1. Introduction to Microorganisms

You see several kinds of plants and animals around you. However, there are other living organisms around us which we normally cannot see. These are called microorganisms or microbes. For example, you might have observed that during the rainy season, moist bread gets spoilt and its surface gets covered with greyish white patches. If you observe these patches through a magnifying glass, you will see tiny, black rounded structures.

These microorganisms are extremely small and cannot be seen with the naked 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 help of a microscope.

2. Classification of Microorganisms

Microorganisms are classified into four major groups. These groups are bacteria, fungi, protozoa and algae.

A. Bacteria

Bacteria are single-celled organisms. They are found almost everywhere. They can be spiral, rod-shaped, spherical, or comma-shaped. Example: Lactobacillus (rod-shaped), Rhizobium.

B. Fungi

Fungi are non-green plants. They can be unicellular (yeast) or multicellular (moulds, mushrooms). They lack chlorophyll and obtain their food from dead and decaying organic matter. Example: Bread mould, Penicillium, Aspergillus.

C. Protozoa

Protozoa are unicellular animals. They are mostly aquatic. Example: Amoeba, Paramecium.

D. Algae

Algae are simple plant-like organisms. They contain chlorophyll and can prepare their own food. Example: Chlamydomonas, Spirogyra.

3. Where do Microorganisms Live?

Microorganisms may be single-celled like bacteria, some algae and protozoa, or multicellular, such as many algae and fungi. They live in all types of environment, ranging from ice cold climate to hot springs; and deserts to marshy lands. They are also found inside the bodies of animals including humans. Some microorganisms grow on other organisms while others exist freely.

4. Microorganisms and Us

Microorganisms play an important role in our lives. Some of them are beneficial in many ways whereas some others are harmful and cause diseases.

A. Friendly Microorganisms

• Making of Curd and Bread: Lactobacillus bacteria promotes the formation of curd. Yeast reproduces rapidly and produces carbon dioxide during respiration, which makes the dough rise for bread, pastries and cakes.
• Commercial Use: Microorganisms are used for the large scale production of alcohol, wine and acetic vinegar. Yeast is used for commercial production of alcohol and wine. This process of conversion of sugar into alcohol is known as fermentation (discovered by Louis Pasteur in 1857).
• Medicinal Use: Whenever you fall ill the doctor may give you some antibiotic tablets, capsules or injections such as of penicillin. The source of these medicines is microorganisms. These medicines kill or stop the growth of the disease-causing microorganisms. Such medicines are called antibiotics. Example: Streptomycin, Tetracycline, Erythromycin.
• Vaccine: When a disease-carrying microbe enters our body, the body produces antibodies to fight the invader. The body also remembers how to fight the microbe if it enters again. If dead or weakened microbes are introduced into a healthy body, the body fights and kills the invading bacteria by producing suitable antibodies. This is how a vaccine works. Diseases like cholera, tuberculosis, smallpox and hepatitis can be prevented by vaccination. Edward Jenner discovered the vaccine for smallpox in 1798.
• Increasing Soil Fertility: Some bacteria (Rhizobium) and blue-green algae are able to fix nitrogen from the atmosphere to enrich soil with nitrogen and increase its fertility.
• Cleaning the Environment: Microorganisms decompose dead organic waste of plants and animals converting them into simple substances. These substances are again used by other plants and animals.

B. Harmful Microorganisms

Microorganisms are harmful in many ways. Some of the microorganisms cause diseases in human beings, plants and animals. Such disease-causing microorganisms are called pathogens.

Disease-causing Microorganisms in Humans: Pathogens enter our body through the air we breathe, the water we drink or the food we eat. They can also get transmitted by direct contact with an infected person or carried by an animal. Microbial diseases that can spread from an infected person to a healthy person through air, water, food or physical contact are called communicable diseases. Examples: Cholera, common cold, chicken pox, tuberculosis.

Disease-causing Microorganisms in Animals: Anthrax is a dangerous human and cattle disease caused by a bacterium. Foot and mouth disease of cattle is caused by a virus.

Disease-causing Microorganisms in Plants: Bacteria, viruses and fungi affect plants like wheat, rice, potato, sugarcane, orange, apple and others reducing the yield of crops.

5. Food Preservation

Microorganisms spoil our food. Spoiled food emits bad smell and has a bad taste and changed colour. We use the following methods to preserve food:

• Chemical Method: Salts and edible oils are the common chemicals generally used to check the growth of microorganisms. Therefore they are called preservatives. We add salt or acid preservatives to pickles to prevent the attack of microbes. Sodium benzoate and sodium metabisulphite are common preservatives.
• Preservation by Common Salt: Common salt has been used to preserve meat and fish for ages. Salting is also used to preserve amla, raw mangoes, tamarind, etc.
• Preservation by Sugar: Jams, jellies and squashes are preserved by sugar. Sugar reduces the moisture content which inhibits the growth of bacteria which spoil food.
• Preservation by Oil and Vinegar: Use of oil and vinegar prevents spoilage of pickles because bacteria cannot live in such an environment.
• Heat and Cold Treatments: Boiling kills many microorganisms. Low temperature inhibits the growth of microbes (refrigeration). Pasteurization is a process where milk is heated to about 70°C for 15 to 30 seconds and then suddenly chilled and stored. This prevents the growth of microbes. It was discovered by Louis Pasteur.

