Download Role of nitrogen in the biosphere

Role of nitrogen in the biosphere
The growth of all organisms depends on the availability of mineral nutrients, and none is more
important than nitrogen, which is required in large amounts as an essential component of proteins,
nucleic acids and other cellular constituents. There is an abundant supply of nitrogen in the earth's
atmosphere - nearly 79% in the form of N2 gas. However, N2 is unavailable for use by most
organisms because there is a triple bond between the two nitrogen atoms, making the molecule
almost inert. In order for nitrogen to be used for growth it must be "fixed" (combined) in the form of
ammonium (NH4) or nitrate (NO3) ions. The weathering of rocks releases these ions so slowly that it
has a neglible effect on the availability of fixed nitrogen. So, nitrogen is often the limiting factor for
growth and biomass production in all environments where there is suitable climate and availability of
water to support life.
Microorganisms have a central role in almost all aspects of nitrogen availability and thus for life
support on earth:
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some bacteria can convert N2 into ammonia by the process termed nitrogen fixation;
these bacteria are either free-living or form symbiotic associations with plants or other
organisms (e.g. termites, protozoa)
other bacteria bring about transformations of ammonia to nitrate, and of nitrate to N 2 or
other nitrogen gases
many bacteria and fungi degrade organic matter, releasing fixed nitrogen for reuse by
other organisms.
All these processes contribute to the nitrogen cycle.
We shall deal first with the process of nitrogen fixation and the nitrogen-fixing organisms, then
consider the microbial processes involved in the cycling of nitrogen in the biosphere.
Nitrogen fixation
A relatively small amount of ammonia is produced by lightning. Some ammonia also is produced
industrially by the Haber-Bosch process, using an iron-based catalyst, very high pressures and fairly
high temperature. But the major conversion of N2 into ammonia, and thence into proteins, is achieved
by microorganisms in the process called nitrogen fixation (or dinitrogen fixation).
Both free-living cyanobacteria and the cyanobacterial associates of lichens initially contributed
nitrogen to the soil by forming a cryptobiotic crust. Now numerous leguminous plants occur in this
desert, with nitrogen-fixing Rhizobium in their root nodules. Examples are the green-stemmed brushlike trees at the right and left of the image (Parkinsonia species, common name "paloverde"), and
several acacias and mesquites.
Mechanism of biological nitrogen fixation
Biological nitrogen fixation can be represented by the following equation, in which two moles of
ammonia are produced from one mole of nitrogen gas, at the expense of 16 moles of ATP and a
supply of electrons and protons (hydrogen ions):
N2 + 8H+ + 8e- + 16 ATP = 2NH3 + H2 + 16ADP + 16 Pi
This reaction is performed exclusively by prokaryotes (the bacteria and related organisms), using an
enzyme complex termed nitrogenase. This enzyme consists of two proteins - an iron protein and a
molybdenum-iron protein, as shown below.
The reactions occur while N2 is bound to the nitrogenase enzyme complex. The Fe protein is first
reduced by electrons donated by ferredoxin. Then the reduced Fe protein binds ATP and reduces the
molybdenum-iron protein, which donates electrons to N2, producing HN=NH. In two further cycles of
this process (each requiring electrons donated by ferredoxin) HN=NH is reduced to H2N-NH2, and this
in turn is reduced to 2NH3.
Depending on the type of microorganism, the reduced ferredoxin which supplies electrons for this
process is generated by photosynthesis, respiration or fermentation.
There is a remarkable degree of functional conservation between the nitrogenase proteins of all
nitrogen-fixing bacteria. The Fe protein and the Mo-Fe protein have been isolated from many of these
bacteria, and nitrogen fixation can be shown to occur in cell-free systems in a laboratory when the Fe
protein of one species is mixed with the Mo-Fe protein of another bacterium, even if the species are
very distantly related.
The nitrogen-fixing organisms
All the nitrogen-fixing organisms are prokaryotes (bacteria). Some of them live independently of other
organisms - the so-called free-living nitrogen-fixing bacteria. Others live in intimate symbiotic
associations with plants or with other organisms (e.g. protozoa). Examples are shown in the table
below.
