Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles that are invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality. Fluids can contain suspended solid matter consisting of particles of many different sizes. While some suspended material will be large enough and heavy enough to settle to the bottom of the container if a liquid sample is left to stand small particles will settle only slowly or not at all if the sample is agitated or the particles are colloidal; these small solid particles cause the liquid to appear turbid. Turbidity is applied to transparent solids such as glass or plastic. In plastic production, haze is defined as the percentage of light, deflected more than 2.5° from the incoming light direction. Turbidity in open water may be caused by growth of phytoplankton. Human activities that disturb land, such as construction and agriculture, can lead to high sediment levels entering water bodies during rain storms due to storm water runoff.
Areas prone to high bank erosion rates as well as urbanized areas contribute large amounts of turbidity to nearby waters, through stormwater pollution from paved surfaces such as roads and parking lots. Some industries such as quarrying and coal recovery can generate high levels of turbidity from colloidal rock particles. In drinking water, the higher the turbidity level, the higher the risk that people may develop gastrointestinal diseases; this is problematic for immunocompromised people, because contaminants like viruses or bacteria can become attached to the suspended solids. The suspended solids interfere with water disinfection with chlorine because the particles act as shields for the virus and bacteria. Suspended solids can protect bacteria from ultraviolet sterilization of water. In water bodies such as lakes and reservoirs, high turbidity levels can reduce the amount of light reaching lower depths, which can inhibit growth of submerged aquatic plants and affect species which are dependent on them, such as fish and shellfish.
High turbidity levels can affect the ability of fish gills to absorb dissolved oxygen. This phenomenon has been observed throughout the Chesapeake Bay in the eastern United States. For many mangrove areas, high turbidity is needed in order to support certain species, such as to protect juvenile fish from predators. For most mangroves along the eastern coast of Australia, in particular Moreton Bay, turbidity levels as high as 600 Nephelometric Turbidity Units are needed for proper ecosystem health; the most used measurement unit for turbidity is the Formazin Turbidity Unit. ISO refers to its units as FNU. ISO 7027 provides the method in water quality for the determination of turbidity, it is used to determine the concentration of suspended particles in a sample of water by measuring the incident light scattered at right angles from the sample. The scattered light is captured by a photodiode, which produces an electronic signal, converted to a turbidity. Open source hardware has been developed following the ISO 7027 method to measure turbidity reliably using an Arduino microcontroller and inexpensive LEDs.
There are several practical ways of checking water quality, the most direct being some measure of attenuation of light as it passes through a sample column of water. The alternatively used Jackson Candle method is the inverse measure of the length of a column of water needed to obscure a candle flame viewed through it; the more water needed, the clearer the water. Of course water alone produces some attenuation, any substances dissolved in the water that produce color can attenuate some wavelengths. Modern instruments do not use candles, but this approach of attenuation of a light beam through a column of water should be calibrated and reported in JTUs; the propensity of particles to scatter a light beam focused on them is now considered a more meaningful measure of turbidity in water. Turbidity measured this way uses an instrument called a nephelometer with the detector set up to the side of the light beam. More light reaches the detector if there are lots of small particles scattering the source beam than if there are few.
The units of turbidity from a calibrated nephelometer are called Nephelometric Turbidity Units. To some extent, how much light reflects for a given amount of particulates is dependent upon properties of the particles like their shape and reflectivity. For this reason, a correlation between turbidity and total suspended solids is somewhat unusual for each location or situation. Turbidity in lakes, reservoirs and the ocean can be measured using a Secchi disk; this black and white disk is lowered into the water. The Secchi disk has the advantages of integrating turbidity over depth, being quick and easy to use, inexpensive, it can provide a rough indication of the depth of the euphotic zone with a 3-fold division of the Secchi depth, however this cannot be used in shallow waters where the disk can still be seen on the bottom. An additional device, which may help measuring turbidity in shallow waters is the turbidity tube; the turbidity tube condenses water in a graded tube which allows determination of tur
Disinfectants are antimicrobial agents that are applied to the surface of non-living objects to destroy microorganisms that are living on the objects. Disinfection does not kill all microorganisms resistant bacterial spores. Disinfectants are different from other antimicrobial agents such as antibiotics, which destroy microorganisms within the body, antiseptics, which destroy microorganisms on living tissue. Disinfectants are different from biocides — the latter are intended to destroy all forms of life, not just microorganisms. Disinfectants work by interfering with their metabolism. Sanitizers are substances that clean and disinfect. Disinfectants kill more germs than sanitizers. Disinfectants are used in hospitals, dental surgeries and bathrooms to kill infectious organisms. Bacterial endospores are most resistant to disinfectants, but some viruses and bacteria possess some tolerance. In wastewater treatment, a disinfection step with chlorine, ultra-violet radiation or ozonation can be included as tertiary treatment to remove pathogens from wastewater, for example if it is to be reused to irrigate golf courses.
