Thursday, April 29, 2021


You have seen several kinds of plants and animals. 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. Observe these patches through a magnifying glass. You will see tiny, black rounded structures. Do you know what these structures are and where do these come from?

Do you find tiny organisms moving around: These observations show that water and soil are full of tiny organisms, though not all of them fall into the category of microbes. These microorganisms or microbes are so small in size that they cannot be seen with the unaided 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. That is why these are called microorganisms or microbes.

What are Microbes or microorganisms?
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 Microorganisms are classified into four major groups. These groups are bacteria, fungi, protozoa and some algae.

Bacteria:Bacteria are small single-celled organisms. Bacteria are found almost everywhere on Earth and are vital to the planet's ecosystems. Some species can live under extreme conditions of temperature and pressure. The human body is full of bacteria, and in fact is estimated to contain more bacterial cells than human cells. Bacteria are one of the oldest life forms. They are unicellular i.e. they are made up of one cell. They appear in a variety of shapes & sizes (Spherical, elongated, spiral etc. They inhabit soil, water, radioactive waste & the deep portions of Earth's crust. In fact, they also live in plant /animal bodies.

They live in colonies. Some of them are autotrophic (prepare their own food), while others are heterotrophic (depend on others for their food). bacteria are single-celled microorganisms with prokaryotic cells, which are single cells that do not have organelles or a true nucleus and are less complex than eukaryotic cells. Bacteria with a capital B refers to the domain Bacteria, one of the three domains of life. The other two domains of life are Archaea, members of which are also single-celled organisms with prokaryotic cells, and Eukaryota. Bacteria are extremely numerous, and the total biomass of bacteria on Earth is more than all plants and animals combined.

Evolution of Bacteria: Bacteria first arose on Earth approximately 4 billion years ago, and they were the first forms of life on Earth. For 3 billion years, bacteria and archaea were the most prevalent kinds of organisms on Earth. Multicellular eukaryotes did not appear until around 1.6-2 billion years ago. Eukaryotic cells, which make up all protists, fungi, animals, and plants, also contain what was once bacteria; it is thought that the mitochondria in eukaryotes, which produce energy through cellular respiration, and chloroplasts in plants and algae, which produce energy through photosynthesis, both evolved from bacteria that got taken up into cells in an endosymbiotic (mutually benefiting) relationship that became permanent over time.

Structure of Bacteria
The structure of bacteria is known for its simple body design. Bacteria are single-celled microorganisms with the absence of the nucleus and other cell organelles; hence, they are classified as prokaryotic organisms. They are also very versatile organisms, surviving in extremely inhospitable conditions. Such organisms are called extremophiles. Extremophiles are further categorized into various types based on the types of environments they inhabit: Thermophiles, Acidophiles, Alkaliphiles, Osmophiles, Barophiles, Cryophiles

Another fascinating feature of bacteria is their protective cell wall, which is made up of a special protein called peptidoglycan. This particular protein isn’t found anywhere else in nature except in the cell walls of bacteria. But few of them are devoid of this cell wall, and others have a third protection layer called capsule. On the outer layer, one or more flagella or pili is attached, and it functions as a locomotory organ. Pili can also help certain bacteria to attach themselves to the host’s cells. They do not contain any cell organelle as in animal or plant cell except for ribosomes. Ribosomes are the sites of protein synthesis. In addition to this DNA, they have an extra circular DNA called plasmid. These plasmids make some strains of bacteria resistant to antibiotics.

Bacteria Characteristics
Bacteria are single-celled organisms. They lack organelles such as chloroplasts and mitochondria, and they do not have the true nucleus found in eukaryotic cells. Instead, their DNA, a double strand that is continuous and circular, is located in a nucleoid. Together, the cell membrane and cell wall are referred to as the cell envelope. Many bacteria need a cell wall in order to survive.

