WBBSE Notes For Class 8 School Science Chapter 7 The Microbial World

Chapter 7 The Microbial World

What are Microbes?


The term microbe is short for microorganisms, which means small organisms visible only under microscopes. A microbe is any living thing that is too tiny to be seen with the naked eye.

Microbes are the oldest form of life on earth. They may live as individuals or cluster together in communities. Microbes live in the water you drink, the food you eat, and the air you breathe.

Right now, billions of microbes are swimming in your belly and mouth, and crawling on your skin Don’t worry; over 95% of microbes are good for you.

Microbes include bacteria, viruses, fungi, algae, and protozoa. These single-cell organisms are invisible to the eye, but they can be seen with microscopes.

WBBSE Solutions For Class 8 School Science Chapter 7 The Mircobial World Mircobes

Microbial Characteristics

Many characteristic features are used in classifying and identifying microorganisms. In general, these characteristic features have been divided into several major categories.

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1. Classical Characteristics

The classical type of approach such as morphological, physiological, biochemical, ecological, and genetic characteristics has been widely employed to study microbial taxonomy and it also provides phylogenetic information about microorganisms.

2. Morphological characteristics

Morphological features are important in microbial taxonomy for many reasons. Morphology is easy to study and analyze both eukaryotic and prokaryotic microorganisms.

Many different morphological features are used in the classification and identification of microorganisms. Some of these features are cell size, cell shape, colonial morphology, ultrastructural characteristics, staining behavior, cilia, and flagella, mechanism of motility, color, etc.

3. Physiological and Metabolic characteristics

Physiological and metabolic characteristics are very useful because they are directly related to the nature and activity of microbial enzymes and transport proteins.

Because proteins are gene products, analysis of these characteristics provides an indirect comparison of microbial genomes.

4. Ecological characteristics

Microorganisms are well-associated and growing in terrestrial freshwater and marine environments. The taxonomically important ecological properties are life cycle patterns,

The nature of the symbiotic relationships, and the ability to cause disease in particular hosts and habitat preferences such as temperature, pH, oxygen, and osmotic concentration.

5. Genetic characteristics

Most eukaryotes are able to reproduce sexually; hence genetic analysis has been of considerable usefulness in the classification of these types of microorganisms.

However, prokaryotes do not reproduce sexually and chromosomal gene exchange (through transformation and -conjugation) is sometimes useful in the classification of prokaryotes.

The major Characteristics of microbes are

1. Microbes are found almost everywhere, i.e. in deserts, polar regions, saline or brackish water wetland, and hot streams, and also found in the alimentary canal of humans as well as termites:

2. The majority of microbes require oxygen for survival. However, yeast and tetanus-causing pathogens can survive under low concentrations of oxygen.

3. Damp places are ideal for their growth and survival. Their growth is better in darker places

4. Some of them cannot survive in direct sunlight. Some of them (Saprophytes) collect their food from dead and decaying organic substances.

5. Some of them (Parasites) find shelter in other animals and collect food from different organs or tissues or cells of those animals. Some are able to synthesize their own food (Autotrophs).

6. Generally, the microbes can grow well between 25’Cto 38°C. A few microbes can even grow and survive at- 108C. Some others can grow and survive even above 100°C.

7. The thermophilic (heat-loving) bacteria can grow and reproduce at 100°C. The thermophilic bacteria are found in hot springs and deep-sea hydrothermal vents.

8. Without staining, microbes are invisible to us under the microscope.

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Classification of Microbes

Microbes Are Classified As Follows:

The term microbe is short for microorganism, which means small organisms. To help people understand the different types of microbes, they are grouped or classified in various ways.

Microbes are very diverse and represent all the great kingdoms of life. In fact, in terms of numbers, most of the diversity of life on earth is represented by microbes.

Here is an outline of the major groups of microorganisms:

  1. Viruses
  2. Bacteria (Monera)
  3. Algae (Plantae)
  4. Fungi
  5. Protozoa (Protista)


A virus is acellular and composed basically of a tiny bundle of genetic material (either DNA or RNA) carried in a shell called the viral coat.

They have no boundary wall, cytoplasm, or nucleus. Thousands of different viruses, which come in many shapes, are parasites and disease-causing agents.

