Class 7 Science Microbial World Notes

Micro- organisms are microscopically small organisms. These include Bacteria, Viruses, Fungi, Protozoa and some Algae. Micro-organisms can flourish in all kinds of environment. Some even live inside human intestine as endoparasites. Few can reproduce independently, while some multiply only by attaching themselves to specific hosts. These are very hard and can survive under texteme unfavourable conditions by forming cysts around them and remain inactive. Under favorable conditions, they come out from cysts and start their regular activity.
Micro- organisms are useful as well as harmful and cause various diseases.
Viruses are in between living and non- living characters. In free state, viruses cannot grow.
They grow on other living cells, viruses are obligate parasites.

Bacteria:-
They are unicellular micro- organisms and their sizes vary between 0.3 and 2  m in diameter. They differ from plant and animal cell in that their nucleus is not a distinct organelle surrounded by a membrane. Bacteria are like plant cells in that they are surrounded by a rigid cell wall. Majority of them posses cilia and flagella for movement. Some are even photosynthetic.

Economic Importance of Bacteria
- Some bacteria increase soil fertility by fixing free atmospheric nitrogen into nitrogenous compounds, e.g., Clostridium, Azotobacter and Rhizobium.
- Some decompose dead animal and plant materials to form nutrients in the soil.
- Acetobacter aceti bacteria produce vinegar from sugar solution.
- Certain bacteria are present in the intestines of cattles and are helpful in the digestion of cellulose.
- Some bacteria help in tanning of leather.
- Some medicines, e.g., streptomycin is produced by Streptomyces grisieus bacteria.
- Some human diseases are caused by bacteria, e.g., Tuberculosis, Typhoid, Diptheria etc.
- Some bacteria help in the disposal of sewage by converting organic faecal matter into manure and humus.

Fungi:
Fungi are simple plants that reproduce mostly by means of spores and which lack chlorophyll, hence are either saprophytes or parasites. Fungi include unicellular as well as multicellular organisms and reproduce asexually and sexually. Moulds and yeast are major representative of fungi.

Economic Importance of Fungi :
- Because of high content of vitamin B 1, B ₁₂ and C, yeast are used for preparation of digestive tablets.
- Bread and cakes are made by the action of yeast.
- Morchella, Agaricus and Lycoperdon are most commnon edible fungi.
- Wines and beers are produced by the action of fungi.
- Few very important medicines, such as penicillin is made by the fungi- Penicillium notatum and penicillium chrysogenum.
- Some fungi cause disease in plant, while others cause athletes foot and ring worm in humans.
- Few fungi also cause diseases in animals.
- Amanitia is very poisonous fungus to man.LSD drug is extracted from the fungi- Claviceps.

Algae :
Algae is a large and extremely diverse group of plants including some prokaryotic forms,although the majority are eukaryotic. The plant body may be unicellular or multicellular.
They are mostly aquatic. Most of them contain chlorophyll and perform photosynthesis.
Their sizes range from 1.0 micron (microscopic unicellular forms) to several meters long sea weeds.

Economic Importance of Algae
- Some algae are utilized as food in Japan and China, e.g., Porphyra.
- Chlorella alga contains proteins and vitamins A and D in abundant quantity.
- Gel i d iu m an d Gr acil ar ia algae p r odu ce Agaragar, often used in the manufacture of ice creams.
- Nostoc and Anabaena algae convert atomospheric nitrogen into nitrogen compounds which are absorbed by higher plants.
- Kelp is a brown algae and is rich source of iodine and potassium.
- Some algae cause water blooms in water reservoirs and emit bad smell.
- Carrageenin extract, which is another product of algae origin, acts as a blood coagulant.
- An effective vermifuge is obtained from the extracts of Codium, Alsidium, Digenea and Durvillen.

Protozoa :
Protozoans are unicellular (single cell) animals and are microscopic. These animals do not posses a cell wall. They have variable structure, such as amoeba does not posses a definite structure and its pseudopodia can change its shape. Paramecium has a slipper like shape and the body is covered by numerous cilia.

