MICROORGANISMS : FRIEND AND FOEMICROORGANISMS : FRIEND AND FOE

You have seen several kinds ofplants and animals. However,there are other living organisms

around us which we cannot see witheyes alone. These are calledmicroorganisms or microbes. Forexample, you might have observed thatduring rainy season moist bread getsspoilt and its surface gets covered withgreyish white patches. Observe thesepatches through a magnifying glass. Youwill see tiny, black rounded structures.Do you know what these structures areand where did these come from?

2.1 Microorganisms

Activity 2.1

Collect some moist soil from thefield in a beaker and add water toit. After soil particles have settled,observe a drop of water from thebeaker under a microscope. Whatdo you see ?

Activity 2.2

Take a few drops of water from apond. Spread on a glass slide andobserve through a microscope.

Do you find tiny organisms movingaround?

These observations show that waterand soil are full of tiny organisms,though not all of them fall into thecategory of microbes. Thesemicroorganisms or microbes are sosmall in size that they cannot be seenwith the unaided eye. Some of these,such as the fungus that grows on bread,can be seen with a magnifying glass.Others cannot be seen without the helpof a microscope. That is why these arecalled microorganisms or microbes.

Microorganisms are classified intofour major groups. These groups arebacteria, fungi, protozoa and somealgae. Some of these commonmicroorganisms are shown inFigs. 2.1 - 2.4.

Viruses are also microscopic. They,however, reproduce only inside the cellsof the host organism, which may be abacterium, plant or animal. Some of theviruses are shown in Fig. 2.5. Commonailments like cold, influenza (flu) andmost coughs are caused by viruses.Serious diseases like polio and chickenpox are also caused by viruses.

Diseases like dysentery and malariaare caused by protozoans whereastyphoid and tuberculosis (TB) arebacterial diseases.

You have learnt about some ofthese microorganisms in Classes VIand VII.

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Fig. 2.4: Fungi

Spiral bacteria Rod shaped bacteria

Fig. 2.1: Bacteria

Chlamydomonas Spirogyra

Fig. 2.2 : Algae

Amoeba

Paramecium

Fig. 2.3 : Protozoa

Bread mould Penicillium Aspergillus

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2.2 Where do MicroorganismsLive?

Microorganisms may be single-celledlike bacteria, some algae and protozoa,or multicellular, such as algae and fungi.They can survive under all types ofenvironment, ranging from ice coldclimate to hot springs and deserts tomarshy lands. They are also foundinside the bodies of animals includinghumans. Some microorganisms grow onother organisms while others existfreely. Microorganisms like amoeba canlive alone, while fungi and bacteria maylive in colonies.

2.3 Microorganisms and Us

Microorganisms play an important rolein our lives. Some of them are beneficialin many ways whereas some others areharmful and cause diseases. Let us studyabout them in detail.

Microorganisms have been used forthe production of alcohol since ages.

Friendly Microorganisms

Microorganisms are used for variouspurposes. They are used in thepreparation of curd, bread and cake.

Curd contains several micro-organisms. Of these, the bacteriumLactobacillus promotes the formationof curd. It multiplies in milk and convertsit into curd. Bacteria are also involvedin the making of cheese, pickles andmany other food items. An importantingredient of rava (sooji ) idlis andbhaturas is curd. Can you guess why?

Activity 2.3

Take ½ kg flour (atta or maida),add some sugar and mix with

Fig. 2.5 : Viruses

I saw that my motheradded a little curd to warm

milk to set curd for thenext day. I wonder why!

They are also used in cleaning upof the environment. For example, theorganic wastes (vegetable peels, remainsof animals, faeces, etc.) are brokendown into harmless and usablesubstances by bacteria. Recall thatbacteria are also used in thepreparation of medicines. In agriculturethey are used to increase soil fertilityby fixing nitrogen.

Making of Curd and Bread

You have learnt in Class VII that milk isturned into curd by bacteria.

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yeast powder to the sugar solution.Keep it covered in a warm place for4-5 hours. Now smell the solution.Could you get a smell?

Louis Pasteurdiscovered

fermentationin 1857.

In 1929, AlexanderFleming was workingon a culture of disease-causing bacteria.Suddenly he found the

spores of a little green mould in one ofhis culture plates. He observed thatthe presence of mould prevented thegrowth of bacteria. In fact, it also killedmany of these bacteria. From this themould penicillin was prepared.

This is the smell of alcohol as sugarhas been converted into alcohol by yeast.This process of conversion of sugar intoalcohol is known as fermentation.

warm water. Add a small amountof yeast powder and knead tomake a soft dough. What do youobserve after two hours? Did youfind the dough rising?

