Microbes in Human Welfare: Class 12 Biology NCERT Guide

Microbes in Everyday Household Products

Our kitchens are teeming with microbial activity that we use to our advantage every day. A classic example is the production of curd from milk. Lactic Acid Bacteria (LAB), such as Lactobacillus, are added to milk as a starter (inoculum). These bacteria multiply and, during their growth, produce lactic acid. This acid causes the partial digestion and coagulation of milk proteins (casein), transforming milk into curd. This process also improves the nutritional quality of curd by increasing its Vitamin B12 content. Similarly, the puffy appearance of dough used for making dosa, idli, and bread is due to microbial fermentation. Bacteria or yeast (Saccharomyces cerevisiae, also known as baker's yeast) ferment the sugars in the dough, producing carbon dioxide (CO2) gas. This gas gets trapped in the dough, causing it to rise and giving it a soft, spongy texture. Even traditional drinks like 'Toddy' from Southern India are made by fermenting sap from palms.

The Process of Sewage Treatment: A Step-by-Step Guide

1. Step 1: Primary Treatment (Physical Process) — Raw sewage first undergoes primary treatment. This involves the physical removal of large and small particles. Initially, floating debris is removed by sequential filtration. Then, grit (soil and small pebbles) is removed by sedimentation. The solid material that settles down forms the 'primary sludge', and the supernatant liquid is called the 'primary effluent'.
2. Step 2: Secondary Treatment (Biological Process) — The primary effluent is passed into large aeration tanks. Here, it is constantly agitated mechanically and air is pumped into it. This encourages the vigorous growth of useful aerobic microbes into 'flocs' – masses of bacteria associated with fungal filaments. These microbes consume the major part of the organic matter in the effluent, significantly reducing its Biochemical Oxygen Demand (BOD). BOD is a measure of the organic matter present in water; a lower BOD indicates less polluted water.
3. Step 3: Sludge Digestion and Biogas Production — Once the BOD is reduced, the effluent is passed into a settling tank where the bacterial 'flocs' are allowed to sediment. This sediment is called 'activated sludge'. A small part of it is pumped back into the aeration tank to serve as an inoculum. The remaining major part of the sludge is pumped into large tanks called anaerobic sludge digesters. Here, anaerobic bacteria digest the organic matter in the sludge, producing a mixture of gases called biogas (mainly methane, hydrogen sulfide, and CO2), which can be used as a source of energy. The treated effluent is then released into natural water bodies.

Industrial Applications: Examples of Microbial Products

• Fermented Beverages: The yeast Saccharomyces cerevisiae (Brewer's Yeast) is fundamental for producing ethanol. It ferments malted cereals and fruit juices to create beverages. Products like wine and beer are produced without distillation, whereas whisky, brandy, and rum require distillation to increase the alcohol concentration.
• Antibiotics: These are life-saving drugs produced by microbes to kill or inhibit other harmful microbes. The first antibiotic, Penicillin, was discovered from the mould Penicillium notatum. Other examples include Streptomycin and Tetracycline, which are crucial in treating bacterial infections.
• Organic Acids & Enzymes: Microbes are used as 'cell factories' to produce chemicals. For example, Aspergillus niger (a fungus) produces citric acid, and Acetobacter aceti (a bacterium) produces acetic acid. Enzymes like lipases (used in detergents to remove oily stains) and pectinases/proteases (used to clarify bottled juices) are also produced microbially.
• Bioactive Molecules: Certain microbes produce complex molecules with medical applications. Cyclosporin A, an immunosuppressive agent used in organ transplant patients, is produced by the fungus Trichoderma polysporum. Statins, produced by the yeast Monascus purpureus, act as blood-cholesterol lowering agents by competitively inhibiting the enzyme for cholesterol synthesis.

Exam Tip: Don't Confuse Biocontrol Agents and Biofertilizers

A very common point of confusion for students is the difference between biocontrol agents and biofertilizers. Remember this simple distinction:

• Biocontrol Agents are for PROTECTION. They are living organisms used to control and eliminate plant pests and pathogens, reducing the need for chemical pesticides. Examples: Bacillus thuringiensis (Bt) controls butterfly caterpillars; the fungus Trichoderma controls soil-borne plant pathogens.

