which is a measure of the number of divisions per cell per unit time.<ref name="Bacanova2008"/> The actual rate of this growth (i.e. the slope of the line in the figure) depends upon the growth conditions, which affect the frequency of cell division events and the probability of both daughter cells surviving. Exponential growth cannot continue indefinitely, however, because the medium is soon depleted of nutrients and enriched with wastes.
# During ''stationary phase'', the growth rate slows as a result of nutrient depletion and accumulation of toxic products. This phase is reached as the bacteria begin to exhaust the resources that are available to them.
#At ''death phase'', bacteria run out of nutrients and die.

This basic batch culture growth model draws out and emphasizes aspects of bacterial growth which may differ from the growth of macrofauna. It emphasizes clonality, asexual binary division, the short development time relative to replication itself, the seemingly low death rate, the need to move from a dormant state to a reproductive state or to condition the media, and finally, the tendency of lab adapted strains to exhaust their nutrients.

In reality, even in batch culture, the four phases are not well defined. The cells do not reproduce in synchrony without explicit and continual prompting (as in experiments with stalked bacteria <ref Novick_1955>{{cite journal |author=Novick A |year=1955 |journal=Annual Review of Microbiology |title=Growth of Bacteria |volume=9 |pages=97-110 | doi = 10.1146/annurev.mi.09.100155.000525 }}</ref>) and their logarithmic phase growth is often not ever a constant rate, but instead a slowly decaying rate, a constant stochastic response to pressures both to reproduce and to go dormant in the face of declining nutrient concentrations and increasing waste concentrations.

Batch culture is the most common laboratory growth environment in which bacterial growth is studied, but it is only one of many. It is ideally spatially unstructured and temporally structured. The bacterial culture is incubated in a closed vessel with a single batch of medium. In some experimental regimes, some of the bacterial culture is periodically removed to a fresh sterile media is added. In the extreme case, this leads to the continual renewal of the nutrients. This is a [[chemostat]] also known as continuous culture. It is ideally spatially unstructured and temporally unstructured, in an equilibrium state defined by the nutrient supply rate and the reaction of the bacteria. In comparison to batch culture, bacteria are maintained in expodential growth phase and the grow growth rate of the bacteria is known. Related devices include [[turbidostat]]s and [[auxostat]]s.

'''Bacterial growth can be suppressed with [[bacteriostat]]s, without necessarily killing the bacteria.''' In a [[community ecology|synecological]], a true-to-nature situation, where more than one bacterial species is present, the growth of microbes is more dynamic and continual.

Liquid is not the only laboratory environment for bacterial growth. Spatially structured environments such as biofilms or agar surfaces present additional complex growth models.

==References==
{{Reflist}}

==External links==
*[http://members.optusnet.com.au/exponentialist/Bacteria.htm An examination of the exponential growth of bacterial populations]
*[http://www.scienceaid.co.uk/biology/microorganisms/populations.html Science aid: Microbial Populations]
*[http://wiki.biomine.skelleftea.se/wiki/index.php/Microbial_growth Microbial Growth, BioMineWiki]

''This article includes material from [http://www.nupedia.com/article/500/ an article] posted on [[26 April]] [[2003]] on [[Nupedia]]; written by Nagina Parmar; reviewed and approved by the Biology group; editor, Gaytha Langlois; lead reviewer, Gaytha Langlois ; lead copyeditors, Ruth Ifcher. and Jan Hogle.''

[[Category:Bacteriology|Bacterial growth]]
[[Category:Population|Bacterial growth]]
[[Category:Microbiology]]

[[cs:Růst bakteriální populace]]
[[pl:Wzrost drobnoustrojów]]
[[uk:Ріст бактерій]]

Tonizität

2008-02-26T16:47:59Z

Vojtech.dostal: cs iw

{{expert}}
{{Unreferenced|date=January 2008}}
[[Image:Osmotic pressure on blood cells diagram.svg|thumb|250px|right|Effect of different solutions on blood cells]]
[[Image:Turgor pressure on plant cells diagram.svg|thumb|251px|right|Plant cell under different environments]]

'''Tonicity''' is a measure of ''effective osmolarity'' or ''effective osmolality'' in [[cell (biology)|cell]] [[biology]]. [[Osmolality]] and [[osmolarity]] are properties of a particular solution, independent of any membrane. Tonicity is a property of a solution in reference to a particular membrane, and is equal to the sum of the concentrations of the solutes which have the capacity to exert an osmotic force across that membrane. Tonicity, also, depends on solute permeability (permeant solutes do not affect tonicity; impermeant solutes do affect tonicity). Tonicity is generally classified in three ranges; hypertonicity, hypotonicity and isotonicity.

==Hypertonicity==
A cell in a hypertonic environment is surrounded by a higher concentration of impermeable solute than exists in the inside of the cell. [[Osmotic pressure]] directs a net movement of water out of the cell, causing it to shrink.
Hypertonic, isotonic and hypotonic solutions are defined in reference to a cell membrane by comparing the tonicity of the solution with the tonicity within the cell.

In [[Eukaryote#Animal cell|animal cells]], being in a hypertonic environment results in [[crenation]], where the shape of the cell becomes distorted and wrinkled as water leaves the cell. Some organisms have evolved methods of circumventing hypertonicity; for example, [[Seawater|saltwater]] is hypertonic to the [[fish]] that live in it. Since they cannot isolate themselves from osmotic water loss, because they need a large surface area in their [[gill]]s for [[gas exchange]], they respond by drinking large amounts of water, and [[excretion|excreting]] the salt. This process is called [[osmoregulation]].

In [[plant cell]]s, the effect is more dramatic. The [[cell membrane]] pulls away from the [[cell wall]], but the cell remains joined to the adjacent cells at points called [[plasmodesmata]]. Thus, the cell takes on the appearance of a [[pincushion]], with the plasmodesmata almost ceasing to function because they have become so constricted. This condition is known as [[plasmolysis]].

==Isotonicity==
A cell in an isotonic environment is in a state of [[Diffusion equilibrium|equilibrium]] with its surroundings. When the amount of impermeable solute is the same on the inside and outside of the cell, osmotic pressure becomes equal; the force of water trying to exit and enter the cell balances out. This pressure is what drives hypertonic or hypotonic cells to become isotonic.

==Hypotonicity==
The opposite of a hypertonic environment is a hypotonic one, where the net movement of water is into the cell. If the cell contains more impermeable solute than its surroundings, water will enter it. In the case of animal cells, they will swell until they burst; plant cells do not burst, due to the reinforcement their cell wall provides.

==See also==
*[[Osmosis]]
*[[Osmole (unit)]]

{{sci-stub}}

[[cs:Tonicita]]
[[es:Hipertónico]]
[[fr:Hypertonique]]
[[ka:ჰიპერტონიული ხსნარი]]
[[nl:Hypertoniciteit]]
[[pl:Roztwór hipertoniczny]]
[[pt:Meio hipertónico]]
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[[vi:Ưu trương]]
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Bakterielles Wachstum

Tonizität