An alternative term used in the sanitation sector for disinfection of waste streams, sewage sludge or fecal sludge is sanitisation or sanitization. A perfect disinfectant would offer complete and full microbiological sterilisation, without harming humans and useful form of life, be inexpensive, noncorrosive. However, most disinfectants are by nature harmful to humans or animals. Most modern household disinfectants contain Bitrex, an exceptionally bitter substance added to discourage ingestion, as a safety measure; those that are used indoors should never be mixed with other cleaning products as chemical reactions can occur. The choice of disinfectant to be used depends on the particular situation; some disinfectants have a wide spectrum, while others kill a smaller range of disease-causing organisms but are preferred for other properties. There are arguments for creating or maintaining conditions that are not conducive to bacterial survival and multiplication, rather than attempting to kill them with chemicals.
Bacteria can increase in number quickly, which enables them to evolve rapidly. Should some bacteria survive a chemical attack, they give rise to new generations composed of bacteria that have resistance to the particular chemical used. Under a sustained chemical attack, the surviving bacteria in successive generations are resistant to the chemical used, the chemical is rendered ineffective. For this reason, some question the wisdom of impregnating cloths, cutting boards and worktops in the home with bactericidal chemicals. Air disinfectants are chemical substances capable of disinfecting microorganisms suspended in the air. Disinfectants are assumed to be limited to use on surfaces, but, not the case. In 1928, a study found. An air disinfectant must be dispersed either as an aerosol or vapour at a sufficient concentration in the air to cause the number of viable infectious microorganisms to be reduced. In the 1940s and early 1950s, further studies showed inactivation of diverse bacteria, influenza virus, Penicillium chrysogenum mold fungus using various glycols, principally propylene glycol and triethylene glycol.
In principle, these chemical substances are ideal air disinfectants because they have both high lethality to microorganisms and low mammalian toxicity. Although glycols are effective air disinfectants in controlled laboratory environments, it is more difficult to use them in real-world environments because the disinfection of air is sensitive to continuous action. Continuous action in real-world environments with outside air exchanges at door, HVAC, window interfaces, in the presence of materials that adsorb and remove glycols from the air, poses engineering challenges that are not critical for surface disinfection; the engineering challenge associated with creating a sufficient concentration of the glycol vapours in the air have not to date been sufficiently addressed. Alcohol and alcohol plus Quaternary ammonium cation based compounds comprise a class of proven surface sanitizers and disinfectants approved by the EPA and the Centers for Disease Control for use as a hospital grade disinfectant.