Viruses are found on or in just about every material and environment on Earth from soil to water to air. Viruses do not show most of the characteristics of living things outside the living host.

But if they come into contact with a suitable plant, animal, or bacterial cell, they spring into action. They infect and take over the cell like pirates hijacking a ship.

Viruses exist to reproduce only. To do that, they have to take over suitable host cells. The new viral genes then come together and assemble into whole new viruses.

The new viruses are either released from the host cell without destroying the cell or eventually build up to a large enough number that they burst the host cell.


WBBSE Solutions For Class 8 School Science Chapter 7 The Mircobial World virus



Bacteria consist of only one cell, but they are a very complex group of living things. Unlike viruses, bacteria feed, move, and respire as well as reproduce on their own.


WBBSE Solutions For Class 8 School Science Chapter 7 The Mircobial World Shape of bacteria


Some bacteria can live in temperatures above the boiling point and in cold below the freezing point. There are thousands of species of bacteria.

Bacteria are classified into 5 groups according to their basic shapes: spherical (cocci), rod (bacilli), spiral (spirilla), comma (vibrios), or corkscrew (spirochaetes). They can exist as single cells, in pairs, chains, or clusters.

Some bacteria can make their own food from sunlight, just like plants. Also like plants, they give off oxygen. Other bacteria absorb food from the material they live on or in. They have no membrane-bound cell organelles.

A single teaspoon of soil contains more than a billion (1,000,000,000) bacteria. Bacteria reproduce by binary fission. In this process the bacterium, which is a single cell, divides into two identical daughter cells. Binary fission begins when the DNA of the bacterium divides into two (replicates).

Differences between bacteria and viruses:

Because bacteria and viruses cause many of the diseases we’re familiar with, people often confuse these two microbes. But viruses are entirely different from bacteria.

For one thing, they differ greatly in size. The biggest viruses are only as large as the tiniest bacteria. Another difference is their structure. Bacteria are complex compared to viruses.

A typical bacterium has a rigid cell wall and a thin, rubbery cell membrane surrounding the fluid, or cytoplasm, inside the cell.

A bacterium contains all of the genetic information needed to make copies of itself—its DNA—in a structure called a chromosome. In addition, it may have extra loose bits of DNA called plasmids floating in the cytoplasm.

Bacteria also have ribosomes, necessary for copying DNA so bacteria can reproduce. Some have threadlike structures called flagella that they use to move.


WBBSE Solutions For Class 8 School Science Chapter 7 The Mircobial World bacteriophage



Algae are found in fresh and saltwater around the world where they can either be free-floating (planktonic) or attached to the bottom.

They can also grow on rocks and trees and in soil when enough water is available. Algae can exist as single cells, an example of which is Chlamydomonas, or joined together in chains like Spirogyra or made up of many cells, for instance, Rhodymenia (red seaweed).


WBBSE Solutions For Class 8 School Science Chapter 7 The Mircobial World Algae


All algae contain a pigment called chlorophyll and they make their own food by photosynthesis. Chlorophyll is contained in the chloroplasts and gives many algae their green appearance. Their cell contains a cell wall, nucleus, and other cell organelles.


Fungi cannot synthesize their own food as plants do. They have cell walls, nuclei, and other organelles but these do not have chloroplast.

Hence they cannot synthesize food. The cell wall is composed of chitin. Fungi include single-celled living things that exist individually, such as yeast, and multicellular clusters, such as molds or mushrooms.

Yeast cells look round or oval under a microscope. They’re too small to be seen as individuals, but you can see large clusters of them as a white powdery coating on fruits and leaves.

Groups called decomposers to grow in the soil or on dead plant matter where they play an important role in the cycling of carbon and other elements.

Some are parasites of plants causing diseases such as mildews, rusts, scabs, or canker. In crops, fungal diseases can lead to significant monetary loss for the farmer.

A very small number of fungi cause diseases in animals. In humans, these include skin diseases such as athlete’s foot, ringworm, and thrush.

Fungi live in the soil and on your body, in your house and on plants and animals, and in freshwater and seawater. A single teaspoon of topsoil contains about 120,000 fungi. They need moist and warm conditions to grow.