Reproduction occurs through binary fission which is the splitting of a bacterial cell after it reaches a certain size. Bacteria reproduce asexually, so the two daughter cells that result from binary fission have the same DNA as the parent cell. However, some bacteria can also exchange genetic material among one another in a process known as horizontal gene transfer. This method involves two already existing bacteria; it is not a form of transmission from parent to child.

Classification of Bacteria
Bacteria can be classified into various categories based on their features and characteristics. The classification of bacteria is mainly based on the following:
Shape: Composition of the cell wall, Mode of respiration Mode of nutrition. Classification of bacteria based on the Composition of the Cell Wall

Bacteria Shapes
Bacteria come in a myriad of shapes. The three main shapes of bacteria are coccus, spiral, and bacillus.
Cocci are bacteria that are spherical or ovoid in shape. Some cocci remain attached after binary fission, even though separate cells have been formed. For example, diplococci are cocci in pairs, streptococci are chains, and staphylococci are clusters of multiple cocci. Tetrads are square arrangements of four cocci, while sarcinae are cubes of eight cocci.
Spiral bacteria are, as the name suggests, spiral-shaped. Spirillums are thick, tough spirals. Spirochetes are spirals that are thin and flexible. Vibrios are comma-shaped rods with a small twist.
Bacilli are rod-shaped bacteria. Like cocci, bacilli can be solitary or arranged together. Diplobacilli are two bacilli arranged next to each other, and streptobacilli are chains of bacilli. Bacteria can also be other shapes such as filamentous (long and thin), square, star-shaped, and stalked. This diagram depicts the numerous shapes of bacteria.

Types of Bacteria: The cell wall also makes Gram staining possible. Gram staining is a method of staining bacteria involving crystal violet dye, iodine, and the counterstain safranin. Many bacteria can be classified into one of two types: gram-positive, which show the stain and appear violet in color under a microscope, and gram-negative, which only show the counterstain, and appear red. Gram-positive bacteria appear violet because they have thick cell walls that trap the crystal violet-iodine complex. The thin cell walls of gram-negative bacteria cannot hold the violet-iodine complex, but they can hold safranin. This makes gram-negative bacteria appear red under Gram staining. Gram staining is used for general identification of bacteria or to detect the presence of certain bacteria; it cannot be used to identify bacteria in any specific way, such as at a species level. Examples of gram-positive bacteria include the genera Listeria, Streptococcus, and Bacillus, while gram-negative bacteria include Proteobacteria, green sulfur bacteria, and cyanobacteria.
Examples of Bacteria
Escherichia coli is one example of a common species of bacteria. It is bacillus-shaped and found naturally in the intestines of many animals including humans, where it produces vitamin K and b-complex vitamins. E. coli is also often used in laboratory research since it reproduces quickly and is hardy. Most strains of E. coli are harmless to humans, but some can cause infection. E. coli infection can result in gastrointestinal problems like diarrhea, and in more severe cases, bacterial meningitis or pneumonia can occur.

Lactobacillus acidophilus is another bacillus-shaped species of bacteria naturally found in places like the intestines and vagina, where it protects against harmful bacteria. It is a probiotic, a bacterium found in certain foods like yogurt and other fermented foods that is consumed in order to help absorb nutrients and replenish the body’s supply of “good” bacteria. It can also be consumed in small amounts by people with lactose intolerance in order to help them consume lactose.

Reproduction in Bacteria: Bacteria follow an asexual mode of reproduction, called binary fission. A single bacterium divides into two daughter cells. These are identical to the parent cell as well as to each other. Replication of DNA within parent bacterium marks the beginning of the fission. Eventually, cell elongates to form two daughter cells. The rate and timing of reproduction depend upon the conditions like temperature and availability of nutrients. When there is a favourable condition, E.coli or Escherichia coli produces about 2 million bacteria every 7 hours. Bacterial reproduction is strictly asexual, but it can undergo sexual reproduction in very rare cases. Genetic recombination in bacteria has the potential to occur through conjugation, transformation, or transduction. In such cases, the bacteria may become resistant to antibiotics since there is variation in the genetic material (as opposed to asexual reproduction where the same genetic material is present in generations)

Prokaryote-An organism that has a simple prokaryotic cell bacteria and archaea are prokaryotes.
Binary fission – The method by which bacteria reproduce asexually through dividing.
Probiotic – A bacterium that helps maintain a healthy digestive tract when consumed. Horizontal gene transfer – Gene transfer between two organisms that are not parent and offspring.

