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Revision Notes on Microorganisms: Friend and Foe

Quick Revision:

What are Microbes?
Microbes or microorganisms are tiny organisms which are so small that we cannot see them with an unaided eye. Some microorganisms can be seen with the help of a magnifying glass (such as fungus that grows on bread) while some can only be seen when you use a microscope (such as bacteria and protozoa). Microorganism were first observed by Anton Van Leeuwenhoek.
Viral, Bacterial, and Protozoan Diseases

Causes Diseases

Common Ailments: Cold, Cough, and Influenza (or Flu)
Serious Diseases: Polio, Chicken Pox, Measles etc


Typhoid and Tuberculosis (TB) etc


Dysentery and Malaria etc

Table 1: Microbial Diseases


There are four major types of microorganisms:

Types of Microorganisms

Fig 1: Types of Microorganisms

  • Bacteria: These are single-celled organisms with a rigid cell wall. They can only be seen under a microscope which enlarges images from 100 to 1000 times.

Ultrastructure of a Bacterial Cell

Fig 2: Ultrastructure of a Bacterial Cell

Types of Bacteria (based on their Shapes) and their Examples

Shape of Bacteria Example Image
Comma-shaped Bacteria

 Vibrio Cholera

Fig 3: Vibrio Cholera

Vibrio Cholerae
Spherical-shaped Bacteria (Cocci)


Fig 3: Streptococcus

Staphylococcus and Streptococcus
Rod-shaped Bacteria (Bacilli)


Fig 4: Salmonella

E.coli and Salmonella
Spiral-shaped Bacteria (Spirilla)


Fig 5: Borrelia

Treponema and Borrelia

Table 2: Shapes of Bacteria (with Examples)

  • Fungi: These are non-green plants and hence, cannot make their own food. They either live as parasites (deriving nutrition from host organisms, for example, Puccinia which causes wheat leaf rust) or grow on the organic matter (such as bread mould). 

Puccinia triticina

Fig 7: Puccinia triticina

Bread Mould

Fig 8: Bread Mould

Fungi, like mushrooms, moulds, mildews, and years, are eukaryotic. It means that they have a true nucleus.

The  main components of fungi are:

i. Hyphae: They are thread-like filaments which penetrate into substrates, secrete enzymes to break down nutrients into smaller molecules, and absorb them.

ii. Spores are a unit of sexual or asexual reproduction. They can adapt for dispersal and survival for extended periods of time in unfavourable conditions.

  • Algae : These are simple plant- like organisms which are usually aquatic in nature. They contain a cell wall and chlorophyll and can make their own food by photosynthesis.Algae can be unicellular or multicellular. Some of the common examples are diatoms, Chlamydomonas, and seaweed.

  • Protozoa: Protozoa are unicellular are organisms. Some of them live independently while others live as parasites. Many of the parasitic protozoans cause diseases in plants, domestic animals, and human beings. Example of some protozoans are  Amoeba, Plasmodium and Paramecium


Fig 9: Paramecium

How are Viruses different from other microbes?

Viruses are microscopic organisms but they are different from other microbes because they reproduce only inside the cells of the host organism (which can be a plant, animal, or a bacterium).

Fig 10: Types of Viruses

Viruses are much smaller than bacteria and come in a wide variety of shapes and sizes. A complete virus particle is known as Virion.
Virion consists of a nucleic acid surrounded by 'capsid'. Capsid is a protective coat made of protein. The subunits of this protein called 'Capsomeres'. Viruses can be seen only by an electron microscope as they are ultramicroscopic in size.

Outside the body of a living organism, they do not show any reaction and hence, can be crystallized and stored like non-living things.

Where do Microorganisms Live?

Microbes can survive in all kinds of environments – from icy cold climates to hot springs (any kind of temperature); and deserts to marshy lands (any humidity level). Some live independently while others live as parasites – inside the bodies of other organisms (including animals and human beings).

Microorganisms and Us

Some microorganisms are beneficial to us while others are harmful and cause diseases.

Microorganisms and Us

How are bacteria useful to us?

How is curd formed?Bacteria are helpful because:

  • It decomposes organic wastes (such as vegetable peels, animal remains, and faeces etc.).

  • It is used in the preparation of medicines.

  • It increases soil fertility by fixing nitrogen.

  • It is used in the setting of curd and making cheese, pickles, and other food items.

How is yeast useful to us?

What is Fermentation?Yeast is used in the baking industry (to make bread, pastries, and cakes) because it helps in fermentation. It reproduces rapidly and produces carbon dioxide during respiration. Bubbles of the carbon-dioxide gas it produces fill the spaces in the dough and increases its volume.

It is also used in the commercial production of alcohol and wine which is done by growing yeast on natural sugars present in fruit juices and grains like rice, wheat, and barley.


What are Antibiotics? What are their uses?

Antibiotics are medicines that can kill or stop the growth of disease-causing microorganisms. For Example, Penicillin.

Who discovered Pencillin and when?Antibiotics are used to:

  • Cure a variety of diseases (such as streptomycin, erythromycin, and tetracycline that are made from bacteria and fungi),

  • Cure microbial infection in animals (by mixing antibiotics with the feed of livestock and poultry), and

  • Control several plant diseases.

What precautions should be followed while taking antibiotics and why?

Antibiotics should be taken only on the advice of the doctor, and one must complete the course the doctor prescribes.

Antibiotics taken in wrong doses may make the body resistant to the drug and it may not be effective in the future. Moreover, antibiotics may also kill the beneficial bacteria in the body.