WBBSE Solutions For Class 8 School Science Chapter 7 The Mircobial World fungi

Fungi absorb nutrients from living or dead organic matter that they grow on. They absorb simple, easily dissolved nutrients, such as sugars, through their cell walls.

They give off special digestive enzymes to break down complex nutrients into simpler forms that they can absorb. Some fungi are quite useful to us.

We’ve used several kinds to take antibiotics to fight bacterial infections. We use baker’s yeast, to make bread rise and to brew beer.

There are some dangerous fungi that cause diseases in plants, animals, and people. Fungi ruin about a quarter to half of harvested fruits and vegetables each year.


Protozoa are single-celled organisms having one or more nuclei. They come in many different shapes and sizes ranging from an Amoeba which can change its shape to its fixed shape and complex structure.

They live in a wide variety of moist habitats including freshwater, marine environments, and soil. They can live freely or in a colony.

Protozoa mainly feed on bacteria, but they also eat other protozoa, and sometimes fungi. Some protozoa absorb food through their cell tissues.


WBBSE Solutions For Class 8 School Science Chapter 7 The Mircobial World Protozoa


Others, surround food and engulf it. Others have openings similar to mouth pores into which they sweep food.
Protozoa can be classified into three general groups based on their shape. One group is the Ciliates, which are generally the largest protozoa. The second group is the Amoebae.

The nutrients from living or dead organic matter that they grow on. They absorb simple, easily dissolved nutrients, such as sugars, third group is the Flagellates, which are usually the smallest of the protozoa.

Most protozoa do us no harm. But there are a few that cause diseases. One type of amoeba can live in human intestines. It feeds on red blood cells and causes a disease known as dysentery.

Another species of protozoa can sicken hundreds of thousands of people when it gets into the tap water. Perhaps the best-known deadly protozoa cause malaria, a terrible disease that leads to about 800,000 deaths each year worldwide.

Nutritional Relationships with Other Organisms

Microbes are related to several other organisms to maintain their life and derive nourishment. Some of these relationships are –

1. Parasitic

They derive their nutrition from the plants and animals on which they grow. Certain enzymes are produced by them which decompose or kill the protoplasm of the host cells.

The interrelationship between the parasitic microbe and the host is called parasitism. Such effects of the parasites on the host become visible to the naked eye as disease symptoms.

Many well-known diseases of human beings like typhoid, tetanus, tuberculosis, pneumonia, and many others are due to parasitic bacteria.

Microbes are also known as pathogenic bacteria. Some bacteria grow well only in the presence of oxygen, while others grow well in absence of oxygen. The former is known as aerobes and the latter are anaerobes.

2. Saprophytic

They grow on dead and decaying plants and animals, dung,’ rotten wood, stagnant water, and many other decaying substances rich in organic matter.

Certain enzymes secreted by the bacteria decompose the complex organic substances of the substrate, converting them into simpler ammonium compounds.

They cause decay and therefore are also known as putrefying bacteria. The souring of milk, the manufacture of cheese, the preparation of butter from milk, and vinegar from sugarcane juice, are various processes completed by the action of certain specific saprophytic bacteria.

Zygomonas ferments glucose-producing alcohol, lactic acid, and carbon dioxide, and plays a significant role in the wine industry.

Acetobacter oxidizes organic compounds to organic acids such as lactic acid thus having a significant role in the vinegar industry. Lactobacillus converts sugars into lactic acid. Canned food is spoiled by some Bacillus and Clostridium bacteria.

3. Symbiotic

Symbiosis is the phenomenon by which two organisms maintain a relationship with each other to be mutually benefitted. In symbiotic mode, organisms develop a special relationship with certain other organisms to obtain nourishment.

Organisms involved in this type of relationship are called symbionts. Rhizobium bacteria is a striking example of this type.

They occur in root nodules of leguminous plants and help in fixing the free nitrogen of the atmosphere in the soil for the plants which in return provides carbohydrates and protection to the bacteria.

They are also called nitrogen-fixing bacteria and add to the fertility of the soil. Azotobacter and Clostridium are other examples of nitrogen-fixing bacteria.

They are found in chalky soil and obtain energy from the carbohydrates present in the soil. The energy so obtained is used in fixing atmospheric nitrogen into amino acids in the soil which react with the calcium salts, forming nitrites and thereafter nitrates.