6. Nitrogen Fixation and Nitrogen Cycle

Nitrogen Fixation: Rhizobium bacterium is involved in the fixation of nitrogen in leguminous plants (pulses). It lives in the root nodules of leguminous plants. Sometimes nitrogen gets fixed through the action of lightning.

Nitrogen Cycle: Our atmosphere has 78% nitrogen gas. Nitrogen is one of the essential constituents of all living organisms as part of proteins, chlorophyll, nucleic acids and vitamins. The atmospheric nitrogen cannot be taken directly by plants and animals. Certain bacteria and blue-green algae present in the soil fix nitrogen from the atmosphere and convert it into compounds of nitrogen. Once nitrogen is converted into these usable compounds, it can be utilised by plants. Animals feeding on plants get these proteins. When plants and animals die, bacteria and fungi present in the soil convert the nitrogenous wastes into nitrogenous compounds to be used by plants again. Certain other bacteria convert some part of them to nitrogen gas which goes back into the atmosphere. As a result, the percentage of nitrogen in the atmosphere remains more or less constant.

Living organisms that are too small to be seen with the naked eye are called microorganisms or microbes.

An instrument used to observe microorganisms.

Microorganisms are classified into bacteria, fungi, protozoa, and algae.

Single-celled microorganisms found almost everywhere. Examples: Lactobacillus, E. coli.

Non-green plants that lack chlorophyll. Examples: Bread mould, Penicillium, Yeast.

Unicellular animals mostly found in water. Examples: Amoeba, Paramecium.

Simple plant-like organisms containing chlorophyll. Examples: Spirogyra, Chlamydomonas.

Microscopic agents that reproduce only inside the cells of a host organism. Different from other microbes.

Microbes live in all environments: ice-cold, hot springs, deserts, marshy lands, and inside bodies.

A bacterium that promotes the formation of curd from milk.

A fungus used in baking (bread, cakes) and brewing (alcohol, wine). It produces CO2 causing dough to rise.

The process of conversion of sugar into alcohol by yeast.

Discovered fermentation in 1857 and pasteurization.

Medicines produced from microorganisms that kill or stop the growth of disease-causing microbes.

Discovered Penicillin in 1929 from a mould called Penicillium notatum.

A preparation of dead or weakened microbes used to stimulate antibody production and immunity.

Discovered the vaccine for smallpox in 1798.

Substances produced by the body to fight invading harmful microbes.

Microbes like Rhizobium fix nitrogen from the air to enrich soil and increase fertility.

The process of converting atmospheric nitrogen into nitrogen compounds usable by plants.

Blue-green algae and symbiotic bacteria that fix nitrogen.

Microbes that break down dead organic matter into simple substances, cleaning the environment.

Disease-causing microorganisms.

Microbial diseases that spread from an infected person to a healthy person (e.g., Cholera, Cold).

An insect or animal that transmits disease-causing microbes (e.g., Housefly, Mosquito).

Female Anopheles mosquito is the carrier of the malaria parasite (Plasmodium).

Female Aedes mosquito is the carrier of the dengue virus.

A dangerous disease affecting humans and cattle, caused by a bacterium (Bacillus anthracis).

Discovered the bacterium (Bacillus anthracis) which causes anthrax disease in 1876.

A bacterial disease of plants spread by air.

A fungal disease of wheat spread by air and seeds.

A viral disease of bhindi (okra) spread by insects.

Illness caused by consuming food contaminated by toxic substances produced by microbes.

Chemicals like sodium benzoate used to check the growth of microorganisms in food.

Heating milk to 70°C for 15-30 secs and chilling it to prevent microbial growth.

Plants like peas and beans that have Rhizobium bacteria in their root nodules.

A relationship where two organisms live together and benefit each other (e.g., Rhizobium and legumes).

Makes up 78% of the Earth's atmosphere.

A disease caused by a virus, preventable by vaccine (Polio drops).

A bacterial disease spread through air.

A bacterial disease spread through contaminated water or food.

A disease caused by a protozoan (Plasmodium), spread by mosquitos.

A viral disease spread by contact or air.

A common fungus (Rhizopus) that grows on moist bread.

A single-celled protozoan that can change its shape.

A filamentous green algae found in fresh water.

The percentage of nitrogen in the atmosphere remains constant due to the nitrogen cycle.

Process of preserving food like meat and fish using dry salt.

Bacteria in soil that convert nitrogen compounds back into free nitrogen gas.

Microorganisms can be single-celled (bacteria, some algae) or multicellular (moulds, algae).