Alcohols are most effective when combined with distilled water to facilitate diffusion through the cell membrane. A mixture of 70% ethanol or isopropanol diluted in water is effective against a wide spectrum of bacteria, though higher concentrations are needed to disinfect wet surfaces. Additionally, high-concentration mixtures are required to inactivate lipid-enveloped viruses; the efficacy of alcohol is enhanced. The synergistic effect of 29.4% ethanol with dodecanoic acid is effective against a broad spectrum of bacteria and viruses. Further testing is being performed against Clostridium difficile spores with higher concentrations of ethanol and dodecanoic acid, which proved effective with a contact time of ten minutes. Aldehydes, such as formaldehyde and glutaraldehyde, have a wide microbiocidal activity and are sporicidal and fu
Water activity or aw is the partial vapor pressure of water in a substance divided by the standard state partial vapor pressure of water. In the field of food science, the standard state is most defined as the partial vapor pressure of pure water at the same temperature. Using this particular definition, pure distilled water has a water activity of one; as temperature increases, aw increases, except in some products with crystalline salt or sugar. Higher aw substances tend to support more microorganisms. Bacteria require at least 0.91, fungi at least 0.7. See fermentation. Water migrates from areas of high aw to areas of low aw. For example, if honey is exposed to humid air, the honey absorbs water from the air. If salami is exposed to dry air, the salami dries out, which could spoil it. Definition of aw: a w ≡ p / p ∗ where p is the partial vapor pressure of water in the solution, p*₀ is the partial vapor pressure of pure water at the same temperature. Alternate definition: a w ≡ l w x w where lw is the activity coefficient of water and xw is the mole fraction of water in the aqueous fraction.
Relationship to relative humidity: The relative humidity of air in equilibrium with a sample is called the Equilibrium Relative Humidity. E R H = a w × 100 % Estimated mold-free shelf life in days at 21° C: M F S L = 10 7.91 − 8.1 a w Water activity is an important consideration for food product design and food safety. Food designers use water activity to formulate shelf-stable food. If a product is kept below a certain water activity mold growth is inhibited; this results in a longer shelf life. Water activity values can help limit moisture migration within a food product made with different ingredients. If raisins of a higher water activity are packaged with bran flakes of a lower water activity, the water from the raisins migrates to the bran flakes over time, making the raisins hard and the bran flakes soggy. Food formulators use water activity to predict. Water activity is used in many cases as a critical control point for Hazard Analysis and Critical Control Points programs. Samples of the food product are periodically taken from the production area and tested to ensure water activity values are within a specified range for food quality and safety.
Measurements can be made in as little as five minutes, are made in most major food production facilities. For many years, researchers tried to equate bacterial growth potential with water content, they found that the values were specific to each food product. W. J. Scott first established that bacterial growth correlated with water activity, not water content, in 1953, it is established that growth of bacteria is inhibited at specific water activity values. U. S. Food and Drug Administration regulations for intermediate moisture foods are based on these values. Lowering the water activity of a food product should not be seen as a kill step. Studies in powdered milk show that viable cells can exist at much lower water activity values, but that they never grow. Over time, bacterial levels decline. Water activity values are obtained by either a resistive electrolytic, a capacitance or a dew point hygrometer. Resistive electrolytic hygrometers use a sensing element in the form of a liquid electrolyte held in between of two small glass rods by capillary force.
The electrolyte changes resistance if it loses water vapor. The resistance is directly proportional to relative air humidity, to water activity of the sample; this relation can be checked by either a verification or calibration using salt-water mixtures, which provide a well-defined and reproducible air humidity in the measurement chamber. The sensor does not have any physically given hysteresis as it is known from capacitance hygrometers and sensors, does not require regular cleaning as its surface is not the sensing element. Volatiles, in principle, influence the measurement performance—especially those that dissociate in the electrolyte and thereby change its resistance; such influences can be avoided by using chemical protection filters that absorb the volatile compound before arriving at the sensor. Capacitance hygrometers consist of two charged plates separated by a polymer membrane dielectric; as the membrane adsorbs water, its ability to hold a charge increases and the capacitance is measured.