Lichens form a very good example of a symbiotic relationship. Lichens are formed by the symbiotic relationship between algae and fungi sharing a single colony.

Fungus provides water, nutrients, and shelter in the form of the network formed by mycelium. Algae synthesize the food for the entire colony through the process of photosynthesis.

Role of Microbes in the Environment

The human body has( 10-100 trillion microbes living on it, making it one giant super-organism. Since the first link between microbes and diseases was made, people have been advised to wash their hands.

Scientists, however, have recently started to investigate more closely how the microbes that call the human body home affect our health. While some microbes cause disease, others are more beneficial, working with our bodies in many subtle ways.

Microbes and Human Health In fact, many of these microbes are actually beneficial. The microbes living in our digestive system break down food and produce useful vitamins.

The millions of microbes that coat our skin and insides form a protective barrier against more dangerous microbes. Without them, our bodies would be open to microbial attack.

In spite of the benefits, a relatively small number of microbes are harmful to humans. Disease-causing micro organisms are called pathogens.

Many diseases and epidemics are caused by microbes: the plague, smallpox, AIDS, influenza, food poisoning, malaria, and anthrax. These diseases result in severe illness or even death.

Those microbial diseases which spread from an infected person to a healthy person through the air, water, food, physical contact, etc are called communicable diseases.

Some of the communicable diseases are cholera, the common cold, chicken pox, Tuberculosis, etc. As scientists learn more about bacteria, fungi, and viruses, they are better able to treat and prevent these diseases. Common treatments include antibiotics that kill bacteria and vaccines that help the body fight off viruses.

2. Microbes and Ecosystems:

Recycling of waste Microbes obtains energy from their environment. Like humans, many microbes do this by eating plant and animal material.

This is called biodegradation. A typical microbe buffet consists of waste from humans and other animals, dead plants and animals, and food scraps.

Bacteria, fungi, and algae all take part in decomposing—or breaking down—this waste material. Without them, the world would quickly be overrun with discarded food scraps, raw sewage, and dead organisms.

Microbial decomposition releases nutrients into the environment that are needed by other organisms. Microbes are also involved in the cycling of many other important compounds in— and between—ecosystems, including oxygen, carbon, and nitrogen.

Many microbes use the energy of sunlight to convert carbon dioxide to oxygen, which we need to breathe. As they do this, they create new organic material— themselves—which is then eaten by other organisms.

In this way, the cycling of nutrients and energy continues. Microbes like methanogenic bacteria produce biogas, an environment-friendly biofuel.

3. Microbes and Food and Agriculture

People have a long history of using microbes in agriculture and food production. Microbes play a part in the production of food, such as yogurt, cheese, wine, bread, vitamins, beans, and chocolate.

Alcohol is produced by microbes like Yeast. In some cases, microbes function as they would in their original environment. For example, bacteria living around the roots of bean plants (Rhizobium, acetobacter, Clostridium, etc) convert nitrogen from the air into a form that the plant can use, like fertilizer.

People have also learned how to use the natural abilities of microbes to create new food, such as by making beer from grains.

4. Industrial Applications of Microbes

Microbes are so efficient at what they do that scientists use them in many industrial applications. Decomposing microbes are active in wastewater treatment plants, composting facilities, and landfills.

They break down food scraps and waste materials into compost or fertilizer that can be used in gardens or in agriculture. Some industrial facilities capture methane gas that is produced during certain types of decomposition.

The methane can then be used to make ethanol for cars or to generate electricity. Microbes are employed to separate jute fiber from jute stem.

Many types of scientific research involve altering the DNA of microbes or using their DNA to alter other organisms. By changing the DNA sequence of microbes, scientists can use them to create compounds that the microbes would not normally produce.

This includes medical products like Insulin or sources of fuel. Microbes are also used to change the DNA sequence of other organisms, such as changing plants to become resistant to insects or viruses.

Microbial Habitat

Microbes live in almost every nook and corner you can think of, from 20 miles beneath the Earth’s surface to 20 miles overhead. They live at temperatures less than -20 degrees Celsius to temperatures hotter than the boiling point.

Microbes thrive on a huge range of food including oil and toxic wastes. Every time you walk on the ground you step on billions of microbes.

