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Chapter 10 Microbes In Human Welfare
Microbes In Household Products
Microbes, including bacteria, fungi, viruses, viroids, and prions, are ubiquitous and diverse, found in soil, water, air, and inside living organisms. While some microbes are pathogenic (cause diseases), many are incredibly beneficial to humans in various ways.
Microbes like bacteria and fungi can be grown in laboratories on nutritive media, forming visible colonies that are useful for studying them.
We use microbes or their products in our daily lives, often without realizing it, particularly in the preparation of many household food items.
- Curd: Produced from milk by microorganisms such as Lactobacillus and other Lactic Acid Bacteria (LAB). LAB grow in milk, producing acids that coagulate and partially digest milk proteins, converting milk into curd. Adding a small amount of curd (starter) to milk introduces millions of LAB. Curd also has improved nutritional quality due to increased Vitamin B12 content. In the stomach, LAB can help check the growth of disease-causing microbes.
- Dough Fermentation: Bacteria ferment the dough used for making idli and dosa. The puffed-up appearance is due to the production of $\textsf{CO}_2$ gas during bacterial fermentation (anaerobic respiration).
- Bread Making: Dough for bread is fermented using baker's yeast (Saccharomyces cerevisiae), which produces $\textsf{CO}_2$ and causes the dough to rise.
- Traditional Foods and Drinks: Many traditional foods and beverages rely on microbial fermentation. Examples include 'Toddy' (fermented palm sap in southern India), fermented fish, soyabean, and bamboo shoots.
- Cheese: One of the oldest food items made using microbes. Different microbes are used to produce various cheese varieties with distinct textures, flavors, and tastes.
- Large holes in Swiss cheese are due to large amounts of $\textsf{CO}_2$ produced by the bacterium Propionibacterium sharmanii.
- Roquefort cheese is ripened by growing specific fungi, which contribute to its unique flavor.
Microbes are essential for these transformations, carrying out specific metabolic pathways that yield the desired products and characteristics.
Microbes In Industrial Products
Microbes are also used on a large scale in industry to synthesise a variety of products valuable to humans. Industrial production often takes place in large vessels called fermentors.
Fermented Beverages
Microbes, particularly yeasts (specifically Saccharomyces cerevisiae, also known as brewer's yeast), have been used for centuries to produce alcoholic beverages like wine, beer, whisky, brandy, and rum.
Yeast ferments malted cereals or fruit juices to produce ethanol. The type of beverage and whether distillation is involved depends on the raw material and processing method.
- Wine and Beer: Produced by fermentation without distillation.
- Whisky, Brandy, and Rum: Produced by distillation of the fermented broth.
Antibiotics
Antibiotics are chemical substances produced by some microbes that can kill or inhibit the growth of other (disease-causing) microbes. They are considered one of the most important discoveries for human welfare in the 20th century.
The term 'antibiotic' means 'against life' (referring to the life of pathogens), but for humans, they are 'pro-life'.
Penicillin was the first antibiotic discovered. It was a chance discovery by Alexander Fleming while working with Staphylococci bacteria. He observed that a mold growing on his culture plate prevented the growth of bacteria around it. He identified the mold as Penicillium notatum and the chemical it produced as Penicillin.
The full potential of Penicillin as a therapeutic antibiotic was later established by Ernest Chain and Howard Florey, who used it extensively during World War II. Fleming, Chain, and Florey shared the Nobel Prize in 1945 for their work on Penicillin.
Since the discovery of Penicillin, many other antibiotics have been purified from different microbes. Antibiotics have been crucial in treating deadly infectious diseases like plague, whooping cough, diphtheria, and leprosy, which previously caused high mortality rates.
Chemicals, Enzymes And Other Bioactive Molecules
Microbes are widely used in industrial production of various chemicals, enzymes, and other bioactive molecules:
- Organic Acids:
- Citric acid: Produced by the fungus Aspergillus niger.
