Besides macroscopic plants and animals, microbes are
the major components of biological systems on this earth.
You have studied about the diversity of living organisms
in Class XI. Do you remember which Kingdoms among
the living organisms contain micro-organisms? Which are
the ones that are only microscopic? Microbes are present
everywhere–in soil, water, air, inside our bodies and that
of other animals and plants. They are present even at sites
where no other life-form could possibly exist–sites such
as deep inside the geysers (thermal vents) where the
temperature may be as high as 100
0
C, deep in the soil,
under the layers of snow several metres thick, and in highly
acidic environments. Microbes are diverse–protozoa,
bacteria, fungi and microscopic animal and plant viruses,
viroids and also prions that are proteinacious infectious
agents. Some of the microbes are shown in Figures 10.1
and 10.2.
Microbes like bacteria and many fungi can be grown
on nutritive media to form colonies (Figure 10.3), that can
be seen with the naked eyes. Such cultures are useful in
studies on micro-organisms.
CHAPTER 10
MICROBES IN HUMAN WELFARE
10.1 Microbes in Household
Products
10.2 Microbes in Industrial
Products
10.3 Microbes in Sewage
Treatment
10.4 Microbes in Production of
Biogas
10.5 Microbes as Biocontrol
Agents
10.6 Microbes as Biofertilisers
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(a)
(b)
(c)
(a) (b)
Figure 10.3 (a) Colonies of bacteria growing in a petri dish;
(b) Fungal colony growing in a petri dish
(a) (b)
(c)
Figure10.1 Bacteria: (a) Rod-shaped,
magnified 1500X; (b) Spherical
shaped, magnified1500X; (c) A rod-
shaped bacterium showing flagella,
magnified 50,000X
Figure 10.2 Viruses: (a) A bacteriophage; (b)
Adenovirus which causes respiratory
infections; (c) Rod-shaped Tobacco
Mosaic Virus (TMV). Magnified about
1,00,000–1,50,000X
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In chapter 8, you have read that microbes cause a large number of
diseases in human beings. They also cause diseases in animals and plants.
But this should not make you think that all microbes are harmful; several
microbes are useful to man in diverse ways. Some of the most important
contributions of microbes to human welfare are discussed in this chapter.
10.1 MICROBES IN HOUSEHOLD PRODUCTS
You would be surprised to know that we use microbes or products
derived from them everyday. A common example is the production of
curd from milk. Micro-organisms such as Lactobacillus and others
commonly called lactic acid bacteria (LAB) grow in milk and convert it
to curd. During growth, the LAB produce acids that coagulate and
partially digest the milk proteins. A small amount of curd added to the
fresh milk as inoculum or starter contain millions of LAB, which at
suitable temperatures multiply, thus converting milk to curd, which
also improves its nutritional quality by increasing vitamin B
12
. In our
stomach too, the LAB play very beneficial role in checking disease-
causing microbes.
The dough, which is used for making foods such as dosa and idli is
also fermented by bacteria. The puffed-up appearance of dough is due to
the production of CO
2
gas. Can you tell which metabolic pathway is
taking place resulting in the formation of CO
2
? Where do you think the
bacteria for these fermentations come from? Similarly the dough, which
is used for making bread, is fermented using baker’s yeast
(Saccharomyces cerevisiae). A number of traditional drinks and foods
are also made by fermentation by the microbes. ‘Toddy’, a traditional
drink of some parts of southern India is made by fermenting sap from
palms. Microbes are also used to ferment fish, soyabean and bamboo-
shoots to make foods. Cheese, is one of the oldest food items in which
microbes were used. Different varieties of cheese are known by their
characteristic texture, flavour and taste, the specificity coming from the
microbes used. For example, the large holes in ‘Swiss cheese’ are due to
production of a large amount of CO
2
by a bacterium named
Propionibacterium sharmanii. The ‘Roquefort cheese’ are ripened by
growing a specific fungi on them, which gives them a particular flavour
.
10.2 MICROBES IN INDUSTRIAL PRODUCTS
Even in industry, microbes are used to synthesise a number of products
valuable to human beings. Beverages and antibiotics are some examples.
Production on an industrial scale, requires growing microbes in very large
vessels called fermentors (Figure 10.4).