This value is proportional to the water activity as determined by a sensor-specific calibration. Capacitance hygrometers are not affected by most volatile chemicals and can be much smaller than other alternative sensors, they are less accurate than dew point hygrometers. They should have regular calibration checks and can be affected by residual water in the polymer membrane; the temperature at which dew forms on a clean surface is directly related to the vapor pressure of the air. Dew point hygrometers work by placing a mirror over a closed sample chamber; the mirror is cooled. This temperature is used to find the rel
Bacterial growth is the asexual reproduction, or cell division, of a bacterium into two daughter cells, in a process called binary fission. Providing no mutational event occurs, the resulting daughter cells are genetically identical to the original cell. Hence, bacterial growth occurs. Both daughter cells from the division do not 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. Models reconcile theory with the measurements. In autecological studies, the growth of bacteria in batch culture can be modeled with four different phases: lag phase, log phase or exponential phase, stationary phase, death phase. During lag phase, bacteria adapt themselves to growth conditions, it is the period where the individual bacteria are not yet able to divide. During the lag phase of the bacterial growth cycle, synthesis of RNA, enzymes and other molecules occurs.
During the lag phase cells change little because the cells do not reproduce in a new medium. This period of little to no cell division is called the lag phase and can last for 1 hour to several days. During this phase cells are not dormant; the log phase is a period characterized by cell doubling. 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 produces a straight line; the slope of this line is the specific growth rate of the organism, a measure of the number of divisions per cell per unit time. The actual rate of this growth depends upon the growth conditions, which affect the frequency of cell division events and the probability of both daughter cells surviving. Under controlled conditions, cyanobacteria can double their population four times a day and they can triple their population.
Exponential growth cannot continue indefinitely, because the medium is soon depleted of nutrients and enriched with wastes. The stationary phase is due to a growth-limiting factor such as the depletion of an essential nutrient, and/or the formation of an inhibitory product such as an organic acid. Stationary phase results from a situation in which death rate are equal; the number of new cells created is limited by the growth factor and as a result the rate of cell growth matches the rate of cell death. The result is a “smooth,” horizontal linear part of the curve during the stationary phase. Mutations can occur during stationary phase. Bridges et al. presented evidence that DNA damage is responsible for many of the mutations arising in the genomes of stationary phase or starving bacteria. Endogenously generated. At death phase, bacteria die; this could be caused by lack of nutrients, environmental temperature above or below the tolerance band for the species, or other injurious conditions. 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 low death rate, the need to move from a dormant state to a reproductive state or to condition the media, the tendency of lab adapted strains to exhaust their nutrients. In reality in batch culture, the four phases are not well defined; the cells do not reproduce in synchrony without explicit and continual prompting and their exponential phase growth is not a constant rate, but instead a 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. Near the end of the logarithmic phase of a batch culture, competence for natural genetic transformation may be induced, as in Bacillus subtilis and in other bacteria. Natural genetic transformation is a form of DNA transfer that appears to be an adaptation for repairing DNA damages. Batch culture is the most common laboratory growth method in which bacterial growth is studied, but it is only one of many.
It is 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 and added to fresh sterile medium. In the extreme case, this leads to the continual renewal of the nutrients; this is a chemostat known as continuous culture. It is ideally spatially unstructured and temporally unstructured, in a steady state defined by the rates of nutrient supply and bacterial growth. In comparison to batch culture, bacteria are maintained in exponential growth phase, the growth rate of the bacteria is known. Related devices include auxostats; when Escherichia coli is growing slowly with a doubling time of 16 hours in a chemostat most cells ha
Escherichia coli known as E. coli, is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus Escherichia, found in the lower intestine of warm-blooded organisms. Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in their hosts, are responsible for product recalls due to food contamination. The harmless strains are part of the normal microbiota of the gut, can benefit their hosts by producing vitamin K2, preventing colonization of the intestine with pathogenic bacteria, having a symbiotic relationship. E. coli is expelled into the environment within fecal matter. The bacterium grows massively in fresh fecal matter under aerobic conditions for 3 days, but its numbers decline afterwards. E. Coli and other facultative anaerobes constitute about 0.1% of gut microbiota, fecal–oral transmission is the major route through which pathogenic strains of the bacterium cause disease. Cells are able to survive outside the body for a limited amount of time, which makes them potential indicator organisms to test environmental samples for fecal contamination.