Useful Bacteria
Not all bacteria are harmful to humans. There are some bacteria which are beneficial in different ways. Listed below are few benefits of bacteria: Convert milk into curd – Lactobacillus or lactic acid bacteria Ferment food products – Streptococcus and Bacillus Help in digestion and improving the body’s immunity system – Actinobacteria, Bacteroidetes, Firmicutes, Proteobacteria Production of antibiotics, which is used in the treatment and prevention of bacterial infections – Soil bacteria

Harmful Bacteria
There are bacteria that can cause a multitude of illnesses. They are responsible for many of the infectious diseases like pneumonia, tuberculosis, diphtheria, syphilis, tooth decay. Their effects can be rectified by taking antibiotics and prescribed medication.

However, precaution is much more effective. Most of these disease-causing bacteria can be eliminated by sterilizing or disinfecting exposed surfaces, instruments, tools and other utilities. These methods include- application of heat, disinfectants, UV radiations, pasteurization, boiling, etc.

You would have seen fungi as a white layer on bread when you leave it outside at room temperature for quite a few days. Fungi get favourable conditions & therefore they grow. But, the same bread when kept inside a refrigerator remains fresh for a longer period of time. Fungi are multicellular e. they are made up of multiple cells They are Heterotrophic (Depend on others for their food). They live in colonies and prefer warm & moist places to grow. They are immobile. Some common examples of fungi are: Yeast, Mushroom, Molds. 1. Organisms of kingdom Fungi have the capacity to become multi-cellular, at some stage of their lives. Some are uni-cellular.

2. They are Eukaryotic.They have cell walls made up of tough complex sugar called “chitin”.Some of these organisms use decaying organic material as food, so they are called “saprotrophs”. Some require a living protoplasm of a host organism for food, they are called “parasites”. Examples: Yeasts (unicellular), Moulds, and Mushrooms. Some fungal species live in permanent, mutually dependent relationship with blue-green algae (cyanobacteria). This relationship is called as “symbiosis”, and the symbiotic life forms are called “litchens”. We can see litchens as the slow growing, multi-coloured patches on the barks of the trees.

Characteristic features of fungi
Fungi are unicellular or multicellular and filamentous. All fungi are made up of fine threads called hyphae. Fungi are plant like organism. They have cell wall.They lack chlorophyll and that’s why do not perform the process of photosynthesis. Store food in the form of glycogen instead of starch. Examples of fungi are yeast, moulds such as Penicillium, bread mould and aspergillus, mushrooms etc. Study of fungi is called mycology.

Habitat of fungi: Fungi are present everywhere and in any habitat. They are found in deserts, cold regions, sea waters, fresh water, in soil, on plants and animals, human skin etc. They grow best in dark warm moist places.

Mode of nutrition in fungi
Fungi look like plant but they cannot make their own food. Fungi are heterotrophic i.e. they depend upon another organism for their food. Fungi generally follow saprophytic or parasitic mode of nutrition. Saprophytic fungi obtain their nutrients from non-living organic matter, usually dead and decaying plants and animals matter by absorbing soluble organic compounds. Parasitic fungi live in or on other organism and get their nutrients from their host. It generally causes disease or illness to the host.

Mode of reproduction in fungi
The Reproduction of fungi can be either sexual or Asexual. In asexual mode of reproduction fungi reproduce by Fragmentation, Budding. Produces spores. The majority of fungi reproduces asexually.

Advantages of fungi in day to day life and it’s usage
Fungi are decomposers, they are good for the soil and cleaning up the environment. Many fungi are used for the production of antibiotics. For example Penicillium is a fungus which produces antibiotic call penicillin. All mushrooms, including the ones we eat, a large fungi. Yeast is another useful fungus which is used in many fermentation processes.