Maida with Yeast Powder

Raised maida

Fig. 2.6

Yeast reproduces rapidly andproduces carbon dioxide duringrespiration. Bubbles of the gas fill thedough and increase its volume (Fig. 2.6).This is the basis of the use of yeast inthe baking industry for making breads,pastries and cakes.

Commercial Use of Microorganisms

Microorganisms are used for the largescale production of alcohol, wine andacetic acid (vinegar). Yeast is used forcommercial production of alcoholand wine. For this purpose yeast isgrown on natural sugars present ingrains like barley, wheat, rice andcrushed fruit juices, etc.

Activity 2.4

Take a 500 mL beaker filled upto ¾with water. Dissolve 2-3 teaspoonsof sugar in it. Add half a spoon of

Medicinal Use of Microorganisms

Whenever you fall ill the doctor maygive you some antibiotic tablets,capsules or injections such as ofpenicillin. The source of thesemedicines is microorganisms. Thesemedicines kill or stop the growth of thedisease-causing microorganisms.Such medicines are called antibiotics.These days a number of antibiotics arebeing produced from bacteria andfungi. Streptomycin, tetracycline anderythromycin are some of the

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commonly known antibiotics which aremade from fungi and bacteria. Theantibiotics are manufactured bygrowing specific microorganisms andare used to cure a variety of diseases.

Antibiotics are even mixed with thefeed of livestock and poultry to checkmicrobial infection in animals. They arealso used to control many plantdiseases.

It is important to remember thatantibiotics should be taken only onthe advice of a qualified doctor. Alsoyou must finish the courseprescribed by the doctor. If you takeantibiotics when not needed or inwrong doses, it may make the drugless effective when you might needit in future. Also antibiotics takenunnecessarily may kill the beneficialbacteria in the body. Antibiotics,however, are not effective againstcold and flu as these are caused byviruses.

Vaccine

we are protected from the disease-causing microbes. This is how a vaccineworks. Several diseases, includingcholera, tuberculosis, smallpox andhepatitis can be prevented byvaccination.

Edward Jennerdiscovered thevaccine for small-pox in 1798.

Why are children/infants givenvaccination?

When a disease-carrying microbe entersour body, the body produces antibodiesto fight the invader. The body alsoremembers how to fight the microbe ifit enters again. So, if dead or weakenedmicrobes are introduced in a healthybody, the body fights and killsthem by producing suitable antibodies.The antibodies remain in the body and

In your childhood, you must havebeen given injections to protect yourselfagainst several diseases. Can youprepare a list of these diseases? Youmay take help from your parents.

It is essential to protect all childrenagainst these diseases. Necessary vaccinesare available in the nearby hospitals. Youmight have seen the advertisement on T.V.and newspapers regarding protection ofchildren against polio under Pulse PolioProgram. Polio drops given to children areactually a vaccine.

A worldwide campaign againstsmallpox has finally led to itseradication from most parts of theworld.

These days vaccines are made on alarge scale from microorganisms toprotect humans and other animals fromseveral diseases.

Increasing Soil Fertility

Some bacteria and blue green algae(Fig. 2.7) are able to fix nitrogen fromthe atmosphere to enrich soil withnitrogen and increase its fertility. Thesemicrobes are commonly calledbiological nitrogen fixers.

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Cleaning the Environment

Boojho and Paheli had observed theschool gardener making manure. Alongwith their friends, they collected wastesof plants, vegetables and fruits fromnearby houses and gardens. They putthem in a pit meant for waste disposal.After some time, it decomposed and gotconverted to manure. Boojho andPaheli wanted to know how this couldhappen.

Activity 2.5

Take two pots and fill each pot halfwith soil. Mark them A and B. Putplant waste in pot A and things likepolythene bags, empty glass bottlesand broken plastic toys in pot B.Put the pots aside. Observe themafter 3-4 weeks.

Do you find any difference in thecontents of the two pots? If so, what isthe difference? You will find that plantwaste in pot A, has been decomposed.How could this happen? The plant wastehas been converted into manure by theaction of microbes. The nutrients

released in the process could be usedby the plants again.

Did you notice that in pot B, thepolythene bags, empty glasses, bottlesand broken toy parts did not undergoany such change? The microbes couldnot ‘act’ on them and convert them intomanure.

You often see large amounts of deadorganic matter in the form of decayingplants and sometimes dead animals onthe ground. You find that theydisappear after some time. This isbecause the microorganismsdecompose dead organic waste of plantsand animals converting them intosimple substances. These substancesare again used by other plants andanimals. Thus, microorganisms canbe used to degrade the harmful andsmelly substances and thereby cleanup the environment.

