- Microorganisms. The living organisms which are very minute and cannot be seen by naked (unaided) eyes are known as microorganisms or microbes.
- Some of the microorganisms are unicellular (consisting of a single cell), for example, bacteria. Some have a well‑organised multicellular body, for example, moulds like Mucor. Microorganisms include virus, bacteria, fungi like yeast and moulds, algae like Chlamydomonas and Spirogyra, and protozoan like Amoeba and Paramecium.
- Some multicellular microorganisms, such as fungus which grows on bread, can be seen with a magnifying glass. Others such as bacteria, cannot be seen without the help of a microscope, so they are called microorganisms or microbes.
- Microorganisms are found in all kinds of environment ranging from ice cold climate to hot springs and desert to marshy lands. They are present in air, water, soil, food, clothes and other things.Some microbes are found on and inside the bodies of plants and animals.
Microorganisms are classified into five major groups:
Virus: You may be surprised to know that the viruses can be crystallized and stored like some non‑living things, such as common salt and sugar. On the other hand, they multiply like living organisms when they are within a living body.
Thus, viruses are strictly parasites and are kept on the borderline of living and non-living. Its structure can only be observed under an electron microscope (size 0.015 to 0.2 micron). A virus consists of a protein coat within which lies nucleic acid.
Different types of virusesA virus always multiplies in host cell either of a plant or of an animal. So, they cause diseases in plants and animals, for example, tobacco mosaic virus disease of tobacco plant, foot and mouth disease of cattle, viral dysentery and polio myelitis in children, common cold etc.
Some viruses use bacteria as host. These viruses are called Bacteriophages (phages—engulf).
Bacteria: Bacteria are single celled microorganisms. They are put under plant kingdom because they have rigid definite cell wall like plant cells. Bacteria can be seen under microscope which enlarges their image from a hundred to thousand times.
Observe the structure of bacteria you will notice, unlike other cells, they do not have definite nucleus, but the nuclear material is present in the central region of a bacterium (singular).
Gut BacteriaFungi: Fungi are non-green plants. They cannot synthesize their own food. They have to depend for their food on others. Most fungi take their food from dead organic matter. Some live on other organisms as parasites. Some fungi like yeast are unicellular; others like Penicillium, Mucor, Aspergillus, etc. are multicellular.
Different types of fungi
Algae: Algae are green substances floating on the surface of a pond, lake, river, stagnant water, moist soil, stones. They tend to grow on wet surfaces. Therefore, they can synthesize their own food. They are found in water or in very moist places. You will notice that some of the algae are unicellular like chlamydomonas but some algae are multicellular. Some multicellular algae are a few metres long, for example kelps, they are found in sea.
Types of green algae in purified water
Protozoa: Protozoa are unicellular animals. Some are free-living, others are parasites. Several parasitic protozoans cause diseases in human beings, domestic animals and plants. For example, Plasmodium, a protozoan, causes malaria. Entamoeba causes amoebic dysentery. Some free living protozoa eat unicellular algae and are, in turn, eaten by bigger animals in the food chain.
Types of Protozoa
- Bacteria are used in making of curd, cheese, pickles and many other food items: Specific bacteria Lactobacillus cause the formation of curd from milk. That is why a little quantity of curd (which includes Lactobacillus) is added to milk for making curd.
- They are used in cleaning up of environment.
Decomposition of waste material:
The most useful bacterial activity is decomposition of dead bodies of plants, and animals, wastes from kitchen, lavatory, sewage etc. Some bacteria decompose organic matter like faecal material, urine, fallen leaves, dead animal and plant bodies.
In the process of decaying, bacteria get their food and energy and release inorganic materials such as carbon dioxide, nitrates, phosphates, etc. These materials are reutilised by green plants in the synthesis of food.
Thus, bacteria which cause decomposition and decay of organic matter help in recycling of the matter and also cleaning up of environment.
Diagrammatic representation of decomposition cycle in terrestrial ecosystem
- In agriculture, microorganisms, such as bacteria, are used to increase soil fertility by fixing free atmospheric nitrogen.