Please Note: Antibiotics cannot cure cold and flu caused by viruses.



Some Definitions to Remember:

Pathogens: Disease-causing microbes are called Pathogens.
Antibodies: Antibodies are substances our body produces to fight disease-causing microbes.
Vaccines: Vaccines are weakened or dead disease-causing microbes that are injected in our body to trigger the production of antibodies. These antibodies remain in the body for a long time to protect it against any attack of disease-causing microbes.
Vaccination: The process of protecting the body from pathogens with the help of vaccines is called Vaccination.

Name some of the diseases which can be prevented by vaccines

Who discovered vaccine for smallpox?Some of the diseases that can be prevented by vaccination are:

  • Cholera,

  • Hepatitis,

  • Smallpox, and

  • Tuberculosis.

One can get necessary vaccines from nearby hospitals.

How do microbes clean up the environment?

Microbes or microorganisms decompose organic waste and dead remains of plants and animals and convert them into simpler substances (which can again be used by other plants and animals) by the process of biodegradation. Thus, they help us in getting rid of harmful and smelly substances and clean up the environment.

Harmful Microorganisms

Some Definitions to Remember:

Communicable Diseases: These are microbial diseases (diseases caused by microbes) that spread from one infected person to a healthy person through air, water, food or physical contact,  such as cholera, chicken pox, common cold and tuberculosis.
Carriers: Insects and animals that carry disease-causing microbes and transfer them from one place to other are called carriers or vectors, such as house flies and mosquitoes.

How do houseflies transfer pathogens?

Houseflies transfer pathogens

A housefly may sit on the garbage and animal excreta and the pathogens stick to their body. When they sit on uncovered food, these pathogens get transferred to the food. When someone eats this contaminated food, he or she may fall sick.

To avoid this, we must keep the food covered.

Name the carriers of:



Female Anopheles mosquito (carries the parasite of malaria called Plasmodium)



Female Aedes mosquito (carries the dengue virus called Flavivirus)

Common Diseases and their Modes of Transmission

Common Diseases and their Modes of Transmission

How to prevent diseases that spread through air or contact?

To prevent diseases that spread through the air, keep the patient in complete isolation and keep his or her personal belongings away from others. Vaccination at the suitable age can prevent the onset of tuberculosis, chicken pox, polio, and measles.

How to prevent diseases that spread through water or food?

To prevent the spread of polio, cholera, typhoid and hepatitis A, vaccination is effective. One should also maintain personal hygiene and good sanitary habits and drink boiled drinking water.

One should also eat properly cooked food to avoid diseases like cholera.

How to prevent diseases that spread through mosquitoes?

We can stop the spreading of diseases caused by mosquitoes (such as malaria and dengue) by not allowing the mosquitoes to breed. We should keep our surroundings clean and dry, and not let water collect anywhere - in coolers, tyres, and flower pots etc.

We should also spray insecticides and use mosquito repellents and mosquito nets to protect ourselves from mosquito bites.

Common Diseases and the Microorganisms that cause them

Common Diseases and the Microorganisms that cause them

Diseases Caused by Microorganisms in Animals

BacillusAnthrax: A dangerous disease that affects human and cattle is caused by a bacterium called Bacillus Anthracis.

Foot and mouth disease in Cattle: It is caused by a virus called Foot-and-mouth-disease Virus (FMDV) or Picornavirus.

Diseases Caused by Microorganisms in Plants

Microorganisms can cause diseases in plants and reduce crop yield. Some of the plants in which they cause diseases are:

Diseases Caused by Microorganisms in Plants

The plants can be protected by using chemicals that kill these microbes.

Common Diseases in Plants caused by Microbes

Citrus Canker is caused by Bacteria and spreads through Air.

Rust of Wheat is caused by Fungi and spreads through Air or Seeds.

Yellow Vein Mosaic of Okra (Bhindi) is caused by Virus and spreads through Insects.

Food Preservation

Why do we need to preserve food?

We need to preserve food because microorganisms that grow on food can sometimes produce toxic substances which are poisonous to us. If we consume this spoilt food, we can become seriously ill or die. Hence, we need to preserve food from being spoilt.

Common Methods of preserving food are:

Common Methods of preserving food are:

Nitrogen Fixation

Nitrogen constitutes 78% of our atmosphere.

In living organisms, it is found in:

  • Proteins,

  • Nucleic Acids,

  • Chlorophyll, and

  • Vitamins.

Atmospheric nitrogen cannot be used directly by the plants and animals. It gets fixed by either lightning or natural nitrogen fixers.

Nitrogen Fixation

Nitrogen Cycle

A step-by-step explanation of Nitrogen Cycle

  • Nitrogen Fixation: Atmospheric nitrogen is converted by lightning or certain bacteria like Rhizobium, Azotobacter and blue-green algae (present in soil) into compounds usable by plants.

  • Nitrification: Ammonia conversion into nitrites by Nitrosomonas and further conversion of nitrites into nitrates by Nitrobacter. Plants take up nitrogen in form of ammonia or nitrates.

  • Assimilation: Roots of plants absorb these nitrogenous compounds from soils and plants use them to synthesize proteins and other compounds.

  • Animals feeding on plants get these proteins and nitrogen compounds.

  • Ammonification: When plants and animals die, bacteria and fungi present in the soil convert the nitrogenous wastes into compounds that can be used by plants again.

  • Denitrification:Nitrates can be converted  into nitrogen gas which is released back in the atmosphere by certain bacteria. Eg. Pseudomonas

Hence, atmospheric nitrogen remains constant.

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