Examples of nitrogen-fixing bacteria (* denotes a photosynthetic bacterium)
Free living
Symbiotic with plants
Aerobic
Anaerobic (see Winogradsky
column for details)
Legumes
Other plants
Azotobacter
Beijerinckia
Klebsiella (some)
Cyanobacteria (some)*
Clostridium (some)
Desulfovibrio
Purple sulphur bacteria*
Purple non-sulphur bacteria*
Green sulphur bacteria*
Rhizobium
Frankia
Azospirillum
Bacteria in waste management
Mahendra Pandey
MICRO-ORGANISMS constitute an antique group of living organisms which
appeared on earth's surface almost 3,000 million years ago. Since they first
began to be studied in 1590, bacteriologists have described only about 5,000
species of bacteria. They are found in soil, water, air, in plants, animals, food
products, in the human body, and even on its surface.
Though they collectively outweigh all other living organisms on earth, and despite
their contributions to environment, health and day-to-day life, bacteria remain
largely ignored and certainly under-appreciated. Bacteria have always been an
integral part of traditional waste and sewage management, but now scientists
have discovered strains that can clean up toxic and radioactive wastes.
The emerging role of bacteria in the field of biotechnology, with countless new
genes and biochemical pathways to sift for enzymes, antibiotics and other useful
molecules, has generated new interest in them. And now, microbiologists have
the means to pro be the entire microbial world for the first time.
Molecular biology has provided many powerful tools to study these organisms,
and one of them is plucking bacterial DNA straight from nature. Most of these
studies revolve around 16S rRNA gene, a gene present in every living creature
that evolves so slowl y that its sequence of nucleotide building blocks is identical
in every member of a given species and even among closely related species.
More recently, researchers have been using the polymerase chain reaction (PCR)
to make millions of copies of this gene from a single bacterium chosen more or
less at random from, say, a sample of soil or water. By repeating the process
many times and com paring the genes from each grab, researchers can begin to
count the bacterial species in the sample.
Some bacteria, during their regular activity, feast on certain toxins. Many strains
of bacteria have been used effectively to control toluene, a carcinogenic organic
compound, found in gasoline, adhesives and household solvents. To solve this
problem, Dr Peter Coschigano, a microbiologist at Ohio University, is currently
studying Thauera aromatica T1, a bacterium that has the capacity to consume
toluene in the absence of oxygen. Likewise, Southern Illinois University's Dr John
Coates has discovered a ba cterium that can break down petroleum compounds in
similar conditions.
Many scientists are using a bacterium Deinococcus radiodurans to clean up
nuclear waste sites as they consume the toxic mercury compounds associated
with nuclear weapon production. Pollution from these sites is a major
environmental threat, and many stra ins of bacteria have the capacity to treat
such sites chemically, but the major problem is that most of them cannot
withstand such high levels of radiation.
Scientists have discovered that Deinococcus radiodurans is able to withstand
3,000 times the lethal radiation dose for a human. Michael Daly, an assistant
professor of pathology at the Uniformed Services University of the Health
Sciences in the US, has d iscovered how to inject the genetic material from other
bacteria into Deinococcus, thus creating different strains with various capabilities
of treating various forms of mercury and toluene. Australia's Road Technologies
International produces a microbe- based product called Road Tech 2000, which
helps to form a weather- and traffic-durable road surface.
It was developed after geologists noticed that gravel and clay roads that were
only a few tears old had already begun to break down, while other old roads were
in good condition. What was happening was that a type of microbe was digesting
certain mineral s in the soil and producing a polymer that helped to bind the clay
particles in the soil, thus protecting and strengthening the roadway.
In Canada, International Bio-Recovery Co. has developed a method of using heatresponsive bacteria to break down organic waste and convert it into a beneficial
organic fertiliser. Even the process of composting is dependent on bacteria.
Thus, that after testing all the physical, chemical and biological means of waste
treatment, scientists have found that in managing certain wastes, the best option
is microbiological treatment, which is more efficient and consumes no energy.
The role of bacteria in the sewage treatment process
One area of sewage treatment that is not well understood is the bacterial decomposition
process.
Bacteria may be aerobic, anaerobic or facultative. Aerobic bacteria require oxygen for life
support whereas anaerobes can sustain life without oxygen. Facultative bacteria have the
capability of living either in the presence or in the absent of oxygen. In the typical sewage
treatment plant, oxygen is added to improve the functioning of aerobic bacteria and to assist
them in maintaining superiority over the anaerobes. Agitation, settling, pH and other
controllable are carefully considered and employed as a means of maximizing the potential of
bacterial reduction of organic in the wastewater.
Single-celled organisms grow and when they have attained a certain size, divide, becoming two.
Assuming an adequate food supply, they then grow and divide again like the
original cell. Every time a cell splits, approximately every 20 to 30 minutes, a new generation
occurs. This is known as the exponential or logarithmic growth phase. At the exponential
growth rate, the largest number of cells are produced in the shortest period of time.