The Rhizobium bacteria found in nodules of legume plants are useful for them, they fix atmospheric nitrogen which otherwise is not possible for plants to get. Thus they make nitrogenous fertiliser available to legume plants. In return, legume plant roots provide shelter and nutrients like water and minerals to these bacteria. Blue green algae are also able to fix free atmospheric nitrogen to increase the fertility of the soil. These microbes are commonly called biological nitrogen fixers.
Roots of legume plantsINDUSTRY
Some fungi like yeast and certain moulds are used for the production of alcohol and bakery products such as bread, cakes and biscuits. Yeast is grown in sugar solution or in moist starch (atta or maida). During its growth yeast breaks sugar or starch incompletely and produces alcohol and carbon dioxide. This process is known as fermentation.
Carbon dioxide produced in the process of fermentation cause frothing of dough which is used in preparing loafs, biscuits and cakes. For the production of alcohol, sugar solution is fermented by yeast. Later alcohol is extracted and used in medicine, different chemicals and alcoholic drinks.
⇒ Louis Pasteur discovered fermentation in 1857.
- Antibiotic Medicines: You must have used antibiotics many a times in the form of capsules, ointments or injection. Actually, some fungi like penicillium produce chemical substances called antibiotics. Antibiotics reduce bacterial growth or kill the disease causing microorganisms.
Alexander Flemming, a British scientist, was the first to discover that Penicillium mould produces an antibiotic called Penicillin. He detected penicillin just by chance, when he observed penicillin grown in his culture of bacteria (Staphylococcus), prevented the growth of bacteria in the culture. That was a great discovery.
Now, several antibiotics such as streptomycin, chloromycetin, terramycin etc. are obtained from various fungi and bacteria. Antibiotics are used to cure a large number of diseases. Antibiotics are also used to cure diseases of animals and plants. Antibiotics do affect viruses.
- Antibody: When a disease-causing microbes enter our body, our defensive mechanism produces substances to fight disease-causing microbes. These substances are called antibodies.
- Vaccine: If dead or weakened microbes for a particular disease are introduced in a healthy body, the body produces suitable antibodies. These antibodies remain in the body for a long time and protect us from that disease causing microbes. The substance which is injected to trigger the body to develop its own defence mechanism (by producing antibodies) is called vaccine.The process of giving vaccine to people is known as vaccination.
- Harmful Microorganisms
- The disease‑causing microorganisms are called Pathogens.
- Microbial disease that can spread from an infected person to healthy person through any agency are called communicable disease. Agencies that can cause spread of communicable diseases may be air, contaminated food, water and other drinks, use of utensil and cloth of infected person and physical contact.
- Flies carry the disease‑causing microbes from excreta (faecal matter, spit etc.) of the diseased person to the foods or drinks of a healthy persons. In this way, flies and some other insects like cockroaches act as carriers of microbes. These carriers of disease-causing microbes are also called vectors.
- Mosquitoes are carriers of microbes of many various diseases. For example, female Anopheles mosquito is the carrier of parasite of malaria (Plasmodium). Female Aedes mosquito acts as carrier of dengue virus.
Female Anopheles mosquito
- Microbes also cause diseases in animals, such as anthrax in cattle by bacterium Bacillus anthracis, and plants like citrus canker by bacteria in citrus fruits.
Food Poisoning: Some microorganisms that grow on our food may produce toxic substances. The toxin make the food poisonous causing serious illness and even death.
Food Preservation: Microbes such as bacteria and fungus spoil our raw or cooked food. To save the food from the attack of microorganisms we use some chemicals called preservatives. Common preservatives used are common salt, sugar, edible oils, vinegar, sodium benzoate and sodium metabisulphite.
Boiling and putting the things at low temperature also help in preserving food items.
Pasteurisation: T he process in which milk is heated to about 70ºC for 15 to 30 seconds and then suddenly chilled and stored is called pasteurisation. This process was discovered by Louis Pasteur. Pasteurised milk can be consumed without boiling as it is free from harmful microbes.
Nitrogen Cycle: Some bacteria and blue green algae present in the soil fix nitrogen gas from the atmosphere and convert it into nitrogenous compounds. On the other hand, certain bacteria convert compounds of nitrogen present in the soil into free nitrogen gas which is released to the atmosphere. Due to this nitrogen cycle, the percentage of nitrogen in the atmosphere remains more or less constant.