Disadvantages of fungi
Many saprophytic fungi spoil our food, clothes, leather articles etc. example you me have noticed the cottony growth on leather or stale bread. Eating such food can make you ill. Many fungus can cause human diseases. Some fungal species produces toxin that affects plants animals and humans.

They are mostly Unicellular i.e. made of one cell. protozoa : a single-celled organism (as an amoeba or paramecium) that is a protist and is capable of movement. protozoan. Organisms of kingdom Protista are Unicellular. They are Eukaryotic. Some of these organisms use appendages, such as hair like silia or whip like flagella for locomotion (moving around). The mode of nutrition can be either by synthesising their own food i.e. autotrophic or getting it from the environment i.e. heterotrophic. Some of them are autotrophic (prepare their own food), while others are heterotrophic (depend on others for their food). They prefer moist & aquatic habitats. They can live singly, unlike bacteria & fungi. Some common examples of protozoa are: Amoeba, Paramecium, TrypanosomaExamples:- Unicellular algae, Diatoms, Protozoans (amoeba, paramoecium, euglena etc.)

Protozoa are single-celled microscopic organism. Protozoa are heterotrophs i.e they don’t produce their own food. They generally feed on organic matter such as other microorganisms or organic tissues and debris. Protozoa are heterotrophic microorganisms which do not fully plant or animals.

Why protozoa were considered animals?
Protozoa are unicellular microorganisms which do not fully dissemble plant or animals. The word Protozoa means “early animals” . Earlier Protozoa were consider as animals as they do not have cell wall and they can move. Apart from few examples of protozoa which contains chlorophyll are either saprophytic and parasitic. the protozoa which contains chlorophyll lack other plant characters.

Habitat of protozoa
protozoa freshwater, saline water, ponds , lakes, dirty water drains, rivers, sea water and also in damp soil. Some parasitic protozoa so live in the bodies of other organisms including human beings. Examples of protozoa Giardia, Amoeba, Paramecium, euglena etc.

How Protozoa can move and feed themselves??!
The most important characteristic of protozoa is that they can move from one place to another.
Amoeba:- this single cell organism looks like mass of Protoplasm with no fixed shape. It can move in any direction by extending pseudopodia. Pseudopodia is also called as false feet. A pseudo podium is formed by the flow of Protoplasm in the direction of movement. Pseudopodia helps Amoeba in feeding. It extends to encircle the food particle which is then taken in by the cell and digested. Amoeba feeds tiny algae and bacteria.

This slipper shaped protozoan which lives in ponds and ditches swims by beating it’s Cilia against water. Movement of Cilia helps to direct food and water into an oral groove in paramoecium. Paramecium feeds on small algae and bacteria. Waste matter of paramoecium is thrown out through an anal pore.

euglena is found in ponds, ditches and other stagnant water bodies.
The body of euglena is spindle shaped slightly elongated and tapers at the top.
One end of euglena has flagellum which they use to move from one place to another. Some euglena has two flagella. Euglena with chlorophyll normally obtain their food through photosynthesis. Euglena which lives in dark condition ingest food particles through a mouth like opening leading to a gullet.
this organism has characteristics of both plants and animals. Euglena contains chlorophyll and that’s why most of them autotrophs. Even euglena has chlorophyll and live as autotroph it doesn’t come under plant Kingdom because they don’t have cell wall. Some euglena don’t have chlorophyll and live as saprophytes or parasites.

Advantages of protozoa
Saprophytic protozoans benefit the environment by decomposing organic matter in the soil. Parasitic protozoans prey on bacteria and algae helps to regulate the population of bacteria and algae.
Disadvantages of protozoa
Many protozoa are parasites and causes many diseases. Diseases like dysentery and malaria are caused by protozoa. For example entamoeba causes amoebic dysentery. Protozoan name plasmodium causes disease malaria inhumans. Plasmodium is commonly known as malarial parasite.

