Useful and Harmful Microbes Class 9 Science Notes Maharashtra Board

Useful and Harmful Microbes Class 9 Science Notes Maharashtra State Board

Useful Micro-Organisms

Smear a drop of fresh buttermilk on a glass slide. Stain it with methylene blue and put a coverslip over it. Observe the smear under the 10X objective of a compound microscope and then with the more powerful 60X objective. Did you notice the blue rod-shaped organisms moving around? They are lactobacilli, a kind of bacteria. They are minute and rectangular. Lactobacilli are anaerobic bacteria i.e. they can produce energy without the use of oxygen.
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The lactobacilli convert lactose, the sugar in the milk, into lactic acid. This process is called fermentation. As a result, the pH of milk decreases causing coagulation of milk proteins. Thus, milk proteins are separated from other constituents of milk. This is what happens when milk changes into yogurt. Yoghurt has a specific sour taste due to lactic acid. The low pH destroys harmful microbes present in the milk.

Useful and Harmful Microbes Class 9 Science Notes Maharashtra Board

What is meant by ‘probiotic’ yogurt and other foodstuffs that are popular nowadays?
Useful microbes like lactobacilli are added to these eatables. Such eatables are healthy because they kill harmful bacteria like clostridium in the alimentary canal and help to improve our immunity.

Uses of Lactobacilli:

  • Various milk products like yogurt, buttermilk, ghee, cheese, shrikhand, etc. can be obtained by fermentation of milk.
  • Lactobacilli fermentation is useful for large-scale production of cider, cocoa, pickles of vegetables, etc.
  • Lactobacilli and some other useful microbes taken together are used to treat abdominal discomfort.
  • Leavened fodder offered to domestic cattle like cows and buffalos, is fodder fermented with the help of lactobacilli.
  • The lactobacilli fermentation process is used to make wine and some types of bread.

Rhizobium: Symbiotic Bacteria
Take a plantlet of fenugreek, groundnut, or any other bean and sterilize it with a 3 to 5% solution of hydrogen peroxide.
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Afterward, keep it in a 70% solution of ethyl alcohol for 4 to 5 minutes. Clean the roots with sterile water and take thin sections of the root nodules. Select a good section and place it in a solution of safranin for 2 to 3 minutes. Place the stained section on a glass slide, cover it with a coverslip, and observe it under the compound microscope. The pinkish rod-shaped organisms are the rhizobium bacteria. Note that we had to search for the root nodules of leguminous plants to obtain these bacteria. Are the rhizobium bacilli useful to these plants or harmful?

Role and Importance of Rhizobium
Rhizobia living in root nodules supply nitrates, nitrites, and amino acids to that plant and in exchange get energy in the form of carbohydrates from it. Such a mutually beneficial relationship is called symbiosis. Rhizobia produces nitrogenous compounds from atmospheric nitrogen. However, for this process of nitrogen fixation, they need leguminous plants like beans, sweet peas, soybeans, etc. as ‘host’. Beans and pulses are rich in proteins due to the nitrogenous compounds made available by rhizobia.

After harvesting a leguminous crop, the leftover roots and other plant parts are deliberately dumped in farm soil to maintain its bacterial population. The use of rhizobium has helped to reduce the use of chemical fertilizers and thereby their adverse effects. It has also helped to reduce expenses on fertilizers and thus works to the benefit of farmers. Nowadays, seeds are coated with rhizobial solution or powder before sowing. After sowing, rhizobia enter the plantlets. This is called ‘rhizobial inoculation’. This experiment has helped in the supply of nitrogen to cereal and other crops, besides leguminous crops.