- Acetic acid: Produced by the bacterium Acetobacter aceti.
- Butyric acid: Produced by the bacterium Clostridium butylicum.
- Lactic acid: Produced by the bacterium Lactobacillus.
- Alcohols: Ethanol is commercially produced using yeast (Saccharomyces cerevisiae).
- Enzymes:
- Lipases: Used in detergents to remove oily stains.
- Pectinases and Proteases: Used to clarify bottled fruit juices, making them clearer than homemade juices.
- Streptokinase: Produced by the bacterium Streptococcus and modified by genetic engineering. Used as a 'clot buster' to dissolve blood clots in patients who have suffered a heart attack (myocardial infarction).
- Bioactive Molecules:
- Cyclosporin A: An immunosuppressive agent used in organ transplant patients to prevent rejection. Produced by the fungus Trichoderma polysporum.
- Statins: Produced by the yeast Monascus purpureus. Used commercially as blood-cholesterol lowering agents. Statins work by competitively inhibiting the enzyme responsible for cholesterol synthesis.
Microbes In Sewage Treatment
Large quantities of waste water, containing human excreta and other organic matter, are generated daily in urban areas. This municipal waste-water is called sewage. Sewage also contains numerous microbes, many of which are pathogenic.
Directly discharging untreated sewage into natural water bodies like rivers and streams causes significant pollution. Therefore, sewage must be treated in Sewage Treatment Plants (STPs) before disposal to reduce its polluting potential.
Sewage treatment relies on the action of heterotrophic microbes naturally present in the sewage. The process typically involves two main stages:
Primary Treatment
This is the initial stage involving the physical removal of large and small particles from the sewage. It's essentially a physical separation process.
- Sequential filtration: Removes floating debris.
- Sedimentation: Grit (soil and small pebbles) is removed by allowing them to settle down.
All the solid material that settles during primary treatment forms the primary sludge. The liquid portion that remains above the sludge is called the effluent. The primary effluent is then transferred for secondary treatment.
Secondary Treatment Or Biological Treatment
This stage is also known as biological treatment because it heavily relies on the activity of microbes. The primary effluent is passed into large tanks called aeration tanks.
- In aeration tanks, the effluent is constantly agitated mechanically, and air is pumped in. This creates aerobic conditions that stimulate the vigorous growth of useful aerobic microbes.
- These microbes form masses associated with fungal filaments, creating mesh-like structures called flocs.
- While growing, these aerobic microbes consume a significant portion of the organic matter present in the effluent.
- This microbial activity significantly reduces the Biochemical Oxygen Demand (BOD) of the effluent.
Biochemical Oxygen Demand (BOD) is a measure of the amount of oxygen that would be consumed by microbes if all the organic matter in one liter of water were oxidised. High BOD indicates high organic pollution and greater polluting potential of the waste water.
Sewage is treated in aeration tanks until its BOD is significantly reduced, meaning the amount of biodegradable organic matter has been decreased.
After BOD reduction, the effluent is moved to a settling tank, where the microbial flocs are allowed to sediment. This sediment is called activated sludge.
- A small part of the activated sludge is pumped back into the aeration tank to serve as an inoculum, providing a source of active microbes for the new batch of effluent.
- The majority of the activated sludge is pumped into large tanks called anaerobic sludge digesters.
In anaerobic sludge digesters, other types of bacteria (anaerobic bacteria) digest the organic matter and the microbes in the activated sludge. During this anaerobic digestion, these bacteria produce a mixture of gases, including methane, hydrogen sulphide, and carbon dioxide. This gas mixture is called biogas, which is inflammable and can be used as a source of energy.
The effluent from the secondary treatment process, which has significantly reduced BOD, is then typically released into natural water bodies.
Microbes play a vital and irreplaceable role in sewage treatment. This microbial method has been used for over a century and remains the most effective technology for dealing with large volumes of waste water. The limited number of STPs compared to the volume of sewage generated due to increasing urbanisation leads to the discharge of untreated sewage into rivers, causing pollution and the spread of water-borne diseases.
