Maharashtra Board Class 9 Science Chapter 8 Useful and Harmful Microbes PDF Download

Useful And Harmful Microbes

Useful micro-organisms: Lactobacilli, Rhizobium, Yeast

Harmful micro-organisms: Clostridium and others.

Can You Recall

1. What is meant by microbes? What are their characteristics?

2. How did you observe 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 everyday life?

Useful Micro-Organisms

How is yoghurt made from milk? What exactly happens in this process?

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.

Lactobacilli

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 bacteria, i.e. they can produce energy without the use of oxygen.

Teacher's Note

Lactobacilli are good bacteria that make yoghurt taste sour. Just like how your mother adds yoghurt to milk to make more yoghurt at home, these tiny bacteria do the same work.

Exam Trick

Remember: Lactobacilli = yoghurt bacteria = rod-shaped. Think of them as tiny rods that sour the milk, just like how salt changes the taste of food.

Points To Remember

Lactobacilli are small rod-shaped bacteria.
They change milk into yoghurt by making lactic acid.
They can work without oxygen in the air.
Yoghurt kills bad bacteria in your stomach.
Lactobacilli are useful, not harmful.

Can You Tell?

1. Why do doctors advise you to have yoghurt or buttermilk if you have indigestion or abdominal discomfort?

2. Sometimes, yoghurt becomes bitter and froths up. Why does this happen?

3. Which different milk products are obtained at home by fermentation of the cream from the milk?

Do You Know?

What is meant by 'probiotic' yoghurt and other foodstuffs that are popular nowadays?

Useful microbes like lactobacilli are added to these eatables. Such eatables are healthy because they kill the harmful bacteria like clostridium in the alimentary canal and help to improve our immunity.

Uses Of Lactobacilli

1. Various milk products like yoghurt, buttermilk, ghee, cheese, shrikhand, etc. can be obtained by fermentation of milk.

2. Lactobacilli fermentation is useful for large scale production of cider, cocoa, pickles of vegetables, etc.

3. Lactobacilli and some other useful microbes taken together are used to treat abdominal discomfort.

4. Leavened fodder offered to domestic cattle like cows and buffalos, is fodder fermented with the help of lactobacilli.

5. The lactobacilli fermentation process is used to make wine and some types of bread.

Research

1. How many different industries depend upon the lactobacilli bacteria?

2. Which types of cottage industries and factories can be started in areas with abundant milk production?

Teacher's Note

Lactobacilli help make many food products we eat every day. In India, dahi and lassi are made using these helpful bacteria.

Exam Trick

Remember: Lactobacilli uses = yoghurt, cheese, pickle, wine, bread. Think of all the foods that taste sour or fermented - they all use lactobacilli!

Points To Remember

Lactobacilli make yoghurt and cheese.
They help make pickles and bread rise.
They treat stomach pain and indigestion.
Many industries use lactobacilli.
Lactobacilli fermentation is called fermentation process.

Rhizobium: Symbiotic Bacteria

Take a plantlet of fenugreek, groundnut or any other bean and sterilize it with a 3 to 5 percent solution of hydrogen peroxide. Afterwards, keep it in a 70 percent 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.

Teacher's Note

Rhizobium bacteria live in the roots of beans and peas. They give nitrogen from the air to the plant, and the plant gives food to the bacteria. This is like a good friendship.

Exam Trick

Remember: Rhizobium = root nodules = symbiosis = both help each other. Think of it like two friends sharing - bacteria gets food, plant gets nitrogen.

Points To Remember

Rhizobium lives in root nodules of beans and peas.
It fixes nitrogen from the air into the soil.
The plant and bacteria help each other (symbiosis).
Beans have more protein because of rhizobium.
This reduces the need for chemical fertilizers.

Let's Try This

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.

Teacher's Note

Yeast is used to make bread rise and become soft and fluffy. When you make bread at home, yeast makes little bubbles in the dough that make the bread spongy.

Exam Trick

Remember: Yeast + sugar + water = alcohol + carbon dioxide. The carbon dioxide bubbles make bread rise, like air balloons puffing up the dough!

Points To Remember

Yeast is a single-celled fungus.
Yeast reproduces by budding.
Yeast ferments sugar into alcohol and carbon dioxide.
Carbon dioxide makes bread rise.
Yeast needs sugar and warm water to work.

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