A growing body of research, has examined environmentally persistent E. coli which can survive for extended periods outside a host. The bacterium can be grown and cultured and inexpensively in a laboratory setting, has been intensively investigated for over 60 years. E. coli is a chemoheterotroph whose chemically defined medium must include a source of carbon and energy. E. coli is the most studied prokaryotic model organism, an important species in the fields of biotechnology and microbiology, where it has served as the host organism for the majority of work with recombinant DNA. Under favorable conditions, it takes up to 20 minutes to reproduce. E. coli is a facultative anaerobic and nonsporulating bacterium. Cells are rod-shaped, are about 2.0 μm long and 0.25–1.0 μm in diameter, with a cell volume of 0.6–0.7 μm3. E. Coli stains Gram-negative because its cell wall is composed of a thin peptidoglycan layer and an outer membrane. During the staining process, E. coli picks up the color of the counterstain safranin and stains pink.
The outer membrane surrounding the cell wall provides a barrier to certain antibiotics such that E. coli is not damaged by penicillin. Strains that possess flagella are motile; the flagella have a peritrichous arrangement. It attaches and effaces to the microvilli of the intestines via an adhesion molecule known as intimin. E. coli can live on a wide variety of substrates and uses mixed-acid fermentation in anaerobic conditions, producing lactate, ethanol and carbon dioxide. Since many pathways in mixed-acid fermentation produce hydrogen gas, these pathways require the levels of hydrogen to be low, as is the case when E. coli lives together with hydrogen-consuming organisms, such as methanogens or sulphate-reducing bacteria. Optimum growth of E. coli occurs at 37 °C, but some laboratory strains can multiply at temperatures up to 49 °C. E. coli grows in a variety of defined laboratory media, such as lysogeny broth, or any medium that contains glucose, ammonium phosphate monobasic, sodium chloride, magnesium sulfate, potassium phosphate dibasic, water.
Growth can be driven by aerobic or anaerobic respiration, using a large variety of redox pairs, including the oxidation of pyruvic acid, formic acid and amino acids, the reduction of substrates such as oxygen, fumarate, dimethyl sulfoxide, trimethylamine N-oxide. E. coli is classified as a facultative anaerobe. It uses oxygen when it is available, it can, continue to grow in the absence of oxygen using fermentation or anaerobic respiration. The ability to continue growing in the absence of oxygen is an advantage to bacteria because their survival is increased in environments where water predominates; the bacterial cell cycle is divided into three stages. The B period occurs between the beginning of DNA replication; the C period encompasses the time it takes to replicate the chromosomal DNA. The D period refers to the stage between the conclusion of DNA replication and the end of cell division; the doubling rate of E. coli is higher. However, the length of the C and D periods do not change when the doubling time becomes less than the sum of the C and D periods.
At the fastest growth rates, replication begins before the previous round of replication has completed, resulting in multiple replication forks along the DNA and overlapping cell cycles. E. coli and related bacteria possess the ability to transfer DNA via bacterial conjugation or transduction, which allows genetic material to spread horizontally through an existing population. The process of transduction, which uses the bacterial virus called a bacteriophage, is where the spread of the gene encoding for the Shiga toxin from the Shigella bacteria to E. coli helped produce E. coli O157:H7, the Shiga toxin-producing strain of E. coli. E. coli encompasses an enormous population of bacteria that exhibit a high degree of both genetic and phenotypic diversity. Genome sequencing of a large number of isolates of E. coli and related bacteria shows that a taxonomic reclassification would be desirable. However, this has not been done due to its medical importance, E. coli remains one of the most diverse bacterial species: only 20% of the genes in a typical E. coli genome is shared among all strains.
In fact, from the evolutionary point of view, the members of genus Shigella (S. dysenteriae, S. fle
Protozoa is an informal term for single-celled eukaryotes, either free-living or parasitic, which feed on organic matter such as other microorganisms or organic tissues and debris. The protozoa were regarded as "one-celled animals", because they possess animal-like behaviors, such as motility and predation, lack a cell wall, as found in plants and many algae. Although the traditional practice of grouping protozoa with animals is no longer considered valid, the term continues to be used in a loose way to identify single-celled organisms that can move independently and feed by heterotrophy. In some systems of biological classification, Protozoa is a high-level taxonomic group; when first introduced in 1818, Protozoa was erected as a taxonomic class, but in classification schemes it was elevated to a variety of higher ranks, including phylum and kingdom. In a series of classifications proposed by Thomas Cavalier-Smith and his collaborators since 1981, Protozoa has been ranked as a kingdom; the seven-kingdom scheme presented by Ruggiero et al. in 2015, places eight phyla under Kingdom Protozoa: Euglenozoa, Metamonada, Choanozoa sensu Cavalier-Smith, Percolozoa and Sulcozoa.