2.4 Harmful Microorganisms

Microorganisms are harmful in manyways. Some of the microorganismscause diseases in human beings, plantsand animals. Such disease-causing

Fig. 2.7 : The Nitrogen fixing blue green algae

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microorganisms are called pathogens.Some microorganisms spoil food,clothing and leather. Let us study moreabout their harmful activities.

Disease— causing Microorganismsin Humans

Pathogens enter our body through theair we breathe, the water we drink orthe food we eat. They can also gettransmitted by direct contact with aninfected person or carried through ananimal. Microbial diseases that canspread from an infected person to ahealthy person through air, water, foodor physical contact are calledcommunicable diseases. Examples ofsuch diseases include cholera, commoncold, chicken pox and tuberculosis.

When a person suffering fromcommon cold sneezes, fine droplets ofmoisture carrying thousands of virusesare spread in the air. The virus may enterthe body of a healthy person whilebreathing.

There are some insects and animalswhich act as carriers of disease-causing microbes. Housefly is one suchcarrier. The flies sit on the garbage andanimal excreta. Pathogens stick to theirbodies. When these flies sit on uncoveredfood they may transfer the pathogens.Whoever eats the contaminated food islikely to get sick. So, it is advisable toalways keep food covered. Avoidconsuming uncovered items of food.Another example of a carrier is thefemale Anopheles mosquito (Fig. 2.8),which carries the parasite of malaria.Female Aedes mosquito acts as carrierof dengue virus. How can we control thespread of malaria or dengue?

Then how do youprevent the spread of

communicablediseases?

We should keep ahandkerchief on the

nose and mouth whilesneezing. It is better tokeep a distance from

infected persons.

Fig. 2.8 : Female Anopheles mosquito

Why does the teacherkeep telling us notto let water collectanywhere in theneighbourhood?

All mosquitoes breed in water. Hence,one should not let water collectanywhere, in coolers, tyres, flower potetc. By keeping the surroundings cleanand dry we can prevent mosquitoes frombreeding. Try to make a list of measureswhich help to avoid the spread ofmalaria.

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Some of the common diseasesaffecting humans, their mode oftransmission and few general methodsof prevention are given in Table 2.1.

Disease— causing Microorganismsin Animals

Several microorganisms not only causediseases in humans and plants, but also

in other animals. For example, anthraxis a dangerous human and cattledisease caused by a bacterium. Footand mouth disease of cattle is causedby a virus.

Disease— causing Microorganismsin Plants

Several microorganisms causediseases in plants like wheat, rice, potato,sugarcane, orange, apple and others.The diseases reduce the yield of crops.See Table 2.2 for some such plantdiseases. They can be controlled by the

Human Disease Causative Mode of Preventive measuresMicroorganism Transmission (General)

Tuberculosis Bacteria Air

Measles Virus Air

Chicken Pox Virus Air/Contact

Polio Virus Air/Water

Cholera Bacteria Water/Food

Typhoid Bacteria Water

Hepatitis B Virus Water

Malaria Protozoa Mosquito

Robert Köch (1876)discovered the bacterium(Bacillus anthracis) whichcauses anthrax disease.

Keep the patient in completeisolation. Keep the personalbelongings of the patient awayfrom those of the others.Vaccination to be given atsuitable age.

Maintain personal hygieneand good sanitary habits.Consume properly cooked foodand boiled drinking water.Vaccination.

Drink boiled drinking water.Vaccination.

Use mosquito net andrepellents. Spray insecticidesand control breeding ofmosquitoes by not allowingwater to collect in thesurroundings.

Table 2.1: Some Common Human Diseases caused by Microorganisms

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use of certain chemicals which kill themicrobes.

Food Poisoning

Boojho was invited by his friend to aparty and he ate a variety of foodstuff.On reaching home he started vomiting.He had to be taken to a hospital. Thedoctor said that this condition could bedue to food poisoning.

make the food poisonous causingserious illness and even death. So, itis very important that we preservefood to prevent it from being spoilt.

2.5 Food Preservation

In Chapter 1, we have learnt about themethods used to preserve and store foodgrains. How do we preserve cooked foodat home? You know that bread leftunused under moist conditions isattacked by fungus. Microorganismsspoil our food. Spoiled food emits badsmell and has a bad taste and changedcolour. Is spoiling of food a chemicalreaction?

Paheli bought some mangoes but shecould not eat them for a few days. Latershe found that they were spoilt androtten. But she knows that the mangopickle her grandmother makes does notspoil for a long time. She is confused.