In nature and in the laboratory, this growth cannot be maintained indefinitely, simply because
the optimum environment of growth cannot be maintained. The amount of growth is the
function of two variables: - environment and food. The pattern which actually results is known
as the bacterial growth rate curve. Initially dehydrated products (dry) must first re-hydrate and
acclimate in a linear growth phase before the exponential rate is reached.
Microorganisms and their enzyme systems are responsible for many different chemical
reactions produced in the degradation of organic matter. As the bacteria metabolize, grow and
divide they produce enzymes. These enzymes are high molecular weight proteins.
It is important to recognize the fact that colonies of bacteria are literally factories for the
production of enzymes. The enzymes which are manufactured by the bacteria will be
appropriate to the substrate in which the enzyme will be working and so you have automatic
production of the right enzyme for the biological reduction of any waste material, provided you
have the right bacteria to start with. Enzymes do not reproduce whereas as bacteria do.
Enzymes in biochemical reactions act as organic catalysts. The enzymes actually become a part
of the action, but after having caused it, split off from it and are themselves unchanged. After
the biochemical reactions are complete and products formed, the enzyme is released to catalyze
another reaction. The rate of reaction may be increase by increasing the quantity of the substrate
or temperature up to a certain point , but beyond this, the rate of reaction ceases to increase
because the enzyme concentration limits it.
All treatment plants should be designed to take advantage of the decomposition
of organic materials by bacterial activity. This is something you can equate to lower costs,
increased capacity, and an improved quality of effluent; even freedom from bad odors which
may typically result when anaerobe bacteria become dominant and in their decomposition
process, produce hydrogen sulfide gas and similar by-products.
Consider the fact that the total organic load of wastewater or sewage is composed of constantly
changing constituent, it would be quite difficult to degrade all of these organics by the addition
of one enzyme, or even several enzymes. Enzymes are specific catalysts and do not reproduce.
What is needed is the addition of an enzyme manufacturing system right in the sewage that can
be pre - determined as to its activity and performance and which has the initial or continuing
capacity to reduce waste.
At the present time, the addition of specifically cultured bacteria seems to be the least expensive
and most generally reliable way to accomplish desirable results. When you add the right
bacteria in proper proportions to the environment, you have established entirely new parameters
of potential for the treatment situation.
Lactic acid bacteria - their uses in food
Lactic acid bacteria have been used to ferment or culture foods for at least 4000 years. They are
used in particular in fermented milk products from all over the world, including yoghurt, cheese,
butter, buttermilk, kefir and koumiss.
Lactic acid bacteria refers to a large group of beneficial bacteria that have similar properties and all produce
lactic acid as an end product of the fermentation process. They are widespread in nature and are also found in
our digestive systems. Although they are best known for their role in the preparation of fermented dairy
products, they are also used for pickling of vegetables, baking, winemaking, curing fish, meats and sausages.
Without understanding the scientific basis, people thousands of years ago used lactic acid bacteria to produce
cultured foods with improved preservation properties and with characteristic flavours and textures different
from the original food.
Similarly today, a wide variety of fermented milk products including liquid drinks such as kefir and semi-solid
or firm products like yoghurt and cheese respectively, make good use of these illustrious microbial allies.
The manufacture involves a microbial process by which the milk sugar, lactose is converted to lactic acid. As
the acid accumulates, the structure of the milk protein changes (curdling) and thus the texture of the product.
Other variables such as temperature and the composition of the milk, also contribute to the particular features of
different products.
Lactic acid also gives fermented milks their slightly tart taste. Additional characteristic flavours and aromas are
often the result of other products of lactic acid bacteria. For example acetaldehyde, provides the characteristic
aroma of yoghurt, while diacetyl imparts a buttery taste to other fermented milks. Additional micro-organisms
such as yeasts can also be included in the culture to provide unique tastes. For example, alcohol and carbon
dioxide produced by yeasts contribute to the refreshing, frothy taste of kefir, koumiss and leben. Other
manufacturing techniques such as removing the whey or adding flavours, also contribute to the large variety of
available products.
For yoghurt, the manufacture depends on a symbiotic relationship between two bacteria, Streptococcus
thermophilus and Lactobacillus bulgaricus, where each species of bacterium stimulates the growth of the other.
This interaction results in a shortened fermentation time and a product with different characteristics than one
fermented with a single species.