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10.2.1 Fermented Beverages
Microbes especially yeasts have been used from
time immemorial for the production of beverages
like wine, beer, whisky, brandy or rum. For this
purpose the same yeast Saccharomyces
cerevisiae used for bread-making and
commonly called brewer’s yeast, is used for
fermenting malted cereals and fruit juices, to
produce ethanol. Do you recollect the metabolic
reactions, which result in the production of
ethanol by yeast? Depending on the type of the
raw material used for fermentation and the type
of processing (with or without distillation)
different types of alcoholic drinks are obtained.
Wine and beer are produced without distillation
whereas whisky, brandy and rum are produced
by distillation of the fermented broth. The
photograph of a fermentation plant is shown in
Figure 10.5.
10.2.2 Antibiotics
Antibiotics produced by microbes are regarded
as one of the most significant discoveries of the
twentieth century and have greatly contributed
towards the welfare of the human society. Anti is
a Greek word that means ‘against’, and bio means
‘life’, together they mean ‘against life’ (in the
context of disease causing organisms); whereas with reference to human
beings, they are ‘pro life’ and not against. Antibiotics are chemical
substances, which are produced by some microbes and can kill or retard
the growth of other (disease-causing) microbes.
You are familiar with the commonly used antibiotic Penicillin. Do you
know that Penicillin was the first antibiotic to be discovered, and it was a
chance discovery? Alexander Fleming while working on Staphylococci
bacteria, once observed a mould growing in one of his unwashed culture
plates around which Staphylococci could not grow. He found out that it
was due to a chemical produced by the mould and he named it Penicillin
after the mould Penicillium notatum. However, its full potential as an
effective antibiotic was established much later by Ernest Chain and
Howard Florey. This antibiotic was extensively used to treat American
soldiers wounded in World War II. Fleming, Chain and Florey were awarded
the Nobel Prize in 1945, for this discovery.
Figure 10.5 Fermentation Plant
Figure 10.4 Fermentors
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After Penicillin, other antibiotics were also purified from other
microbes. Can you name some other antibiotics and find out their
sources? Antibiotics have greatly improved our capacity to treat deadly
diseases such as plague, whooping cough (kali khansi), diphtheria (gal
ghotu) and leprosy (kusht rog
), which used to kill millions all over the
globe. Today, we cannot imagine a world without antibiotics.
10.2.3 Chemicals, Enzymes and other Bioactive Molecules
Microbes are also used for commercial and industrial production of
certain chemicals like organic acids, alcohols and enzymes. Examples of
acid producers are Aspergillus niger (a fungus) of citric acid, Acetobacter
aceti (a bacterium) of acetic acid; Clostridium butylicum (a bacterium) of
butyric acid and Lactobacillus (a bacterium) of lactic acid.
Yeast (Saccharomyces cerevisiae) is used for commercial production
of ethanol. Microbes are also used for production of enzymes. Lipases are
used in detergent formulations and are helpful in removing oily stains
from the laundry. You must have noticed that bottled fruit juices bought
from the market are clearer as compared to those made at home. This is
because the bottled juices are clarified by the use of pectinases and
proteases. Streptokinase produced by the bacterium Streptococcus and
modified by genetic engineering is used as a ‘clot buster’ for removing
clots from the blood vessels of patients who have undergone myocardial
infarction leading to heart attack.
Another bioactive molecule, cyclosporin A, that is used as an
immunosuppressive agent in organ-transplant patients, is produced by
the fungus Trichoderma polysporum. Statins produced by the yeast
Monascus purpureus have been commercialised as blood-cholesterol
lowering agents. It acts by competitively inhibiting the enzyme responsible
for synthesis of cholesterol.
10.3 MICROBES IN SEWAGE TREATMENT
We know that large quantities of waste water are generated everyday in
cities and towns. A major component of this waste water is human excreta.
This municipal waste-water is also called sewage. It contains large
amounts of organic matter and microbes. Many of which are pathogenic.
Have you ever wondered where this huge quantity of sewage or urban
waste water is disposed off daily? This cannot be discharged into natural
water bodies like rivers and streams directly – you can understand why.
Before disposal, hence, sewage is treated in sewage treatment plants (STPs)
to make it less polluting. Treatment of waste water is done by the
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heterotrophic microbes naturally present in
the sewage. This treatment is carried out in
two stages:
Primary treatment : These treatment
steps basically involve physical removal of
particles – large and small – from the sewage
through filtration and sedimentation. These
are removed in stages; initially, floating debris
is removed by sequential filtration. Then the
grit (soil and small pebbles) are removed by
sedimentation. All solids that settle form the
primary sludge, and the supernatant forms
the effluent. The effluent from the primary
settling tank is taken for secondary treatment.