Plant Micro- Mode of FiguresDiseases organism Transmission

Citrus Bacteria Air

canker

Rust of Fungi Air,

wheat seeds

Yellow vein Virus Insect

mosaic of

bhindi (Okra)

Paheli wonders howfood can become a

‘poison’.

Table 2.2: Some Common Plant Diseases caused by Microorganisms

Food poisoning could be due to theconsumption of food spoilt by somemicroorganisms. Microorganismsthat grow on our food sometimesproduce toxic substances. These

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Let us study the common methodsto preserve food in our homes. We haveto prevent it from the attack ofmicroorganisms.

Chemical Method

Salts and edible oils are the commonchemicals generally used to check thegrowth of microorganisms. Thereforethey are called preservatives. We addsalt or acid preservatives to pickles toprevent the attack of microbes. Sodiumbenzoate and sodium metabisulphite arecommon preservatives. These are alsoused in the jams and squashes to checktheir spoilage.

Preservation by Common Salt

Common salt has been used to preservemeat and fish for ages. Meat and fishare covered with dry salt to check thegrowth of bacteria. Salting is also usedto preserve amla, raw mangoes,tamarind, etc.

Preservation by Sugar

Jams, jellies and squashes are preservedby sugar. Sugar reduces the moisturecontent which inhibits the growth ofbacteria which spoil food.

Preservation by Oil and Vinegar

Use of oil and vinegar prevents spoilageof pickles because bacteria cannot livein such an environment. Vegetables,fruits, fish and meat are often preservedby this method.

Heat and Cold Treatments

You must have observed your motherboiling milk before it is stored or used.Boiling kills many microorganisms.

Similarly, we keep our food in therefrigerator. Low temperature inhibitsthe growth of microbes.

Pasteurized milk can be consumedwithout boiling as it is free from harmfulmicrobes. The milk is heated to about700C for 15 to 30 seconds and thensuddenly chilled and stored. By doingso, it prevents the growth of microbes.This process was discovered by LouisPasteur. It is called pasteurization.

Storage and Packing

These days dry fruits and evenvegetables are sold in sealed air tightpackets to prevent the attack ofmicrobes.

2.6 Nitrogen Fixation

You have learnt about the bacteriumRhizobium in Classes VI and VII. It isinvolved in the fixation of nitrogen inleguminous plants (pulses). Recall thatRhizobium lives in the root nodules ofleguminous plants (Fig. 2.9), such asbeans and peas, with which it has asymbiotic relationship. Sometimesnitrogen gets fixed through the actionof lightning. But you know that theamount of nitrogen in the atmosphereremains constant. You may wonderhow? Let us understand this in the nextsection.

Why does the milk thatcomes in packets not spoil?My mother told me that themilk is ‘pasteurized’. What

is pasteurization?

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nitrogen cannot be taken directly byplants and animals. Certain bacteria andblue green algae present in the soil fixnitrogen from the atmosphere andconvert into compounds of nitrogen.Once nitrogen is converted into theseusable compounds, it can be utilised byplants from the soil through their rootsystem. Nitrogen is then used for thesynthesis of plant proteins and othercompounds. Animals feeding on plantsget these proteins and other nitrogencompounds (Fig. 2.10).

When plants and animals die,bacteria and fungi present in the soilconvert the nitrogenous wastes intonitrogenous compounds to be used byplants again. Certain other bacteriaconvert some part of them to nitrogengas which goes back into theatmosphere. As a result, the percentageof nitrogen in the atmosphere remainsmore or less constant.

Fig. 2.10 : Nitrogen cycle

Fig. 2.9 : Roots of a leguminous plant with rootnodules

2.7 Nitrogen cycle

Our atmosphere has 78% nitrogen gas.Nitrogen is one of the essentialconstituents of all living organisms aspart of proteins, chlorophyll, nucleicacids and vitamins. The atmospheric

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KEYWORDS

ALGAE

ANTIBIOTICS

ANTIBODIES

BACTERIA

CARRIER

COMMUNICABLE-

DISEASES

FERMENTATION

FUNGI

LACTOBACILLUS

MICROORGANISM

NITROGEN CYCLE

NITROGEN FIXATION

PASTEURIZATION

PATHOGEN

PRESERVATION

PROTOZOA

RHIZOBIUM

VACCINE

VIRUS

YEAST

WHAT YOU HAVE LEARNT

� Microorganisms are too small and are not

visible to the unaided eye.

� They can live in all kinds of environment,

ranging from ice cold climate to hot springs

and deserts to marshy lands.

� Microorganisms are found in air, water and

in the bodies of plants and animals.