Economic Importance of Protozoa
- Some protozoans feed on bacteria and fungi, which are decomposers of organic matter, thus finally degrade the wastes.
- Some are found in intenstine of termites and help in the digestion of cellulose.
- Serveral protozoans cause human diseases.
- Some animals feed on diatoms and other protozoans

Viruses :
Viruses are submicroscopic parasitic micro organisms often called microbes. They comprise of a protein or protein / lipid sheath containing nucleic acid (DNA / RNA). Viruses are inert outside the living host cell and can multiply only in host cells. They are in between non- living and living as they possess both the characters. Tobacco plant and foot and mouth disease (FMDV) in cattles. Viruses parasitize bacteria are called Bacteriophage.

Micro- organism and Diseases :
Disease causing micro- organisms are called Pathogenic micro- organisms. Microbial diseases which can spread through air, water, food or physical contact are known as Communicable Diseases. For example, common cold, cholera, and chicken pox etc. Sometimes insects act as carriers of pathogenic micro-organisms, e.g., housefly spreads cholera. Anopheles mosquito spreads malarial fever. Carriers of specific micro-organisms are called Vectors. Thus, Anopheles and Aedes are mosquito vectors causing malaria and yellow fever respectively.

List of Bacterial diseases
1. Cholera
2. Typhoid
3. Tuberculosis
4. Diphtheria
5. Pneumonia
6 .Tetanus
7. Leprosy

List of Viral (Virus) diseases
1. Small pox
2. Common cold
3. Chicken pox
4. Influenza
5. Measles
6. Mumps
7. Poliomyelitis
8. Rabies
9. AIDS

List of diseases caused by Protozoans
1. Malaria
2. Amoebic dysentery
3. Sleeping sickness
4. Kala-azar

List of diseases caused by Fungi
1. Ringworm (tinea)
2. Athlete’s food

Uses of Micro-organisms :

1. Medicinal uses of micro-organisms
It has been scientifically found that the lethal effect of one micro- organism can check the growth of another micro- organism. Thus substances produced by micro- organisms that kill or prevent growth of other micro- organisms are called Antibiotics. This property of microorganism is made use of in the treatment of bacterial and fungal infections. Alexander Fleming first noted this property of micro- organisms in 1928. The production of first antibiotic drug for medicinal use was Penicillin by Chain and Florey. Other well known antibiotics are Streptomycin, Tetracycline, Gentamycin etc. Some of them are made from bacteria and others from fungi.

2. Vaccination:-
Vaccination is the method of inducing immunity to infectious disease caused by bacteria and viruses. Starting from the information that farm-workers, who had accidently acquired cowpox by milking infected cows, were resistant to smallpox. Jenner inoculated cowpox material into non- immune persons who then showed resistance to smallpox. Later Pasteur extended this work to cholera, human Anthrax and Rabies. Pasteur established that smallpox and few other diseases were caused by viruses. Thus, Pasteur started the practice of injecting a small amount of pathogens into the body to develop resistance against a specific disease, and applied the same to other diseases, such as cholera and tuberculosis and called this practice Vaccination. The term vaccination became general for all methods of inducing immunity by inoculation of products of the infectious organism. Antitoxins were soon developed in which specific immunity to disease toxins was induced. One of the body’s defence mechanisms is called antibodies (proteins) which are made by specialized white cells to counter foreign proteins known as antigens (pathogens). Vaccination leads to the formation of antibodies and the ability to produce large quantities rapidly at the time of disease infection. A specific antibody is made for each antigen. After vaccination, the injected viral cells (pathogens) are encountered by the specialized white blood corpuscles and kill the pathogens by producing antibodies. These antibodies once formed remain for some time in the body and fight any infection by that pathogen in future. This type of resistance created in the body is called immunity and the process of vaccination against diseases is called immunization.

3. Commercial uses:-
Certain substances are made by the help of micro- organisms which are very useful to us such as curd, bread, penicillin, alcohol and organic acids. Yeasts are used for the commercial production of wines and alcohols from carbohydrates. Yeasts (fungi) break down the carbohydrate molecules by their enzymes and produce alcohol by chemical reaction. Certain bacteria break down carbohydrates (potato and corn) to produce organic acids, e.g. acetic acid and lactic acid. Certain leguminous plants (pulse) have nitrogen fixing bacteria in their root nodules which fix atmospheric nitrogen into soluble nitro-genous compounds (nitrates) . Plants fulfil their nitrogenous need in the form of nitrates and for proteins. These nitrogen fixing nodule bacteria are mostly Rhizobium. All pulses are mostly rich in proteins.