Activity: Bring ‘active dry yeast’ from the market. Mix a spoonful of yeast, and two spoonfuls of sugar with a sufficient quantity of lukewarm water in a bottle. Fix a colorless, transparent balloon on the mouth of that bottle. What changes do you observe after 10 minutes? Mix limewater with the gas accumulated in the balloon. Collect that limewater in a beaker and observe it. What do you notice?
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Take a drop of the solution from the bottle on a glass slide, put a cover slip over it, and observe it under the compound microscope. Store the solution in the bottle carefully. Do you see the colorless, oval cells of yeast on the slide? Some of those cells may have small round bodies attached to them. These are new daughter cells of yeast in the process of formation. This method of asexual reproduction is called ‘budding’. Yeast is a heterotrophic fungal microbe that grows on carbon compounds. Yeast is a unicellular fungus with 1500 different species in existence. The yeast cell is a eukaryotic type of cell.

Useful and Harmful Microbes Class 9 Science Notes Maharashtra Board

In the above experiment, yeast grows and multiplies very quickly due to the carbon compounds in the sugar solution. In the process of obtaining nutrition, yeast cells convert the carbohydrates in that solution into alcohol and carbon dioxide. This process is called fermentation.

How is Bread made?
Find out how to use the solution prepared in the above experiment, to make bread. Follow the recipe and make the bread. Find out and note down the reasons why the dough rises and makes the bread spongy.

Often, alcohol is produced along with sugar in sugar factories. Molasses is produced from sugarcane juice. It is rich in carbohydrates. Molasses is fermented with the help of the yeast called Saccharomyces. In this process, ethanol (C2H5OH) alcohol is produced as a primary product, and ester and other alcohols as secondary products. Spirit and other alcohols can be obtained from ethanol. Ethanol is also a smokeless and high-quality fuel. Besides molasses, maize, barley, and other grains are also used for industrial production of ethanol. Glucose and fructose, the sugars present in grape juice, are also fermented with the help of yeast to produce the alcohol that is used for making wines.

A yeast, Yarrowia lipolytica is used to absorb the toxins released during the production of palm oil and the heavy metals and minerals released in some other industrial processes. Saccharomyces cerevisiae is used for the absorption of a pollutant, arsenic. Oil spills in oceans are cleaned with the help of Alcanivorax bacteria.

Carbon compounds obtained from some bacteria and fungi for destroying or preventing the growth of harmful micro-organisms are called ‘antibiotics’. Antibiotics, a discovery of the 20th century, have brought about a revolution in the field of medicine. Even a disease like tuberculosis has been almost completely eradicated from some countries. Antibiotics mainly act against bacteria. Some antibiotics can destroy protozoa. Some antibiotics are useful against a wide variety of bacteria. They are called broadspectrum antibiotics. Examples are ampicillin, amoxicillin, tetracycline, etc. When the pathogen cannot be identified even though the symptoms of the disease are visible, broad-spectrum antibiotics are used. Whenever a pathogenic micro-organism is known, then narrow-spectrum antibiotics are used. Examples are penicillin, gentamycin, erythromycin, etc.

Penicillin is a group of antibiotics obtained from a fungus, Penicillium, and is used for controlling infections caused by bacteria like Staphylococci, Clostridia, Streptococci, etc. Medicines containing penicillin are useful to treat certain bacterial infections of the ear, nose, throat, and skin as well as diseases like pneumonia and scarlet fever.


  • Antibiotics should be taken only when prescribed by a doctor.
  • Don’t purchase any antibiotics from medical stores without a prescription from a doctor.
  • Don’t consume antibiotics on your own to treat common diseases like a throat infection, common cold, or influenza.
  • Even if you feel well before completing the prescribed course of the antibiotic you must continue and complete it.
  • Don’t suggest to others the antibiotics which were useful to you.

Alexander Fleming, a professor of microbiology at St Mary’s Hospital had cultured varieties of bacteria and fungi in Petri dishes in his laboratory. On 3 September 1928, while observing staphylococci cultures, he made an interesting observation in one Petri dish. In that Petri dish, fungal colonies had grown but the area around those colonies was clean and clear. i.e. bacteria had been destroyed.
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After further studies, he confirmed that the fungus growing there was Penicillium and its secretion had destroyed the bacterial colonies. Thus, the first antibiotic – penicillin had been discovered accidentally and this formed the basis for attempts to find cures for incurable diseases. Shouldn’t we always be grateful to Alexander Fleming for discovering life-saving antibiotics?