Secondary treatment or Biological treatment : The primary
effluent is passed into large aeration tanks (Figure 10.6) where it is
constantly agitated mechanically and air is pumped into it. This allows
vigorous growth of useful aerobic microbes into flocs (masses of
bacteria associated with fungal filaments to form mesh like
structures). While growing, these microbes consume the major part
of the organic matter in the effluent. This significantly reduces the
BOD (biochemical oxygen demand) of the effluent. BOD refers to
the amount of the oxygen that would be consumed if all the organic
matter in one liter of water were oxidised by bacteria. The sewage
water is treated till the BOD is reduced. The BOD test measures the
rate of uptake of oxygen by micro-organisms in a sample of water
and thus, indirectly, BOD is a measure of the organic matter present
in the water. The greater the BOD of waste water, more is its polluting
potential.
Once the BOD of sewage or waste water is reduced significantly, the
effluent is then passed into a settling tank where the bacterial ‘flocs’ are
allowed to sediment. This sediment is called activated sludge. A small
part of the activated sludge is pumped back into the aeration tank to
serve as the inoculum. The remaining major part of the sludge is pumped
into large tanks called anaerobic sludge digesters. Here, other kinds
of bacteria, which grow anaerobically, digest the bacteria and the fungi
in the sludge. During this digestion, bacteria produce a mixture of gases
such as methane, hydrogen sulphide and carbon dioxide. These gases
form biogas and can be used as source of energy as it is inflammable.
The effluent from the secondary treatment plant is generally released
into natural water bodies like rivers and streams. An aerial view of such
a plant is shown in Figure 10.7.
Figure 10.6 Secondary treatment
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You can appreciate how microbes play a major
role in treating millions of gallons of waste water
everyday across the globe. This methodology has
been practiced for more than a century now, in
almost all parts of the world. Till date, no man-
made technology has been able to rival the
microbial treatment of sewage.
You are aware that due to increasing
urbanisation, sewage is being produced in much
larger quantities than ever before. However the
number of sewage treatment plants has not
increased enough to treat such large quantities.
So the untreated sewage is often discharged directly into rivers leading to
their pollution and increase in water-borne diseases.
The Ministry of Environment and Forests has initiated Ganga Action
Plan and Yamuna Action Plan to save these major rivers of our country
from pollution. Under these plans, it is proposed to build a large number
of sewage treatment plants so that only treated sewage may be discharged
in the rivers. A visit to a sewage treatment plant situated in any place
near you would be a very interesting and educating experience.
10.4 MICROBES IN PRODUCTION OF BIOGAS
Biogas is a mixture of gases (containing predominantly methane) produced
by the microbial activity and which may be used as fuel. You have learnt
that microbes produce different types of gaseous end-products during
growth and metabolism. The type of the gas produced depends upon the
microbes and the organic substrates they utilise. In the examples cited in
relation to fermentation of dough, cheese making and production of
beverages, the main gas produced was CO
2.
. However, certain bacteria,
which grow anaerobically on cellulosic material, produce large amount
of methane along with CO
2
and H
2
. These bacteria are collectively called
methanogens, and one such common bacterium is Methanobacterium.
These bacteria are commonly found in the anaerobic sludge during
sewage treatment. These bacteria are also present in the rumen (a part of
stomach) of cattle. A lot of cellulosic material present in the food of cattle
is also present in the rumen. In rumen, these bacteria help in the
breakdown of cellulose and play an important role in the nutrition of
cattle. Do you think we, human beings, are able to digest the celluose
present in our foods? Thus, the excreta (dung) of cattle, commonly called
gobar, is rich in these bacteria. Dung can be used for generation of biogas,
commonly called gobar gas.
The biogas plant consists of a concrete tank (10-15 feet deep) in which
bio-wastes are collected and a slurry of dung is fed. A floating cover is
Figure 10.7 An aerial view of a sewage plant
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placed over the slurry, which
keeps on rising as the gas is
produced in the tank due to the
microbial activity. The biogas
plant has an outlet, which is
connected to a pipe to supply
biogas to nearby houses. The
spent slurry is removed through
another outlet and may be used
as fertiliser. Cattle dung is
available in large quantities in
rural areas where cattle are used
for a variety of purposes. So
biogas plants are more often
built in rural areas. The biogas
thus produced is used for
cooking and lighting. The
picture of a biogas plant is
shown in Figure 10.8. The
technology of biogas production
was developed in India mainly
due to the efforts of Indian Agricultural Research Institute (IARI) and
Khadi and Village Industries Commission (KVIC). If your school is situated
in a village or near a village, it would be very interesting to enquire if there
are any biogas plants nearby. Visit the biogas plant and learn more about
it from the people who are actually managing it.