Storage and Preservation:
Many micro- organisms grow and spoil useful materials, such as wood, leather, textile,paper and grains. Thus these substances are well kept and treated to avoid the attack of microorganisms.Some methods of preservation are described below,

Chemical method:-
Useful substances are often treated with chemicals to inhibit the growth of harmful microorganisms. Insecticides and fungicides are often used to protect the substances from the attack of these microbes. It is observed that food and pickles are often spoiled by the growth of micro- organisms. To check the spoilage of edible substances, certain preservatives are used. Acid preservatives are mostly used to check the activities of micro-organisms. These acids check the enzymes action of microbes. Salt is also used as preservatives. Salt draw water from substances and thus check the growth of microbes. Salts and acetic acid are used as preservatives in pickles, while sodium benzoate or sodium metasulphate is used in jams and squashes for preservation.

Heat and cold treatments :
Water is often boiled and then cooled before drinking. Thus the harmful microbes present are eliminated. Similarly milk is also boiled before keeping it for some duration. Boiling is a way of killing microbes although the spores of micro- organisms are not destroyed by boiling. Edible substances are kept in refrigerators. Both cold and hot treatments are used to keep things for longer duration. By both ways the enzymes present in micro- organisms are inactivated, thus microbes are unable to spoil the food. Pasteurization of milk is a process of sterilizing milk for improving storage qualities. This is done by keeping the mik at about 65°C for 30 minutes and then suddenlly chilling it. Disease producing bacteria, particularly those causing tuberculosis, are thus destroyed with a minimum effect on the flavour.

Isolation :
Generally materials are stored in closed, sometimes airtight containers to check the growth of harmful microbes. After harvestation, grains are kept in large containers called ‘silos’ he chances of microbial growth are reduced in this way but cannot be checked entirely. Best way of storing materials, the containers must be sterilized by different methods. Microbes often spoil edible things by breaking their molecules and producing amines; which have foul smell. Some microbes completely spoil the food by producing poisonous toxins. These toxins produce illness and even sometimes prove fatal. Therefore, it is quite neccessary to store food properly.

Points to Remember:-
- Blue green algae or cyanobacteia are prokaryotic and autotrophic organisms.
Ex. Anabaena, Nostoc, Oscillatoria, Spirulina
- Blue green algae fix atmospheric nitrogen and increase soil fertility. They have specialized cells to fix nitrogen, such cell named heterocysts.
- Diatoms are unicellular eukaryotic organisms. Their cell wall contain cellulose but is largely made up of silica. This silicated cell wall is called shell or frustule.
- When diatoms die, their covers (cell wall) settle at the bottom of the oceans or lake. Over the year these form diatomaceous earth or diatomite or keiselgurh.
- Diatoms are also known as golden aglae or jewels of sea.
- The wine (brewery) and bread (bakery) industries are based on fermentation activity of yeast.
- Yeast is used in the production of riboflavin (vitamin-B 1)
- Mycoplasmas are also known as cell- wall less bacteria.
- The first polio vaccine was prepared by Jonas Salk (1953) by killing polio virus with formaldehyde.
- Sabin prepared an oral polio vaccine (OPV). It consists of living but attenuated polio virus.
- Mycoderma aceti and Acetobacter aceti used for large scale production of acetic acid (vinegar).
- Mycorrhiza are the symbiotic associations of fungal hyphae and roots of higher plants.
- Lichens are the symbiotic associations of fungi and algae.
- Bacillus, Pseudomonas and Proteus are used for the treatmemt of sewage.
- Christian Gram developed a technique to stain bacteria, called Gram staining.
- Gram positive bacteria retain the stain while the Gram negative ones do not. The two types of bacteria primarily differ in the structure and composition of cell walls.
- Gram positive bacteria e.g. Salmonella, Spirillum, Escherichia coli.
- Gram negative bacteria e.g. Staphylococcus, Streptococcus.
- Kelps (Sea weeds algae) are the sources of iodine and potassium.
- An antibiotic chlorellin is obtained from the algae chlorella.
- Claviceps purpurae: A narcotic drug- LSD (Lysergic acid Diethylamide) is obtained from it.
- Neurospora- red or pink mold known as Drosophila of Plant Kingdom.
- Bioremediation- The use of microorganisms to clean up the environment or to remove pollutants from the environment.
- Plant diseases caused by bacteria Citrus canker, fire blight of pear, Leaf blight of rice, Crown gall, Black root of cabbage, Red stripe of sugarcane, Wilt of potato.
- Plant disease caused by viruses. Yellow vein mosaic of ladyfinger, Tobbaco mosaic, Bunchy top of banana, Leaf roll of potato.
- Plant diseases caused by fungi, Late blight of potato caused by Phytophthora infestans.
This disease is known as Famine of Ireland (1845).

