We know that the microorganisms are livings things, but, What are they? Are they harmful or useful? What do they produce? We are going to know about the world of the microorganisms.

• Microorganisms are unicellular or multicolour organisms, that are microscopic (you can not see them with your naked eye). Micro-organisms are classified into four major groups; Bacteria; Fungi; Protozoa; and Viruses.

There are four main types of microorganisms. They are :- bacteria, fungi, algae and protozoans.

Viruses are also considered as microorganisms. They are different from other microorganisms

because they reproduce only in the body of host organisms like bacteria, plants or animals.

E.g. :- Algae – Chlamydomonas, Spirogyra etc.

Fungi – Bread mould, Penicillium, Aspergilles etc.

Protozoans – Amoeba, Paramaecium etc.

Bacteria  singular: bacterium) constitute a large domain of prokaryotic microorganisms. Typically a fewmicrometres in length, bacteria have a number of shapes, ranging from spheres to rods and spirals. Bacteria were among the first life forms to appear on Earth, and are present in most of its habitats. Bacteria inhabit soil, water, acidic hot springs, radioactive waste,[2] and the deep portions of Earth's crust. Bacteria also live in symbiotic and parasitic relationships with plants and animals. They are also known to have flourished in manned spacecraft.[3] 

A virus is a small infectious agent that replicates only inside the living cells of other organisms. Viruses can infect all types of life forms, from animals and plants to bacteria and archaea.[1]

Since Dmitri Ivanovsky's 1892 article describing a non-bacterial pathogen infecting tobacco plants, and the discovery of the tobacco mosaic virus by Martinis Beijerinck in 1898,[2] about 5,000 viruses have been described in detail,[3] although there are millions of different types.[4]Viruses are found in almost every ecosystem on Earth and are the most abundant type of biological entity.[5][6] The study of viruses is known as virology, a sub-speciality

VIRUS

Protozoa are a diverse  group of unicellular eukaryotic organisms,[1] many of which are motile. Historically, protozoa were defined as unicellular protests with animal-like behavior, such as movement. Protozoa were regarded as the partner group of protests toprotophyta, which have plant-like behavior, e.g. photosynthesis. The term protozoan has become highly problematic due to the introduction of modern ultra structural, biochemical, and genetic techniques. Today, protozoan are usually single-celled andheterotrophic eukaryotes containing non-filamentous structures that belong to any of the major lineages of protests.

A fungus  plural: fungi[3] or funguses[4]) is any member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds (British English: moulds), as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, protists, and bacteria. One major difference is that fungal cells have cell walls that contain chitin, unlike the cell walls of plants and some protists, which contain cellulose, and unlike the cell walls of bacteria. These and other differences show that the fungi form a single group of related organisms, named theEumycota (true fungi or Eumycetes), that share a common ancestor (is a monophyletic group)

• Micro-organisms may be single-celled like Bacteria, some Viruses and some Protozoa, or Multicolor, such as Virus and Fungi.• They can survive under all types of environment, ranging from ice cold climate to hot springs and deserts to marshy lands. • Microorganisms like amoeba can live alone, while fungi and bacteria may live in colonies.

In agriculture they are used to increase soil fertility by fixing nitrogen.

Milk is turned into curd by bacteria. Of these, the bacterium Lactobacillus promotes the

formation of curd. Bacteria are also involved in the making of cheese, pickles and many other food items

Microorganisms are used for the large scale production of alcohol, wine and acetic acid (vinegar. For this purpose yeast is grown on natural sugars present in grains like barley, wheat, rice and crushed fruit juices, etc.

The process of conversion of sugar into alcohol is known as fermentation.

Microorganisms are used in the production of antibiotic tablets, capsules or injections such as of penicillin.

Such medicines are called antibiotics. These days a number of antibiotics are being produced from bacteria and fungi. Streptomycin, tetracycline and erythromycin are some of the commonly known antibiotics.

When a disease-carrying microbe enters our body, the body produces antibodies to fight the invader.

Children are given injections to protect themselves against several diseases.

Polio drops given to children are actually a vaccine.

These days vaccines are made on a large scale from microorganisms to protect humans and other animals from several diseases.

Some bacteria and blue green algae are able to fix nitrogen from the atmosphere to enrich soil with nitrogen and increase its fertility. These microbes are commonly called biological nitrogen fixers.

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.

Examples of such diseases include cholera, common cold, chicken pox and tuberculosis.

There are some insects and animals which act as carriers of disease-causing microbes.

Housefly is one such carrier. Examples of carriers are: the female Anopheles mosquito which carries the parasite of malaria; Female Aides mosquito acts as carrier of dengue virus.

Aedes mosquito

Food poisoning is a common, yet distressing and sometimes life-threatening problem for millions of people in the U.S., and throughout the world. People infected with foodborne organisms may be symptom-free or may have symptoms ranging from mild intestinal discomfort to severe dehydration and bloody diarrhea. Depending on the type of infection, people can even die as a result of food poisoning.More than 250 different diseases can cause food poisoning. Some of the most common diseases are infections caused by bacteria, such as Campylobacter, Salmonella, Shigella, E. coli O157:H7, Listeria, and botulism.

Boiling kills many microorganisms in milk. Low temperature inhibits the growth of microbes.

The milk is heated to about 70 Degrees Celsius for 15 to 30 seconds and then suddenly chilled and stored. By doing so, it prevents the growth of microbes. This process was discovered by Louis Pasteur. It is called pasteurization.

Food is spoilt by microorganisms. Spoilt food has bad smell and bad taste and causes food poisoning. Food can be preserved by protecting it from microorganisms. Food

can be preserved by different methods. They are :-i) Chemical method :- Food like pickles can be preserved by using chemicals like salts and edible oils. Jams and squashes can be preserved by using sodium benzoate, sodium metabisulphite. These chemicals are called

preservatives.ii) By using common salt :- Food items like fish, meat, amla, raw mangoes, tamarind etc can be preserved by

using common salt. It prevents the growth of microbes.iii) By using sugar :- Food items like jams, jellies,

squashes etc. can be preserved by using sugar solution. It prevents the growth of microbes.

iv) By using oil and vinegar :- Food items like pickles, vegetables, fish, meat etc. are preserved by using oil and

vinegar. It prevents the growth of microbes.v) By heat and cold treatments :- Heating food items kills

microbes. Similarly storing food items at low temperatures prevents growth of microbes.

Pasteurization :- The process of heating milk to about 70oC for 15 to 30 seconds and then suddenly

chilling it to prevent the growth of microbes is called pasteurization. This process was discovered by

Louis Pasteur.

vi) By storage and packing :- Many food items are stored in air tight containers to protect them from

microbes.

Rhizobium bacteria is involved in the fixation of nitrogen in leguminous plants (pulses). Rhizobium lives in the root nodules of leguminous plants , such as beans and peas, with which it has a symbiotic relationship.

Sometimes nitrogen gets fixed through the action of lightning.