Algae: They are multicellular i.e. Made of multiple cells. They prefer mostly aquatic habitats. Some common examples of protozoa are: Spirogyra, Ulothrix, Chlamydomonas
Structure of algae
Algae are unicellular or multicellular. Some algal cell join each other to form filaments and some have branches or fronds. Both unicellular and multicellular algae live as individual or in colonies. Algae is being found in different forms, Unicellular forms, Multicellular forms A colonial form- volvox.

Unicellular forms:-
Examples of unicellular Algae are as follows—
Chalamydomonas: It is a green single Cellular alga. They are generally oval, spherical or peared shape. Flagella is used for locomotion.
Habitat of algae
Algae live mostly in an aquatic environment floating on the water. Many times they are found in ponds, lakes and seawater. Some algae leave on bark of tree and some on moist soil and rocks. Some Algae are also found in snow and some in hot spring.

Mode of nutrition in Algae
Algae are considered autotrophs as all of them have chlorophyll in it. Most Algal groups are considered photoautotrophs, they depend entirely upon their photosynthetic Apparatus for their food production, using sunlight as a source of energy and CO2 as a source of carbon to produce their own food. Many of the algae have a symbiotic relationship with plants and animals.

Types of Algae: On the basis of the pigment found cell they are classified into following groups
Blue green Algae: 
They have blue pigment along with chlorophyll. Nucleus is not present in blue green algae. They are more like bacteria. And therefore they are called as Cyanobacteria. They live in freshwater.
Green algae: 
This contains mainly chlorophyll. They live in freshwater. Green algae (singular: green alga) are photosynthetic algae that are characterized by having chlorophylls a and b as the predominant pigments, thus rendering them green in colour. They store food as starch within plastids. They include the chlorophytes and the charophytes.

Brown algae: Contents brown pigment in addition to chlorophyll. Brown algae generally marine. Many seaweeds are brown algae, which are almost 50 to 60-meter long.
Red algae: Contains red pigments besides chlorophyll. Red algae are generally marine. Red algae, or Rhodophyta, are one of the oldest groups of eukaryotic algae. The Rhodophyta also comprises one of the largest phyla of algae, containing over 7,000 currently recognized species with taxonomic revisions ongoing.
Uses of Algae
Algae are the major source of food for fishes, prawns and many aquatic animals. Sea Algae is eaten in many countries. Most of the oxygen in the atmosphere is released by algae. Red algae yield algin, substance used to thicken cosmetics and food items like ice cream, jelly coma chocolates. Agar a solidifying substance obtained from red algae used in microbiological research. Algae also consumes huge amount of carbon dioxide which is good for our ecosystem.
Disadvantages of Algae
Algal blooms can be toxic and it harms aquatic ecosystem and human health.

Viruses: An exception
Viruses are also microscopic, but they are not considered as micro-organism. They behave like non-living when outside host cell, and reproduce only when inside the host cell. you might know microbes only for causing harm to human beings by causing diseases and making us ill. But, the truth is microbes are also beneficial to us in a variety of ways. They help in preparation of several household & industrial products like curd, cake, bread, antibiotics & beverages. They also help the environment acting as decomposers and biofertilizers. They play an important in sewage treatment as well.

Viruses are the smallest microorganism can be seen only under an electron microscope.
Characteristics of viruses:  viruses do not show any characteristic of living things. Viruses do not have organised cell structure Viruses do not respire, feed, grow, excrete, move on their own. Viruses can only reproduce if they enter in a living cell of other organism( host cell). When viruses enter into any living cell it starit behaving like a living thing by carrying out the process of reproduction. Outside the living cell dresses behave as non living thing. Due to all this reasons viruses are considered to be an intermediate form between the living and nonliving.

Structure of virus
Viruses do not have an organisation structure. Viruses are acellular i.e their body has no cellular substances or nucleus. Viruses consist of nuclear material coated with proteins. Viruses typically have one or two strands of DNA or RNA. The Genetic material of virus are covered with a protective coat of protein called as CAPSID.