Question: Which of the following is used as culture medium of bacterial growth?
a) Iodine
b) Agar-agar
c) Sugar
d) Alcohol
Answer: b

Question: LSD drug is extracted from a –
a) Algae
b) Fungi
c) Virus
d) Bacteria
Answer: b

Question: Bacteria are –
a) Aerobic only
b) Non-aerobic only
c) Aerobic and non-aerobic both
d) None of the options
Answer: c

Question: Streptomycin is produced by –
a) Fungi
b) Yeast
c) Bacteria
d) Virus
Answer: c

Question: Mushroom is a type of –
a) Yeast
b) Algae
c) Mould
d) Protozoa
Answer: c

Question: Micro-organisms can be seen by the aid of –
a) Microscope
b) Hand lens
c) Naked eye
d) None
Answer: a

Question: Bacteria generally multiply by –
a) Binary fission
b) Multiple fission
c) Longitudinal fission
d) All of the options
Answer: a

Question: Antibiotic penicillin is obtained from –
a) Bacteria
b) Moulds
c) Yeasts
d) Algae
Answer: b

Question: Which of the following is used in the preparation of digestive tablets?
a) Yeast
b) Mould
c) Algae
d) None of these
Answer: a

Question: Typhoid is caused by –
a) Fungi
b) Virus
c) Bacteria
d) Protozoa
Answer: c

Question: Yeasts cause dough in maida, due to the formation of –
a) Oxygen
b) Nitrogen
c) Carbon dioxide
d) Carbon monoxide
Answer: c

Question: The bacterial action in milk is –
a) Alkaline
b) Acidic
c) Neutral
d) Saline
Answer: b

Question: Which of the following beneficial bacteria is found in the colon of man?
a) Lactobacillus
b) Pseudomonas
c) Clostridium
d) Escherichia coli
Answer: d

Question: Edible fungi is –
a) Morchella
b) Agaricus
c) Lycoperdon
d) All the above
Answer: d

Question: Which of the following grows on wet bread?
a) Yeast
b) Mould
c) Both A and B
d) None
Answer: b

Question: Which bacteria produces vinegar in sugar solutions?
a) Escherichia
b) Azotobacter
c) Acetobacter aceti
d) Rhizobium
Answer: c

Question: Viruses are –
a) Living organisms
b) Non-living organisms
c) In between living and non-living organisms
d) None of the options
Answer: c

Question: Yeast helps in the production of –
a) Oxygen
b) Glucose
c) Alcohol
d) Salts
Answer: c

Question: Bacteria is regarded as plant cell, because –
a) It has clear nucleus
b) It has rigid cell wall
c) It reproduces by binary fission
d) It is parasitic
Answer: b

Question: Which of the following is present in the intestines of termites to digest cellulose?
a) Yeasts
b) Bacteria
c) Protozoans
d) Algae
Answer: b

Question: China grass is –
a) Fungi
b) Algae
c) Both A and B
d) None of these
Answer: b

Question: The host for malaria causing protozoan is –
a) Anopheles mosquito
b) Dog
c) Earthworm
d) Housefly
Answer: a

Question: Most algae can make their own food because they contain –
a) Clear nucleus
b) Pigments
c) Chlorophyll
d) None of the options
Answer: c