With yoghurt and other fermented milks there are considerable opportunities for exploiting lactic acid bacteria
as probiotic cultures. These supplement and help our normal gut bacteria to function more efficiently. The
world-wide market for these products continues to increase in response to the demands of an increasingly
health-conscious public.
Lactic acid bacteria are therefore excellent ambassadors for an often maligned microbial world. They are not
only of major economic significance, but are also of value in maintaining and promoting human health.
Natural dairy starters: microbiological aspects and importance in the
field
Today, industrial fermented food is produced by using starter cultures. The basic role of such cultures,
which are mainly composed of a limited number of lactic acid bacteria (LAB) species, is to drive the
fermentation process, leading to a rapid pH drop of the product. Concomitantly, microbial starters often
contribute to aroma compound accumulation during product ripening and may exert beneficial health effects.
Since the starter industry relies on the use of selected strains of given species with known metabolic properties,
the introduction of starter cultures has undoubtedly improved the commercial and hygienic quality of the
product and the overall process standardization. On the other hand, the limited number of available strains with
high technological performances and the constant risk of bacteriophage attack justify the continuous need to
search for new strains for product diversification.
Many fermented foods, however, are still based on the use of undefined cultures of LAB or yeasts
including bread varieties, traditional dried sausages, and cheeses (Hansen, 2002). The production of such
cultures, also defined as "natural starters", is derived from the ancient backslopping practice. Together with
important advantages, backslopping presents a number of drawbacks, the most important of which is the day-today microbial variability of the cultures, which determines a fluctuation in their technological performances. In
spite of this, the production of many traditional cheeses in Europe still relies on the use of natural mixed strain
starters, which are prepared in either cheese whey (called “sieroinnesto”) or milk (“lattoinnesto”) and
reproduced daily at the cheese plant. Concerning cheesemaking technology, backslopping is still practiced to
process traditionally-made, farm house goat cheeses – Rocamadour, Picodon –, or some Swiss-type cheeses and
hard cheeses – Parmigiano Reggiano, Grana Padano, Comté, Sbrinz. More specifically, artisan whey cultures
for the production of traditional hard cheeses are the most frequently produced natural starters. Natural whey
starters ("sieroinnesto") are prepared by removing some of the whey drained from the cheese vat at the end of
cheese-making, which is then left to cool at room temperature or incubated at 40-45°C until the necessary
acidity is reached. The concomitant pressure of both chemical and physical actions leads to the selection of a
thermophilic, aciduric, and moderately heat resistant LAB community, generally dominated by Lactobacillus
helveticus, Lactobacillus delbrueckii subsp. lactis, Lactobacillus fermentum, and Streptococcus thermophilus
(Parente and Cogan, 2004). Most LAB found in natural whey starters are originating from raw milk, thus
suggesting that a link may exist between the geographical origin of the cheese and the microbiological
composition of the starters, although studies on this link are often inconclusive. Natural starters could be an
important source of strains with potentially novel properties for dairy industry and mainly for historical and
cultural reasons are unlikely to be replaced by defined strain starters in the production of traditional cheeses.
A common trait of the natural whey cultures is that the microbial composition and diversity are
generally undefined, with few dominating species and strains in natural rotation during starter preparation and
cheesemaking. Concerning the whey starters for Grana Padano cheese, recent investigations indicated that a
limited number of genotypically diverse LAB strains dominate these cultures (Rossetti et al., 2008), although
few studies are available to evaluate strain-specific phenotypic traits, population composition, and dynamics
during whey starter preparation and later, under cheesemaking conditions. In recent years, a number of reports
indicated that a certain strain variability in peptidase and acidifying activities, phage resistance, and lysogenic
state exists within L. helveticus strains associated with Grana and Parmigiano Reggiano cheese whey starters
(Carminati et al., 1997; Gatti et al., 2003; 2004; Zago et al., in press). In addition, the demonstrated lack of
cultivability of a portion of the thermophilic LAB microflora would indicate that the recovery of all the strains
associated with the whey starters is not always feasible (Cattivelli et al., 2002; Fornasari et al., 2006; Lazzi et
al., 2007). This seems to suggest that the actual microbial composition of the whey starters is partially biased by
ecological phenomena, and justifies the need for more updated knowledge about the LAB community
associated with these cultures as well as the application of more performing, culture-independent methods, to
reach the uncultivable portion of this community.
