8. Useful and Harmful Microbes

You are familiar with different types of microbes which are all around us but cannot be seen with our eyes. In what way are they related to our everday life?

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 in shape. Lactobacilli are anaerobic i.e. they can produce energy without the use of oxygen.

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 yoghurt. Yoghurt has a specific sour taste due to lactic acid. The low pH destroys harmful microbes present in the milk.

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.

Afterwards, 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 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 produce nitrogenous compounds from atmospheric nitrogen. However, for this process of nitrogen fixation, they need leguminous plants like beans, sweet pea, soyabean, 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 left over 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.

Activity : Bring ‘active dry yeast’ from the market. Mix a spoonful of yeast, two spoonfuls sugar with a sufficient quantity of lukewarm water in a bottle. Fix a colourless, 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?

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 colourless, 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. 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 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.


 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 broad[1]spectrum antibiotics. Examples are ampicillin, amoxicillin, tetracycline, etc. When the pathogen cannot be identified even though the symptoms of disease are visible, broad spectrum antibiotics are used.

Whenever a pathogenic micro-organism is definitely 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 the 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.

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 (fibres of the fungus) penetrate deep into the material to obtain nutrition and to 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.

Sometimes, you may notice a black powder or white discs floating on the pickle or murabba, when a jar is opened after a long time. What exactly is this? Why are such food items not good to eat?

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 which spoil cooked food are 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.

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 learnt 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.