Government initiatives like the Ganga Action Plan and Yamuna Action Plan aim to reduce river pollution by increasing the number of STPs that treat sewage before discharge.
Microbes In Production Of Biogas
Biogas is a mixture of gases, primarily methane, produced by the breakdown of organic matter through microbial activity under anaerobic conditions. It is a valuable renewable fuel source, often used for cooking and lighting in rural areas.
Microbes produce various gaseous end-products during their metabolism, depending on the type of microbes and the substrate they utilise. In fermentation processes (like curd, cheese, beverages), $\textsf{CO}_2$ is a major gas produced. However, certain bacteria, called methanogens, produce large amounts of methane along with $\textsf{CO}_2$ and $\textsf{H}_2$ while growing anaerobically on cellulosic material.
- A common methanogenic bacterium is Methanobacterium.
- Methanogens are found in anaerobic environments, such as the anaerobic sludge digesters used in sewage treatment.
- They are also present in the rumen (part of the stomach) of cattle. They help break down the cellulose in the cattle's food, contributing to their nutrition.
- Cattle dung (gobar) is rich in these methanogenic bacteria.
Therefore, cattle dung is a primary source for generating biogas, commonly called gobar gas.
A biogas plant typically consists of a large concrete tank (10-15 feet deep) where biodegradable organic wastes (especially cattle dung) are collected and mixed with water to form a slurry.
- A floating cover is placed over the slurry in the digester tank.
- Methanogenic bacteria in the slurry break down the organic matter anaerobically, producing biogas.
- The floating cover rises as biogas accumulates.
- An outlet pipe connects the tank to supply biogas for use.
- The digested slurry (spent slurry) is removed through another outlet and can be used as valuable fertiliser.
Biogas production technology in India was largely developed through efforts of the Indian Agricultural Research Institute (IARI) and Khadi and Village Industries Commission (KVIC). Biogas plants are particularly popular in rural areas where cattle dung is readily available, providing a sustainable source of energy for cooking and lighting.
Microbes As Biocontrol Agents
Controlling plant diseases and pests using biological methods is called biocontrol. This is an alternative to using chemical insecticides and pesticides, which are toxic, harmful to humans, animals, and the environment, and can pollute soil and water.
Organic farming practices emphasise biodiversity and natural predation for pest control, aiming for a balanced ecosystem rather than complete eradication of 'pests'. Biocontrol aims to keep pest populations at manageable levels using natural checks and balances.
Biological Control Of Pests And Diseases
Biocontrol measures significantly reduce dependence on toxic chemicals. Understanding the life cycles, feeding patterns, and habitats of both pests and their natural predators/parasites is key to effective biocontrol.
Examples of biocontrol agents:
- Insects as predators: Ladybirds (red and black beetles) are used to control aphids. Dragonflies are used to control mosquitoes.
- Bacteria: Bacillus thuringiensis (Bt) bacteria are used to control butterfly caterpillars. Bt spores are mixed with water and sprayed on plants. When insect larvae eat the plant, the bacteria are consumed. A toxin released in the alkaline gut of the larvae kills them. This Bt toxin is specific to certain insects and harmless to other organisms. Genetic engineering has allowed the transfer of Bt toxin genes into plants (e.g., Bt-cotton), making the plants themselves resistant to specific insect pests.
- Fungi: Trichoderma species are free-living fungi common in root ecosystems. They are effective biocontrol agents against several plant pathogens.
- Viruses: Baculoviruses, particularly those in the genus Nucleopolyhedrovirus, are pathogens that infect insects and other arthropods. They are excellent species-specific, narrow-spectrum insecticides, meaning they target specific pests without harming beneficial insects or other non-target organisms (plants, mammals, birds, fish). This is highly desirable for integrated pest management (IPM) and treating ecologically sensitive areas.