10.5 MICROBES AS BIOCONTROL AGENTS
Biocontrol refers to the use of biological methods for controlling plant
diseases and pests. In modern society, these problems have been tackled
increasingly by the use of chemicals – by use of insecticides and pesticides.
These chemicals are toxic and extremely harmful, to human beings and
animals alike, and have been polluting our environment (soil, ground
water), fruits, vegetables and crop plants. Our soil is also polluted through
our use of weedicides to remove weeds.
Biological control of pests and diseases: In agriculture, there is a
method of controlling pests that relies on natural predation rather than
introduced chemicals. A key belief of the organic farmer is that biodiversity
furthers health. The more variety a landscape has, the more sustainable
it is. The organic farmer, therefore, works to create a system where the
insects that are sometimes called pests are not eradicated, but instead
are kept at manageable levels by a complex system of checks and balances
within a living and vibrant ecosystem. Contrary to the ‘conventional’
farming practices which often use chemical methods to kill both useful
Figure 10.8 A typical biogas plant
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and harmful life forms indiscriminately, this is a holistic approach that
seeks to develop an understanding of the webs of interaction between the
myriad of organisms that constitute the field fauna and flora. The organic
farmer holds the view that the eradication of the creatures that are often
described as pests is not only possible, but also undesirable, for without
them the beneficial predatory and parasitic insects which depend upon
them as food or hosts would not be able to survive. Thus, the use of
biocontrol measures will greatly reduce our dependence on toxic chemicals
and pesticides. An important part of the biological farming approach is
to become familiar with the various life forms that inhabit the field,
predators as well as pests, and also their life cycles, patterns of feeding
and the habitats that they prefer. This will help develop appropriate means
of biocontrol.
The very familiar beetle with red and black markings – the Ladybird,
and Dragonflies are useful to get rid of aphids and mosquitoes,
respectively. An example of microbial biocontrol agents that can be
introduced in order to control butterfly caterpillars is the bacteria Bacillus
thuringiensis (often written as Bt). These are available in sachets as dried
spores which are mixed with water and sprayed onto vulnerable plants
such as brassicas and fruit trees, where these are eaten by the insect
larvae. In the gut of the larvae, the toxin is released and the larvae get
killed. The bacterial disease will kill the caterpillars, but leave other insects
unharmed. Because of the development of methods of genetic engineering
in the last decade or so, the scientists have introduced B. thuringiensis
toxin genes into plants. Such plants are resistant to attack by insect pests.
Bt-cotton is one such example, which is being cultivated in some states
of our country. You will learn more about this in chapter 12.
A biological control being developed for use in the treatment of plant
disease is the fungus Trichoderma. Trichoderma species are free-living
fungi that are very common in the root ecosystems. They are effective
biocontrol agents of several plant pathogens.
Baculoviruses are pathogens that attack insects and other arthropods.
The majority of baculoviruses used as biological control agents are in the
genus Nucleopolyhedrovirus. These viruses are excellent candidates for
species-specific, narrow spectrum insecticidal applications. They have
been shown to have no negative impacts on plants, mammals, birds, fish
or even on non-target insects. This is especially desirable when beneficial
insects are being conserved to aid in an overall integrated pest
management (IPM) programme, or when an ecologically sensitive area is
being treated.
10.6 MICROBES AS BIOFERTILISERS
With our present day life styles environmental pollution is a major cause
of concern. The use of the chemical fertilisers to meet the ever-increasing
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demand of agricultural produce has contributed significantly to
this pollution. Of course, we have now realised that there are problems
associated with the overuse of chemical fertilisers and there is a
large pressure to switch to organic farming – the use of biofertilisers.
Biofertilisers are organisms that enrich the nutrient quality of the soil.
The main sources of biofertilisers are bacteria, fungi and cyanobacteria.