A key role in the generation and maintenance of microbial diversity in natural starters seems to be
played by bacteriophages. We recently devoted a number of studies to describe the bacteria and phage
communities and their relationships within the whey starter cultures for Grana cheese (Zago et al., 2006; 2007;
2008). A common trait seems that a small fraction of the different populations is sensitive to bacteriophages
infecting such cultures and that, after their lysis, they are replaced by resistant strains, which are constantly
present and in natural rotation with sensitive bacteria, in accordance with well known ecological phage-host
interactions (Jessup and Forde, 2008). The ecological complexity of natural whey starter for Grana cheese
explains, therefore, the adaptation of these cultures to different technological and stress factors. Within the
latter, phage infection may have an active role in selecting, among the different populations, phage-resistant
strains able to counteract the loss of the sensitive ones, thus preserving the overall technological performance of
the culture (Zago et al., in press). Moreover lysogeny, a phage DNA integrated into the bacterial chromosome
that confers resistance to homologous phages, is widespread in lactobacilli and contributes to the genetic
individuality of a bacterial strain and community evolution (Canchaya et al., 2003; 2004). Studies carried out
from our group on thermophilic lactobacilli (e.g. L. helveticus and L. delbrueckii subsp . lactis) associated with
Grana cheese whey starters have demonstrated high frequencies of lysogenic strains (Carminati et al., 1997;
Zago et al., 2007). Both natural balance between phage-sensitive and phage-resistant strains and the wide
presence of lysogenic strains may contribute to explain the relatively minor susceptibility of natural starters to
phage attack with respect to industrial starters, which need of rotation programs and the use of direct-to-vat
starters to assure their perfect functionality.
To respond to the increasing demand for product diversification, the use of bacterial strains with
expanded characteristics (such as flavor production, probiotic traits or antimicrobial activity) is needed. More
specifically, the interest in the microbiota of traditional cheeses, which often contains heterogeneous pools of
bacterial strains, should be encouraged for potential future applications in functional foods. As stated earlier,
thermophilic LAB species present in natural starters for hard cheeses could meet this need for the following
reasons: (i) they often display useful dairy-grade (such as antibiotic susceptibility) or pro-technological (e.g.
broad phage-resistance) traits which make them interesting candidates for new starter formulations; (ii)
lactobacilli, and L. helveticus in particular, have relatively strong (among LAB) proteinase and peptidase
activities that have been shown to influence cheese quality and functionality as well as the production of
bioactive peptides from casein (Korkhonen and Pilanto, 2006; Broadbent et al., 2008). Concerning this latter
aspect, a marked strain-to-strain variability has been observed in commercial functionality, tendency in
biopeptide accumulation, and range in bioactive peptides release from caseins among L. helveticus (Broadbent
et al., 2008). It is envisaged, therefore that natural starters could represent a source of interesting LAB isolates
with novel properties for applications in food and health. Functional and comparative genomics will be useful
tools for effective screening of wide strain collections to detect technologically relevant strains or to better
elucidate physiology and identity targets for future functional genetics experiments in LAB.
Bacteria and Disease
Bacteria cause disease. Most of these bacteria, like those listed below make their presence felt immediatley and
may or may not result in death. Some people though, can be infected with a bacterium that normally causes a
disease and not show any harmfull effects at all, people like this are called carriers. A sad example of this was
Typhoid Mary who was identified as a carrier for typhoid fever in 1906.
Some bacteria attack us in a manner that is not immediately painful in fact the pain may not be apparent for
years. The primary example of this is tooth decay. Tooth decay is caused by dental plaque which is a build up of
4 main species of bacteria on the teeth around the gums: Streptococcus sanguis, S. sobrinus, S. mitis and S.
mutans. In later stages, this plaque can also involve species of Fusobacterium, Borrelia and Actinomycetes.
Tooth decay results from the corrosive action of organic acids released by the bacteria as they metabolise sugars
in your food as part of their normal life.
Introduction
During the 1990s Tuberculosis was the single greatest cause of death in humans by a bacterium. The World
Health Organisation estimates that nearly 3 000 000 people each year die from tuberculosis. Respiratory and
Diarrhoeal diseases cause a similar amount or even more deaths but only some of these are caused by bacteria,
others being caused by viruses, protozoa and fungi. Apart from these diseases, Aids, malaria and the various
forms of Hepatitis all kill between 1 and 3 million people each year, but none of these are caused by bacteria
either so I will say no more about them here.
As scientific research and WHO immunisation programmes continue some of these diseases are being brought
under control and the toll on human life is decreasing. Others, like AIDS, are still on the increase.