By using microbes and other biological agents for pest and disease control, we can minimise the use of harmful synthetic pesticides.
Microbes As Biofertilisers
Environmental pollution is a major concern, exacerbated by the overuse of chemical fertilisers in agriculture to meet increasing food demands. There is a growing shift towards organic farming and the use of biofertilisers as a more sustainable approach.
Biofertilisers are microorganisms that enhance the nutrient quality of the soil, improving its fertility. The main sources of biofertilisers are bacteria, fungi, and cyanobacteria.
Examples of biofertilisers and their actions:
- Bacteria:
- Rhizobium: Forms symbiotic associations in the root nodules of leguminous plants. They fix atmospheric nitrogen into organic forms (ammonia), which plants can absorb as nutrients.
- Free-living nitrogen-fixing bacteria in the soil, such as Azospirillum and Azotobacter, also fix atmospheric nitrogen, enriching the soil's nitrogen content.
- Fungi:
- Form symbiotic associations with plants called mycorrhiza. Many members of the genus Glomus form mycorrhiza.
- The fungal partner in mycorrhiza absorbs phosphorus from the soil (which is often immobile) and transfers it to the plant, enhancing phosphorus uptake.
- Mycorrhizal associations also provide other benefits to plants, including increased resistance to root-borne pathogens, tolerance to salinity and drought, and overall improved growth. In return, the fungus receives carbohydrates and other organic nutrients from the plant.
- Cyanobacteria (Blue-green algae): Autotrophic microbes found in aquatic and terrestrial environments. Many can fix atmospheric nitrogen (e.g., Anabaena, Nostoc, Oscillatoria).
- In paddy fields, cyanobacteria are important biofertilisers, fixing nitrogen and adding organic matter to the soil, which increases fertility.
Biofertilisers are commercially available and increasingly used by farmers to replenish soil nutrients and reduce dependence on chemical fertilisers, contributing to more sustainable agricultural practices.
Exercises
Question 1. Bacteria cannot be seen with the naked eyes, but these can be seen with the help of a microscope. If you have to carry a sample from your home to your biology laboratory to demonstrate the presence of microbes with the help of a microscope, which sample would you carry and why?
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Question 2. Give examples to prove that microbes release gases during metabolism.
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Question 3. In which food would you find lactic acid bacteria? Mention some of their useful applications.
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Question 4. Name some traditional Indian foods made of wheat, rice and Bengal gram (or their products) which involve use of microbes.
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Question 5. In which way have microbes played a major role in controlling diseases caused by harmful bacteria?
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Question 6. Name any two species of fungus, which are used in the production of the antibiotics.
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Question 7. What is sewage? In which way can sewage be harmful to us?
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Question 8. What is the key difference between primary and secondary sewage treatment?
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Question 9. Do you think microbes can also be used as source of energy? If yes, how?
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Question 10. Microbes can be used to decrease the use of chemical fertilisers and pesticides. Explain how this can be accomplished.
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Question 11. Three water samples namely river water, untreated sewage water and secondary effluent discharged from a sewage treatment plant were subjected to BOD test. The samples were labelled A, B and C; but the laboratory attendant did not note which was which. The BOD values of the three samples A, B and C were recorded as 20mg/L, 8mg/L and 400mg/L, respectively. Which sample of the water is most polluted? Can you assign the correct label to each assuming the river water is relatively clean?
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Question 12. Find out the name of the microbes from which Cyclosporin A (an immunosuppressive drug) and Statins (blood cholesterol lowering agents) are obtained.
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Question 13. Find out the role of microbes in the following and discuss it with your teacher.
(a) Single cell protein (SCP)
(b) Soil
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Question 14. Arrange the following in the decreasing order (most important first) of their importance, for the welfare of human society. Give reasons for your answer.
Biogas, Citric acid, Penicillin and Curd
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Question 15. How do biofertilisers enrich the fertility of the soil?
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