Microbes live in the soil, on rocks, inside roots, buried under miles of earth, in compost piles, and toxic waste all over the earth’s surface.

Microbes are found in boiling hot springs and on frozen snow fields. Most animals interact with microbes in important ways, and all animals, as well as all plants and fungi, depend on microbes for their survival.

Microbes live in their digestive systems, in their mouths, and on their skin. Microbes are important for the good health of animals.

Animals provide microbes with food and shelter. That is why animals are called “hosts” to microbes. For example, cows depend on microbes for their food.

Cows and other animals called ruminants have special stomachs called rumens which are host to billions of microbes that help these animals survive.

Also, we depend on microbes to clean up our environment. Without microbes to get rid of all the dung from animals, the world would be a really smelly place. Microbes make this world a cleaner place.

Useful and Harmful Microbes

People often use the word germ instead of microbe, so you might think that microbes are all harmful. But some are useful to us.

Useful microbes

  1. Some useful microbes are listed below-
  2. Bacillus thuringiensis – a common soil bacterium that is a natural pest-killer in gardens and on crops.
  3. Arbuscular mycorrhizas – fungus living in the soil that helps crops take up nutrients from the soil.
  4. Saccharomyces cerevisiae – fungus, ba maker’s yeast that makes bread rise.
  5. Escherichia coli-one of many kinds of microbes (bacteria) that live in your digestive system to help you digest your food every day.
  6. Streptomyces – bacteria in soil that makes an antibiotic used to treat infections.
  7. Pseudomonas putida – one of many microbes that clean wastes from sewage water at water treatment plants.

We’ve been using microbes for thousands of years to make products we need and enjoy.

For example, you can thank fungi for the cheese on your cheeseburger and yeast for your bun. Cheese and bread are two microbe-made food people have been enjoying since time began.

In pollution control, researchers are using bacteria that eat methane gas to clean up hazardous waste dumps and landfills.
Fungi and bacteria produce antibiotics such as penicillin and tetracycline.

These are medicines we use to fight off harmful bacteria that cause infections, and other discomforts. Scientists have changed the genetic material of bacteria and yeasts to turn them into medicine.

Harmful microbes

Microbes cause disease when they are able to reproduce in the body. They produce harmful Lactobacillus acidophilus – one of the bacteria that turn milk into yogurt.

There are many other important jobs microbes do. They are used to make medicine. They break down the oil from oil spills. They make up about half of the oxygen we breathe.

They are the foundation of the food chain that feeds all living thin harmful disease-causing microbes in them are infected.

A few harmful microbes, for example, less than 1% of bacteria, can invade our body (the host) and make us ill. Microbes cause infectious diseases such There is also strong evidence that microbes may contribute to many non-infectious chronic diseases such as some forms of cancer and coronary heart disease.

Different diseases are caused by different types of microorganisms. Microbes that cause disease are called pathogens.


Infectious disease Microbe that causes the disease Type of microbe
Cold Rhinovirus Virus
Chickenpox Varicella zoster Virus
German measles Rubella Virus
Whooping cough Bordetella pertussis Bacterium
Bubonic plague Yersinia pestis Bacterium
TB (Tuberculosis) Mycobacterium tuberculosis Bacterium
Malaria Plasmodium falciparum Protozoan
Ringworm Trichophyton rubrum Fungus
Athletes’ foot Trichophyton mentagrophytes Fungus


To cause an infection, microbes must enter our bodies. The site at which they enter is known as the portal of entry. Microbes can enter the body through the four sites listed below:

  1. Respiratory tract (mouth and nose) e.g. Influenza virus which causes the flu.
  2. Gastrointestinal tract (mouth oral cavity) e.g. Vibrio cholerae which causes cholera.
  3. Urogenital tract e.g. Escherichia coli which causes cystitis.
  4. Breaks in the skin surface e.g. Clostridium tetani which causes tetanus.

It is important to remember that:

  1. A pathogen is a microorganism that has the potential to cause disease.
  2. An infection is the invasion and multiplication of pathogenic microbes in an individual or population.
  3. The disease is when the infection causes damage to the individual’s vital functions or systems.
  4. An infection does not always result in disease.


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