Environmental disasters such as flood, draught and earthquake, etc, increase the death toll from disease by
making people more vulnerable through shock, weakness and reduced sanitation. War, for similar reasons,
increases the incidence of disease. All in all, during the year 2000, bacterial diseases will probably have killed 5
million people which accounts for only 10% of the 52 million human deaths occurring on average every year.
This is a huge number of our fellow human beings dieing, however it is trivial compared with the number of
people who will suffer some form of illness or other caused by bacteria even though they are not killed by it.
Bacterial and other infectious diseases are far more significant in third world countries, a direct result of poverty
reducing sanitary practices, medical care and awarness. In more developed countries, like the USA or the UK,
Heart Disease and Cancer are responsible for more than 70% of deaths.
How Do Bacteria Cause Disease?
The first thing the bacterium has to do is enter your system. We come into contact with millions of bacteria
every day. They are in the air we breathe, in and on the food we eat and on the surfaces of most things we
touch. Apart from our normal flora, bacteria that come into contact with us have to pass our various defence
mechanisms, our dry skin and our acid stomach. Physical actions such as the movement of matter through our
alimentary canal, brushing our teeth and washing all help to make life difficult for bacteria. Those bacteria
which do colonise our system generally do so by breaking through the mucus barrier that lines most of our
alimentary canal (mouth to anus), or entering through damaged tissue, ie wounds and bites, etc.
Once a bacterium has entered the system it is free to grow and spread - nearly all infectious diseases start out as
small localised infections and will only spread through the system if the bacteria gain access to the blood
stream.
Infection simply means the bacterial or other agent entering the system. It does not equate with disease or
damage. You can be infected by an organism that never makes you ill. An infectious agent is simply an
organism that is capable of getting past your defences and then living/growing inside or you.
Bacteria rarely, if ever, cause disease merely by being present, even the virulence factors they produce to help
them invade the body often do little real harm.
Virulence factors are normally enzymes. Their role is to make it easier for the bacterium to invade your body. A
good example is 'hyaluronidase' an enzyme secreted by a number of bacteria that breaks down hyaluronic acid,
a sort of organic cement that holds the different tissues of your body together. By destroying this cemment
bacteria can make a passage for themselves through your tissues.
The pathogenicity of invasive bacteria, or their ability to cause disease is generally the result of toxins.
Substances produced by the bacterial cell, sometimes simply as a by-produce of its normal metabolism, which
interact negatively with our body, by interfering with the normal functioning. This is often done by simply
damaging the specific cells, blocking the transmission of some sort of internal signals or other or by
overstimulating some sorts of cells so they malfunction. The ecological reasons for the production of these
toxins is not always understood. Also, because of the vagaries of bacterial genetic reproduction, otherwise
harmless species can acquire a gene which causes them to secrete a toxin, thus making them a pathogenic strain
of a normally harmless species. A good example of this is Escherichia coli which lives harmlessly in most
people's intestines but which occasionally makes us sick.
Toxins that are leaked or secreted out of the bacteria cell and into its host (you and me) is called an Exotoxin.
An Exotoxin that is secreted in the intestines is called an Enterotoxin. Bacterial cells can also produce
substances which, though toxic, are not secreted into the host but remain bound to the bacterial cell wall. These
substances only fulfil their toxic potential if and when the bacterial cell dies and is lysed (broken open and its
contents released).
Exotoxins come in three different forms called cytalytic toxins, A-B toxins and superantigen toxins. Cytalytic
toxins work by causing lysis of host cells thus damaging tissue. A-B toxins are two or more molecules working
as a team. The A molecule binds to a cell wall where it forms a channel to allow molecule B access to the cell.
Superantigen toxins work by overstimulating the body's immune response system. Diphtheria is a cytalytic
toxin. Tetanus is an A-B toxin. Cholera is an A-B toxin that is also an Enterotoxin because it works in the
intestine. Cholera toxin works by disrupting the ionic balance of cells' membranes which results in the cells of
the small intestine secreting large amounts of water into the intestine. This keeps happening and has two effects;
all the water in the small intestine causes diarrhoea and thus the body dehydrates. Eventually the small intestine
loses water faster than the large intestine can reabsorb it and death follows from dehydration.
Endotoxins are generally much less pathogenic than Exotoxins and rarely cause death. Many fevers are caused
by endotoxins whereas exotoxins never produce a fever.
Clostridium botulinum produces toxins which are among the most poisonous or toxic substances known. One
milligram of pure Botulinum toxin is enough to kill 1 million guinea pigs.