The living organisms which cannot be seen with the naked eyes (unaided eyes) and can only be observed through a microscope are called microorganisms or microbes.
The branch of science that deals with the study of microorganisms is known as microbiology.
Microorganisms may be single-celled like bacteria, some algae, and protozoa, or multicellular, such as algae and fungi.
Microorganisms occur everywhere. They are found in air, water (ponds, lakes, rivers, and oceans), soil, marshlands, inside our bodies. They are present in adverse environments like polar regions, hot deserts, in cracks in the sea floor, volcanic eruption, hot spring deserts, snow, and deep oceans. They remain inactive under such adverse conditions and become active whether favourable conditions are available again.
In adverse conditions like temperature and dryness microorganisms, form a hard outer covering called a cyst around themselves. They survive by remaining inactive within the cyst until conditions are favourable again.
Classification of Microorganisms:
Microorganisms are classified into five major groups as follows:
Viruses are microscopic. They reproduce only inside the cells of the host organism, which may be a bacterium, plant or animal.
Bacteria are single-celled (unicellular), simplest organisms. As they have a cell wall, they are considered to be related more to plants than animals.
They are included in Kingdom Monera and are considered to be the first living organisms that evolved on the earth.
The place, where an organism occurs, grows and reproduce successfully is termed as its habitat.
They are present almost everywhere air, water, soil, inside and on our body.
They are the smallest microorganisms of a size of 2 micrometer long and 0.5 micrometer thick.
On the basis of shape, bacteria are classified as
Cocci (spherical shaped)
Spirilla (helical spring shaped)
Vibrio (comma shaped)
They live single. Some occur in pairs (diplococci) or in long-chain (streptococci) or in clusters (staphylococci)
The organism may be unicellular as in eubacteria and filamentous as in cyanobacteria. A cell of a bacterium has the cell wall made up of peptidoglycan and not of cellulose as in case of plants. It encloses cell membrane and cytoplasm. Cell membrane and cytoplasm is collectively called as a protoplast. Cell membrane encloses cytoplasm.
A well-developed nucleus is not found, but the nuclear material i.e., chromosomes, is found scattered in the cytoplasm. Besides nuclear material cytoplasm consists of vacuoles, granules and in very few cases green pigments.
Sometimes the cell is enclosed within a gelatin-like envelope called capsule.
They may have flagella which are a thread like structure that help in movement.
Most bacteria can not move about on their efforts. They have to depend on wind, water or contact for their movement.
Some of them can move in a liquid medium very easily with the help of whip-like structures called flagella.
Most bacteria lack chlorophyll hence they depend on other organisms for food (heterotrophic).Therefore, they either live as saprophytes (get nutrition from dead and decaying organisms) or parasites (get nourishment from the body of the living host).
Some bacteria live in symbolic relationship with the other organisms. For example, a bacterium called Escherichia coli lives in the intestine of man.
Some bacteria in spite of the absence of chlorophyll make their own food, with the help of chemicals like sulphur, hydrogen sulphide etc. The food production process is named as chemosynthesis.
Bacteria secrete powerful enzymes from their cells into surrounding material making it soluble and then can readily be absorbed.
Some bacteria are aerobic (require oxygen for respiration), while some are anaerobic (do not require oxygen for respiration).
Anaerobic bacteria get killed when exposed to oxygen.
Under favourable conditions i.e., suitable temperature, moisture, and enough food, bacteria divide rapidly. They multiply by fission.
A bacterium simply divides into two organisms. Bacteria may divide as often as once in every 20 to 40 minutes.
Fission can be of two types binary and multiple.In multiple fission, many daughter cells are formed. In binary fission bacterial the DNA duplicates and the cell grows. the two DNAs are pulled apart. The cell constricts in the middle separating the two cells. Depending upon the type of bacteria, the two daughter cells may remain attached or may separate from each other as independent cells.
Bacteria divide at very fast rate, once in every half an hour. Let us consider a single bacteria undergoing division. At the end of 24 hours there will be 224 bacteria = 281,514,871,750,656 bacteria.
Spore formation is not a method of reproduction but to survive in adverse conditions. Whenever unfavourable conditions like drying up of vegetation prevail, the bacterial cell draws its content in a spherical mass which is protected by a thick, hard protective wall. This spherical body is called spore. these spores are contained in the cell wall.
The spores can be carried out by wind or water or contact.
On obtaining favourable conditions, the spores germinate. Their protoplasm becomes active and comes out of the ruptured wall to form a new bacterium.
Very few bacteria show sexual reproduction and it is extremely simple. In this method, two bacteria of different strains but of the same species come together for conjugation. The plasmid the donor is transferred into the recipient through a hollow tubelike extension of the donor cell.