How viruses grow or how viruses reproduce???
Viruses cannot reproduce by themselves. The virus attaches itself to a specific host cell or the cell in which it can reproduce. After attaching to the host cell virus injects it’s nuclear material( DNA or RNA) inside the host cell. The nuclear material of virus attaches to the nuclear material of the host cell. The nucleic material of virus take the custody of the manufacturing system of the cell. The host cell ignore its own need and switches to making new viruses. The host cell sometimes destroyed in the process and many of the new viruses are released to infect other host cells.

Advantages of viruses
Even though viruses has lots of disadvantages but some viruses plays very important role in the ecosystem. For example the bacteriophages which are also called as bacterial virus please an important role in Marine Ecology and carbon cycling. Bacteriophages in the sewage has the ability to to kill harmful bacteria and keep the bacteria level in control. Scientist are trying to find out viruses which can be used to kill bacteria, and can be used instead of antibiotics.
Disadvantages of viruses
Viruses have many disadvantages as they are the agent of many diseases. Virus causes diseases too many animals and plants. Human diseases such as common cold, influenza, measles, chicken pox, smallpox are all caused by viruses. HIV or immunodeficiency virus causes AIDS in human beings and some animals.

Where do Microorganisms Live?
Microorganisms are found in air, water and in the bodies of plants and animals. They can live in all kinds of environment, ranging from ice cold climate to hot springs, deserts and marshy lands. They are also found inside the bodies of animals including humans. Some microorganisms grow on other organisms while others exist freely.

Microorganisms may be single-celled like bacteria, some algae and protozoa, or multicellular, such as many algae and fungi. So they may be unicellular or multicellular. Viruses though different from the above-mentioned living organisms are referred to as microbes. Viruses are quite different from other microorganisms. They reproduce only inside the cells of the host organism: bacterium, plant or animal.

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. Let us study about them in detail.
Friendly Microorganisms
Microorganisms are used for various purposes. They are used in the preparation of curd, bread and cake. Microorganisms have been used for the production of alcohol since ages. They are also used in cleaning up of the environment. For example, the organic wastes (vegetable peels, remains of animals, faeces, etc.) are broken down into harmless and usable substances by bacteria. Recall that bacteria are also used in the preparation of medicines. In agriculture they are used to increase soil fertility by fixing nitrogen.

Microorganisms are useful in the following ways:
In food and beverage industry
In making medicines and vaccines
In agriculture
In cleaning the environment
Making of Curd and Bread:You have learnt in Class VII that milk is turned into curd by bacteria. Curd contains several microorganisms. Of these, the bacterium, Lactobacillus promotes the formation of curd. It multiplies in milk and converts it into curd. Bacteria are also involved in the making of cheese, pickles and many other food items. An important ingredient of rava (sooji ) idlis, cakes and bhaturas is curd. Can you guess why?
Bacteria and yeast are also helpful for fermentation of rice idlis and dosa batter. Many fungi like Mushrooms and morels are edible which are rich. Yeast is also used in breweries for making wines by the process of fermentation.

Commercial Use of Microorganisms
Microorganisms are used for the large scale production of alcohol, wine and acetic acid (vinegar). Yeast is used for commercial production of alcohol and wine. For this purpose yeast is grown on natural sugars present in grains like barley, wheat, rice, crushed fruit juices, etc. Take a 500 mL beaker filled upto ¾ with water. Dissolve 2-3 teaspoons of sugar in it. Add half a spoon of yeast powder to the sugar solution. Keep it covered in a warm place for 4-5 hours. Now smell the solution. Can you get a smell? This is the smell of alcohol as sugar has been converted into alcohol by yeast. This process of conversion of sugar into alcohol is known as fermentation. Louis Pasteur discovered fermentation in 1857.

Medicinal Use of Microorganisms
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. These days a number of antibiotics are being produced from bacteria and fungi. Streptomycin, tetracycline and erythromycin are some of the commonly known antibiotics which are made from fungi and bacteria. The antibiotics are manufactured by growing specific microorganisms and are used to cure a variety of diseases. Antibiotics are even mixed with the feed of livestock and poultry to check microbial infection in animals. They are also used to control many plant diseases.