Notably, this kingdom excludes several major groups of organisms traditionally placed among the protozoa, including the ciliates, dinoflagellates and the parasitic apicomplexans, all of which are classified under Kingdom Chromista. Kingdom Protozoa, as defined in this scheme, does not form a natural group or clade, but a paraphyletic group or evolutionary grade, within which the members of Fungi and Chromista are thought to have evolved; the word "protozoa" was coined in 1818 by zoologist Georg August Goldfuss, as the Greek equivalent of the German Urthiere, meaning "primitive, or original animals". Goldfuss created Protozoa as a class containing; the group included not only single-celled microorganisms but some "lower" multicellular animals, such as rotifers, sponges, jellyfish and polychaete worms. The term Protozoa is formed from the Greek words πρῶτος, meaning "first", ζῶα, plural of ζῶον, meaning "animal"; the use of Protozoa as a formal taxon has been discouraged by some researchers because the term implies kinship with animals and promotes an arbitrary separation of "animal-like" from "plant-like" organisms.
In 1848, as a result of advancements in cell theory pioneered by Theodor Schwann and Matthias Schleiden, the anatomist and zoologist C. T. von Siebold proposed that the bodies of protozoans such as ciliates and amoebae consisted of single cells, similar to those from which the multicellular tissues of plants and animals were constructed. Von Siebold redefined Protozoa to include only such unicellular forms, to the exclusion of all metazoa. At the same time, he raised the group to the level of a phylum containing two broad classes of microorganisms: Infusoria, Rhizopoda; the definition of Protozoa as a phylum or sub-kingdom composed of "unicellular animals" was adopted by the zoologist Otto Bütschli—celebrated at his centenary as the "architect of protozoology"—and the term came into wide use. As a phylum under Animalia, the Protozoa were rooted in the old "two-kingdom" classification of life, according to which all living beings were classified as either animals or plants; as long as this scheme remained dominant, the protozoa were understood to be animals and studied in departments of Zoology, while photosynthetic microorganisms and microscopic fungi—the so-called Protophyta—were assigned to the Plants, studied in departments of Botany.
Criticism of this system began in the latter half of the 19th century, with the realization that many organisms met the criteria for inclusion among both plants and animals. For example, the algae Euglena and Dinobryon have chloroplasts for photosynthesis, but can feed on organic matter and are motile. In 1860, John Hogg argued against the use of "protozoa", on the grounds that "naturalists are divided in opinion—and some will continue so—whether many of these organisms, or living beings, are animals or plants." As an alternative, he proposed a new kingdom called Primigenum, consisting of both the protozoa and unicellular algae, which he combined together under the name "Protoctista". In Hoggs's conception, the animal and plant kingdoms were likened to two great "pyramids" blending at their bases in the Kingdom Primigenum. Six years Ernst Haeckel proposed a third kingdom of life, which he named Protista. At first, Haeckel included a few multicellular organisms in this kingdom, but in work he restricted the Protista to single-celled organisms, or simple colonies whose individual cells are not differentiated into different kinds of tissues.