You have studied about the nodules on the roots of leguminous plants
formed by the symbiotic association of Rhizobium. These bacteria fix
atmospheric nitrogen into organic forms, which is used by the plant as
nutrient. Other bacteria can fix atmospheric nitrogen while free-living in
the soil (examples Azospirillum and Azotobacter), thus enriching the
nitrogen content of the soil.
Fungi are also known to form symbiotic associations with plants
(mycorrhiza). Many members of the genus Glomus form mycorrhiza.
The fungal symbiont in these associations absorbs phosphorus from
soil and passes it to the plant. Plants having such associations show
other benefits also, such as resistance to root-borne pathogens, tolerance
to salinity and drought, and an overall increase in plant growth and
development. Can you tell what advantage the fungus derives from
this association?
Cyanobacteria are autotrophic microbes widely distributed in aquatic
and terrestrial environments many of which can fix atmospheric nitrogen,
e.g. Anabaena
, Nostoc, Oscillatoria, etc. In paddy fields, cyanobacteria
serve as an important biofertiliser. Blue green algae also add organic matter
to the soil and increase its fertility. Currently, in our country, a number
of biofertilisers are available commercially in the market and farmers use
these regularly in their fields to replenish soil nutrients and to reduce
dependence on chemical fertilisers.
SUMMARY
Microbes are a very important component of life on earth. Not all
microbes are pathogenic. Many microbes are very useful to human
beings. We use microbes and microbially derived products almost every
day. Bacteria called lactic acid bacteria (LAB) grow in milk to convert it
into curd. The dough, which is used to make bread, is fermented by
yeast called Saccharomyces cerevisiae. Certain dishes such as idli and
dosa, are made from dough fermented by microbes. Bacteria and fungi
are used to impart particular texture, taste and flavor to cheese. Microbes
are used to produce industrial products like lactic acid, acetic acid
and alcohol, which are used in a variety of processes in the industry.
Antibiotics like penicillins produced by useful microbes are used to kill
disease-causing harmful microbes. Antibiotics have played a major role
in controlling infectious diseases like diphtheria, whooping cough and
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EXERCISES
1. Bacteria cannot be seen with the naked eyes, but these can be seen
with the help of a microscope. If you have to carry a sample from your
home to your biology laboratory to demonstrate the presence of microbes
with the help of a microscope, which sample would you carry and why?
2. Give examples to prove that microbes release gases during metabolism.
3. In which food would you find lactic acid bacteria? Mention some of
their useful applications.
4. Name some traditional Indian foods made of wheat, rice and Bengal
gram (or their products) which involve use of microbes.
5. In which way have microbes played a major role in controlling diseases
caused by harmful bacteria?
6. Name any two species of fungus, which are used in the production of
the antibiotics.
7. What is sewage? In which way can sewage be harmful to us?
8. What is the key difference between primary and secondary sewage
treatment?
9. Do you think microbes can also be used as source of energy? If yes, how?
10. Microbes can be used to decrease the use of chemical fertilisers and
pesticides. Explain how this can be accomplished.
11. Three water samples namely river water, untreated sewage water and
secondary effluent discharged from a sewage treatment plant were
subjected to BOD test. The samples were labelled A, B and C; but the
laboratory attendant did not note which was which. The BOD values
of the three samples A, B and C were recorded as 20mg/L, 8mg/L and
400mg/L, respectively. Which sample of the water is most polluted?
Can you assign the correct label to each assuming the river water is
relatively clean?
pneumonia. For more than a hundred years, microbes are being used
to treat sewage (waste water) by the process of activated sludge formation
and this helps in recycling of water in nature. Methanogens produce
methane (biogas) while degrading plant waste. Biogas produced by
microbes is used as a source of energy in rural areas. Microbes can also
be used to kill harmful pests, a process called as biocontrol. The
biocontrol measures help us to avoid heavy use of toxic pesticides for
controlling pests. There is a need these days to push for use of
biofertilisers in place of chemical fertilisers. It is clear from the diverse
uses human beings have put microbes to that they play an important
role in the welfare of human society.
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12. Find out the name of the microbes from which Cyclosporin A (an
immunosuppressive drug) and Statins (blood cholesterol lowering
agents) are obtained.
13. Find out the role of microbes in the following and discuss it with your teacher.
(a) Single cell protein (SCP)
(b) Soil
14. Arrange the following in the decreasing order (most important first) of
their importance, for the welfare of human society. Give reasons for
your answer.
Biogas, Citric acid, Penicillin and Curd
15. How do biofertilisers enrich the fertility of the soil?
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