Germs are all around us, both in our environment and in our bodies. When a person is susceptible and they encounter a harmful organism, it can lead to disease and death. The body has many ways of defending itself against pathogens (disease-causing organisms). Skin, mucus, and cilia (microscopic hairs that move debris away from the lungs) all work as physical barriers to prevent pathogens from entering the body in the first place. When a pathogen does infect the body, our body’s defences, called the immune system, are triggered and the pathogen is attacked and destroyed or overcome.

Why are children/infants given vaccination?
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. The antibodies remain in the body and we are protected from the disease-causing microbes forever. This is how a vaccine works. Several diseases, including cholera, tuberculosis, smallpox and hepatitis can be prevented by vaccination. In your childhood, you must have been given injections to protect yourself against several diseases.

Edward Jenner discovered the vaccine for smallpox in 1798. It is important to remember that antibiotics should be taken only on the advice of a qualified doctor. Also you must complete the course prescribed by the doctor. If you take antibiotics when not needed or in wrong doses, it may make the drug less effective when you might need it in future. Also antibiotics taken unnecessarily may kill the beneficial bacteria in the body. Antibiotics, however, are not effective against cold and flu as these are caused by viruses.

How vaccines help:
Vaccines contain weakened or inactive parts of a particular organism (antigen) that triggers an immune response within the body. Newer vaccines contain the blueprint for producing antigens rather than the antigen itself. Regardless of whether the vaccine is made up of the antigen itself or the blueprint so that the body will produce the antigen, this weakened version will not cause the disease in the person receiving the vaccine, but it will prompt their immune system to respond much as it would have on its first reaction to the actual pathogen. Some vaccines require multiple doses, given weeks or months apart. This is sometimes needed to allow for the production of long-lived antibodies and development of memory cells. In this way, the body is trained to fight the specific disease-causing organism, building up memory of the pathogen so as to rapidly fight it if and when exposed in the future.

The Herd Immunity Imperative:
Vaccines don't just work on an individual level, they protect entire populations. Once enough people are immunized, opportunities for an outbreak of disease become so low even people who aren't immunized benefit. Essentially, a bacteria or virus simply won't have enough eligible hosts to establish a foothold and will eventually die out entirely. This phenomenon is called "herd immunity" or "community immunity," and it has allowed once-devastating diseases to be eliminated entirely, without needing to vaccinate every individual.

Vaccines and your immune system
Vaccines give you immunity to a disease without you getting sick first. They are made using killed or weakened versions of the disease-causing germ or parts of the germ (called antigens). For some vaccines, genetic engineering is used to make the antigens used in the vaccine. It’s much safer to get a vaccine than to get the disease it prevents. When you get a vaccine, your immune system responds to the vaccine the same way it would to the real germ. It: Recognizes the germ in the vaccine as being foreign. Responds by making antibodies to the germ in the vaccine, just as it would for the real germ. Remembers the germ and how to destroy it. That way, if you are ever exposed to the disease-causing germ in the future, your immune system will be able to quickly destroy it before it has a chance to make you sick. This is how you get immunity from vaccines.

In 1929, Alexander
Fleming was working on a culture of disease causing bacteria. Suddenly he found the spores of a little green mould in one of his culture plates. He observed that the presence of mould prevented the growth of bacteria. In fact, it also killed many of these bacteria. From this the mould penicillin was prepared. It is important to remember that antibiotics should be taken only on the advice of a qualified doctor. Also you must complete the course prescribed by the doctor. If you take antibiotics when not needed or in wrong doses, it may make the drug less effective when you might need it in future. Also antibiotics taken unnecessarily may kill the beneficial bacteria in the body. Antibiotics, however, are not effective against cold and flu as these are caused by viruses.