� They may be unicellular or multicellular.

� Microorganisms include bacteria, fungi,

protozoa and some algae. Viruses, though

different from the above mentioned living

organisms, are considered microbes.

� Viruses are quite different from other

microorganisms. They reproduce only inside

the host organism; bacterium, plant or animal

cell.

� Some microorganisms are useful for

commercial production of medicines and

alcohol.

� Some microorganisms decompose the organic

waste and dead plants and animals into simple

substances and clean up the environment.

� Protozoans cause serious diseases like

dysentery and malaria.

� Some of the microorganisms grow on our food

and cause food poisoning.

� Some microorganisms reside in the root

nodules of leguminous plants. They can fix

nitrogen from air into soil and increase the

soil fertility.

� Some bacteria and blue green algae present

in the soil fix nitrogen from the atmosphere

and convert into nitrogenous compounds.

� Certain bacteria convert compounds of

nitrogen present in the soil into nitrogen gas

which is released to the atmosphere.

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Exercises

1. Fill in the blanks:

(a) Microorganisms can be seen with the help of a ____________.

(b) Blue green algae fix __________ directly from air to enhance fertilityof soil.

(c) Alcohol is produced with the help of __________.

(d) Cholera is caused by __________.

2. Tick the correct answer:

(a) Yeast is used in the production of

(i) sugar (ii) alcohol (iii) hydrochloric acid (iv) oxygen

(b) The following is an antibiotic

(i) Sodium bicarbonate (ii) Streptomycin (iii) Alcohol (iv) Yeast

(c) Carrier of malaria-causing protozoan is

(i) female Anopheles mosquito (ii) cockroach(iii) housefly (iv) butterfly

(d) The most common carrier of communicable diseases is

(i) ant (ii) housefly (iii) dragonfly (iv) spider

(e) The bread or idli dough rises because of

(i) heat (ii) grinding (iii) growth of yeast cells (iv) kneading

(f) The process of conversion of sugar into alcohol is called

(i) nitrogen fixation (ii) moulding (iii) fermentation (iv) infection

3. Match the organisms in Column I with their action inColumn II.

Column I Column II

(i) Bacteria (a) Fixing Nitrogen

(ii) Rhizobium (b) Setting of curd

(iii) Lactobacillus (c) Baking of bread

(iv) Yeast (d) Causing Malaria

(v) A protozoan (e) Causing Cholera

(vi) A Virus (f) Causing AIDS

(g) Producing antibodies

4. Can microorganisms be seen with the naked eye? If not, how can they beseen?

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5. What are the major groups of microorganisms?

6. Name the microorganisms which can fix atmospheric nitrogen in the soil.

7. Write 10 lines on the usefulness of microorganisms in our lives.

8. Write a short paragraph on the harms caused by microorganisms.

9. What are antibiotics? What precautions must be taken while takingantibiotics?E

X E

R C

I S

E S

A B

Extended Learning — Activities and Projects

1. Pull out a gram or bean plant from the field. Observe its roots. Youwill find round structures called root nodules on the roots. Draw adiagram of the root and show the root nodules.

2. Collect the labels from the bottles of jams and jellies. Write downthe list of contents printed on the labels.

3. Visit a doctor. Find out why antibiotics should not be overused.Prepare a short report.

4. Project : Requirements – 2 test tubes, marker pen, sugar, yeastpowder, 2 balloons and lime water.

Take two test tubes and mark them A and B. Clamp these tubes ina stand and fill them with water leaving some space at the top. Puttwo spoonfuls of sugar in eachof the test tubes. Add a spoonfulof yeast in test tube B. Inflate thetwo balloons incompletely. Nowtie the balloons on the mouthsof each test tube. Keep them ina warm place, away fromsunlight. Watch the setup everyday for next 3-4 days. Recordyour observations and think ofan explanation.

Now take another test tubefilled 1/4 with lime water.Remove the balloon from testtube B in such a manner thatgas inside the balloon does notescape. Fit the balloon on thetest tube and shake well.Observe and explain.

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Did You Know?

Bacteria have lived on the earth for much longer than human beings.They are such hardy organisms that they can live under extreme conditions.They have been found living in boiling mudpots and extremly cold icy waters.They have been found in lakes of caustic soda and in pools of concentratedsulphuric acid. They can survive at depths of several kilometres. Theyprobably can survive in space, too. A kind of bacterium was recoveredfrom a camera which stood on the moon for two years.There is probably noenvironment in which bacteria cannot survive.

5. For more information, visit the following websites:� www.microorganisms� www.biology4kids.com/files/micro_main html