Despite these proposals, Protozoa emerged as the preferred taxonomic placement for heterotrophic microorganisms such as amoebae and ciliates, remained so for more than a century. In the course of the 20th century, the old "two kingdom" system began to weaken, with the growing awareness that fungi did not belong among the plants, that most of the unicellular protozoa were no more related to the animals than they were to the plants. By mid-century, some biologists, such as Herbert Copeland, Robert H. Whittaker and Lynn Margulis, advocated the revival of Haeckel's Protista or Hogg's Protoctista as a kingdom-level eukaryotic group, alongside Plants and Fungi. A variety of multi-kingdom systems were proposed, Kingdoms Protista and Protoctista became well est
A chemostat is a bioreactor to which fresh medium is continuously added, while culture liquid containing left over nutrients, metabolic end products and microorganisms are continuously removed at the same rate to keep the culture volume constant. By changing the rate with which medium is added to the bioreactor the specific growth rate of the microorganism can be controlled within limits. One of the most important features of chemostats is that microorganisms can be grown in a physiological steady state under constant environmental conditions. In this steady state, growth occurs at a constant specific growth rate and all culture parameters remain constant. In addition, environmental conditions can be controlled by the experimenter. Microorganisms growing in chemostats reach a steady state because of a negative feedback between growth rate and nutrient consumption: if a low number of cells are present in the bioreactor, the cells can grow at growth rates higher than the dilution rate as they consume little nutrient so growth is less limited by the addition of limiting nutrient with the inflowing fresh medium.
The limiting nutrient is a nutrient essential for growth, present in the medium at a limiting concentration. However, the higher the number of cells becomes, the more nutrient is consumed, lowering the concentration of the limiting nutrient. In turn, this will reduce the specific growth rate of the cells which will lead to a decline in the number of cells as they keep being removed from the system with the outflow; this results in a steady state. Due to the self-regulation, the steady state is stable; this enables the experimenter to control the specific growth rate of the microorganisms by changing the speed of the pump feeding fresh medium into the vessel. Another important feature of chemostats and other continuous culture systems is that they are well-mixed so that environmental conditions are homogenous or uniform and microorganisms are randomly dispersed and encounter each other randomly; therefore and other interactions in the chemostat are global, in contrast to biofilms. The rate of nutrient exchange is expressed as the dilution rate D.
At steady state, the specific growth rate μ of the micro-organism is equal to the dilution rate D. The dilution rate is defined as the flow of medium per unit of time, F, over the volume V of culture in the bioreactor D = medium flow rate culture volume = F V Specific growth rate μ is inversely related to the time it takes the biomass to double, called doubling time td, by: μ = ln 2 t d Therefore, the doubling time td becomes a function of dilution rate D in steady state: t d = ln 2 D Each microorganism growing on a particular substrate has a maximal specific growth rate μmax. If a dilution rate is chosen, higher than μmax, the cells cannot grow at a rate as fast as the rate with which they are being removed so the culture will not be able to sustain itself in the bioreactor, will wash out. However, since the concentration of the limiting nutrient in the chemostat cannot exceed the concentration in the feed, the specific growth rate that the cells can reach in the chemostat is slightly lower than the maximal specific growth rate because specific growth rate increases with nutrient concentration as described by the kinetics of the Monod equation.
The highest specific growth' rates cells can attain is equal to the critical dilution rate: D = μ max S K S + S, where S is the substrate or nutrient concentration in the chemostat and KS is the half-saturation constant. Chemostats in research are used for investigations in cell biology, as a source for large volumes of uniform cells or protein; the chemostat is used to gather steady state data about an organism in order to generate a mathematical model relating to its metabolic processes. Chemostats are used as microcosms in ecology and evolutionary biology. In the one case, mutation/selection is a nuisance, in the other case, it is the desired process under study. Chemostats can be used to enrich for specific types of bacterial mutants in culture such as auxotrophs or those that are resistant to antibiotics or bacteriophages for further scientific study. Variations in the dilution rate permit the study of the metabolic strategies pursued by the organisms at different growth rates. Competition for single and multiple resources, the evolution of resource acquisition and utilization pathways, cross-feeding/symbiosis, antagonism and competition among predators have all been studied in ecology and evolutionary biology using chemostats.
Chemostats are used in the industrial manufacturing of ethanol. In this case, several chemostats are used in series, each maintained at decreasing sugar concentrations; the chemostat serves as an experimental model of continuous cell cultures in the biotechnological industry. Foaming results in overflow with the vol