Can you prepare a list of these diseases?
You may take help from your parents. It is essential to protect all children against these diseases. Necessary vaccines are available in the nearby hospitals. You might have seen the advertisement on TV and newspapers regarding protection of children against polio under the Pulse Polio Programme. Polio drops given to children are actually a vaccine. A worldwide campaign against smallpox has finally led to its eradication from most parts of the world. These days vaccines are made on a large scale from microorganisms to protect humans and other animals from several diseases. Bacteria and Yeast are used in making vitamin B complex tablets. The human hormonal called insulin can also be obtained from bacteria. Edward Jenner discovered the vaccine for smallpox in 1798.

Increasing Soil Fertility
Some bacteria (Fig. 2.7) 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. Biological nitrogen fixation (BNF) is the term used for a process in which nitrogen gas (N2) from the atmosphere is incorporated into the tissue of certain plants. Only a select group of plants is able to obtain N this way, with the help of soil microorganisms.

In Agriculture:
Some bacteria like Rhizobium, blue green algae like Nostoc, live in the root nodules of plants such as gram, pea etc. These bacteria absorb the nitrogen of the atmosphere and convert it into nitrate which serves as natural fertilizers for plants, hence, enhancing soil fertility.

In cleaning the environment:
Microbes help to keep our environment clean by decomposing dead matter. They decomposed substances are recycled as these get reused from the soil by plants. You often see large amounts of dead organic matter in the form of decaying plants and sometimes dead animals on the ground. You find that they disappear after some time. This is because the microorganisms decompose dead organic waste of plants and animals converting them into simple substances. These substances are again used by other plants and animals. Thus, microorganisms can be used to degrade the harmful and smelly substances and thereby clean up the environment. Some bacteria decompose sewage and other waste in water. This is nature’s method of keeping the environment free from pollution.

Harmful Microorganisms
Microorganisms are harmful in many ways. Some of the microorganismscause diseases in human beings, plants and animals. Such disease-causing microorganisms are called pathogens. Some microorganisms spoil food, clothing and leather. Let us study more about their harmful activities.

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

Then how do you prevent the spread of communicable diseases?
When a person suffering from common cold sneezes, fine droplets of moisture carrying thousands of viruses are spread in the air. The virus may enter the body of a healthy person while breathing and cause infection. We should keep a handkerchief on the nose and mouth while sneezing. It is better to keep a distance from infected persons. There are some insects and animals which act as carriers of disease causing microbes. Housefly is one such carrier. The flies sit on the garbage and animal excreta. Pathogens stick to their bodies. When these flies sit on uncovered food they may transfer the pathogens.

Whoever eats the contaminated food is likely to get sick. So, it is advisable to always keep food covered. Avoid consuming uncovered items of food. Another example of a carrier is the female Anopheles mosquito (Fig. 2.8), which carries the parasite of malaria (Plasmodium). Female Aedes mosquito acts as carrier of dengue virus.

How can we control the spread of malaria or dengue?
All mosquitoes breed in water. Hence, one should not let water collect anywhere, in coolers, tyres, flower pot, etc. By keeping the surroundings clean and dry we can prevent mosquitoes from breeding. Try to make a list of measures which help to avoid the spread of malaria. Viruses are also microscopic but are different from other microorganisms. They, however, reproduce only inside the cells of the host organism, which may be a bacterium, plant or animal. Some of the viruses are in Common ailments like cold, influenza (flu) and most coughs are caused by viruses. Serious diseases like polio and chicken pox are also caused by viruses.

Diseases like dysentery and malaria are caused by protozoa(protozoans) whereas typhoid and tuberculosis (TB)are bacterial diseases.

Some of the common diseases affecting humans, their mode of transmission and few general methods of prevention.

Disease causing Microorganisms in Animals
Several microorganisms not only causecdiseases in humans and plants, but also in other animals. For example, 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
Several microorganisms cause diseases in plants like wheat, rice, potato, sugarcane, orange, apple and others. The diseases reduce the yield of crops. See Table 2.2 for some such plant diseases. They can be controlled by the use of certain chemicals which kill the microbes.

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