Useful and Harmful Microbes Class 9 Science Notes Maharashtra Board

Ants grow fungi in their termitarium (anthill) and obtain food from it. Some species of wasps and other insects lay their eggs in the fungal bodies growing on trees thus ensuring a food supply for their larvae.

Harmful Micro-Organisms

Microscopic spores of fungi are present in the air. If there is sufficient moisture, spores germinate on cotton fabric, gunny bags, leather, wooden items, etc. The fungal hyphae (fibers of the fungus) penetrate deep into the material to obtain nutrition and reproduce. This causes the materials to wear and become weak. As a result, gunny bags, leather items like shoes, purses, belts, etc. on which fungi have grown do not last long. Wooden items also get spoilt. Various species of fungi grow on food items like pickles, murabba, jam, sauce, chutney, etc. They use the nutrients in these food items for growth and reproduction. During this activity, fungi release mycotoxins, certain poisonous chemicals, into the food, and thus food becomes poisonous. Hence, the food on which fungi have grown cannot be eaten.

Sometimes, cases of food poisoning occur during community feasts. How does the food become poisonous all of a sudden? The bacteria that spoil cooked food is Clostridium. Out of about 100 different species of this bacterium, some are free living in the soil whereas some live in the alimentary canal of humans and other animals. These bacteria are rod-shaped and produce bottle-shaped endospores in adverse conditions. One special characteristic of these bacteria is that they cannot withstand the normal oxygen level of the air because they grow in anaerobic conditions.
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Other Harmful Micro-Organisms
Do only the Clostridium bacteria cause illness?
Other kinds of micro-organisms like bacteria, viruses, protozoa, and fungi are also responsible for different diseases that affect humans. You have learned about viruses which are smaller than bacteria and can grow and reproduce only in living cells. Let us see how they are harmful to us.

Useful and Harmful Microbes Class 9 Science Notes Maharashtra Board

It was believed that meat gets spoiled due to some bacteria of the bacillus type but the exact type of bacillus was not known. Van Ermengem proved that the anaerobic bacterium Clostridium botulinum is responsible for food poisoning. Ida Bengston obtained higher education in biochemistry at Chicago University. She conducted important research on the toxin responsible for gas gangrene and the antitoxin useful for treating it. While she was researching the dreaded disease called typhus, she contracted the infection. But, she overcame it and continued her research. For this work, she was honored with the ‘Typhus Medal’ in 1947.

The Spread and Prevention of Disease

Name of Disease Pathogen Mode of Infection Preventive Measures
AIDS Virus Through the blood and semen of the infected person and the milk of a mother suffering from AIDS. Safe sexual contact, avoid reuse of needles and injections.
Hepatitis Virus Contaminated water and food. Clean and filtered water, and proper storage of food.
Influenza Virus Contact with an infected person. Personal hygiene and avoiding contact with infected persons.
Measles and Chickenpox Virus Contact with an infected person. Vaccination, uncontaminated water, clean food.
Bird Flu (H7N9)
Swine Flu (H1N1)
Virus Contact with infected birds and animals. Personal hygiene, properly cooked meat.
Dengue Virus Mosquito bite Cleanliness of surroundings preventing stagnation of water, mosquito control.
Pneumonia Bacteria Droplets spread in the air by an infected person. Vaccination, avoiding contact with infected person.
Leprosy Bacteria Long-term contact with an infected person. Avoiding contact with infected persons and their belongings.
Cholera Bacteria Contaminated food and water. Clean food and water.
Malaria Protozoa Mosquito bite, uncleaned surroundings. Cleanliness of surroundings, preventing stagnation of water, controlling mosquitoes.
Dandruff, Ringworm, Scabies Fungi Contact with an infected person or his/her belongings like clothes. Personal hygiene, avoiding contact with infected person.

Well-structured Maharashtra State Board Class 9 Science Notes Useful and Harmful Microbes can reduce anxiety during exams.

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