Question: Leprosy is caused by –
a) Bacteria
b) Fungi
c) Protozoan
d) Virus
Answer: a

Question: Kelp is –
a) Red algae
b) Blue-green algae
c) Brown algae
d) All the above
Answer: c

Question: Which of the following protozoan possesses definite shape?
a) Amoeba
b) Paramecium
c) Both A and B
d) None of these
Answer: b

Question: Most fungi cannot make their own food due to lack of –
a) Chlorophyll
b) Pigments
c) Clear nucleus
d) All the above
Answer: a

Question: The red colour of red sea is due to –
a) Brown algae
b) Green algae
c) Red algae
d) All the above
Answer: c

Question: AIDS is a –
a) Bacterial disease
b) Viral disease
c) Protozoan disease
d) Fungal disease
Answer: b

Question: Rich source of iodine is –
a) Red algae
b) Green algae
c) Blue-green algae
d) Brown algae
Answer: d

Question: Which of the following algae produce Agar-agar?
a) Gelidium
b) Gracilaria
c) Both A and B
d) None of these
Answer: c

Question: Salting the food helps to preserve it by –
a) Extracting the water from it
b) Increasing alkalinity
c) Lowering the temperature
d) None of the options
Answer: a

Question: Bacteriophage virus –
a) Parasitize man
b) Parasitize bacteria
c) Parasitize cattle
d) Parasitize dogs
Answer: b

Question: Which of the following is an antibiotic?
a) Insulin
b) Streptomycin
c) Dextrose
d) Alcohol
Answer: b

Question: Pasteurization of milk destroys its –
a) Vitamins
b) Proteins contents
c) Fat contents
d) Pathogenic bacteria
Answer: d

Question: Malaria is caused by a –
a) Virus
b) Bacteria
c) Protozoa
d) Fungus
Answer: c

Question: Athletes’ foot and ringworm are caused by –
a) Bacteria
b) Fungi
c) Protozoa
d) None
Answer: b

Question: Disease causing micro-organisms are known as –
a) Microbes
b) Pathogens
c) Antibody
d) All the above
Answer: b

Question: The most common carrier of communicable diseases is –
a) Mosquito
b) Cow
c) Dog
d) Housefly
Answer: d

Question: Viruses are inert –
a) Outside host cell
b) Inside host cell
c) Both A and B
d) None of the options
Answer: a

Question: Pickles are mostly preserved by adding –
a) Sodium benzoate
b) Acetic acid
c) Hydrochloric acid
d) Sulphuric acid
Answer: b

Question: Plants take nitrogen in the form of –
a) Nitrates
b) Nitrites
c) Ammonia
d) All the above
Answer: a

Question: The process of immunization is known as –
a) Pathogenesis
b) Sterilization
c) Immunity
d) All the above
Answer: c

Question: The most ancient group of organisms on the earth is –
a) Eubacteria
b) Cyanobacteria
c) Archaebacteria
d) PPLO
Answer: c

Question: Jams and squashes are preserved by adding –
a) Acetic acid
b) Sodium benzoate
c) Hydrochloric acid
d) Sulphuric acid
Answer: b

Question: The similarity between bacterium and cyanobacterium is in the presence of –
a) Flagella
b) Nucleoid
c) Size
d) 80S ribosomes
Answer: b

Question: Atmospheric nitrogen is fixed in leguminous roots by –
a) Chlorophyll
b) Carbon dioxide
c) Bacteria
d) All the above
Answer: c

Question: Rocky mountain spotted fever is caused by –
a) Rickettsia
b) Fungi
c) Bacteria
d) PPLO
Answer: a

Question: In blue green algae, the structure specialized for nitrogen fixation is –
a) Thylakoid
b) Hormogonia
c) Heterocyst
d) Endospore
Answer: c

Question: Which one of the following statements is correct?
a) Legumes fix nitrogen through specialised bacteria that lives on their leaves.
b) Legumes are incapable of fixing nitrogen
c) Legumes fix nitrogen only through specialised bacteria that lives in their roots
d) Legumes fix nitrogen independently of the specialised bacteria that live in their roots
Answer: c