• Biofertilizers are for NUTRITION. They are organisms that enrich the nutrient quality (like nitrogen and phosphorus) of the soil, promoting plant growth and reducing the need for chemical fertilizers. Examples: Rhizobium bacteria in the root nodules of legumes fix atmospheric nitrogen; Mycorrhiza (fungi) helps plants absorb phosphorus from the soil.

Practice Questions with Solutions

• Q: A sewage treatment plant is not functioning properly, leading to a high BOD in the treated effluent. What does this indicate, and which stage of treatment is likely affected? A: Step 1: High BOD (Biochemical Oxygen Demand) indicates that there is a large amount of organic matter remaining in the water. This means the water is still highly polluted. Step 2: The breakdown of organic matter is the primary function of the Secondary (or Biological) Treatment stage. In this stage, aerobic microbes in 'flocs' consume the organic waste. Final answer: A high BOD indicates that the water is still polluted with organic waste. This suggests that the Secondary (Biological) Treatment stage is not functioning effectively, as the microbes are not adequately breaking down the organic matter.
• Q: Match the microbe in Column I with its commercial product in Column II. Column I: (a) Aspergillus niger, (b) Monascus purpureus, (c) Acetobacter aceti, (d) Lactobacillus Column II: (i) Acetic Acid, (ii) Curd, (iii) Statin, (iv) Citric Acid A: Step 1: Recall the industrial and household products of each microbe. Aspergillus niger is a fungus used to produce an organic acid. Step 2: Monascus purpureus is a yeast that produces a bioactive molecule for lowering cholesterol. Step 3: Acetobacter aceti is a bacterium that produces the main component of vinegar. Step 4: Lactobacillus is the Lactic Acid Bacterium used in dairy fermentation. Final answer: The correct matches are: (a) - (iv) Citric Acid, (b) - (iii) Statin, (c) - (i) Acetic Acid, (d) - (ii) Curd.
• Q: Farmers have been advised to stop using chemical pesticides. Suggest two microbial biocontrol agents they can use to protect their crops from pests and diseases. A: Step 1: Identify the purpose - to protect crops from pests (insects) and diseases (pathogens). Step 2: Recall a microbe that acts as a bio-pesticide. Bacillus thuringiensis (Bt) is a bacterium whose spores are available in sachets as a dry powder. When sprayed on plants and ingested by insect larvae (like butterfly caterpillars), the toxin is released in their gut and kills them. Step 3: Recall a microbe that acts against pathogens. Fungi of the genus Trichoderma are free-living in root ecosystems and are very effective biocontrol agents for several soil-borne plant pathogens. Final answer: Two microbial biocontrol agents are: 1. Bacillus thuringiensis (Bt) to control insect pests like caterpillars. 2. Trichoderma species (a fungus) to control soil-borne plant diseases.
• Q: Explain the role of Saccharomyces cerevisiae in the baking and brewing industries. Why do the products (bread vs. beer) differ so much? A: Step 1: Identify the common process. Saccharomyces cerevisiae (also known as Baker's or Brewer's Yeast) performs fermentation in both processes. Step 2: Explain its role in baking. In dough, the yeast ferments sugars to produce CO2 and ethanol. The CO2 gas gets trapped, causing the dough to rise and become spongy. The ethanol evaporates during baking. Step 3: Explain its role in brewing. In brewing, the yeast ferments sugars in malted cereals or fruit juices to produce ethanol as the main product. The CO2 is often a by-product. Step 4: Explain the difference. The final products differ based on the raw material used and the post-fermentation processing. In baking, the goal is leavening (rising). In brewing, the goal is alcohol production. Furthermore, some alcoholic beverages undergo distillation to increase ethanol content, which is not done for bread. Final answer: Saccharomyces cerevisiae ferments sugars to produce CO2 and ethanol. In baking, the CO2 is desired to make bread rise. In brewing, the ethanol is the desired product. The products differ due to the raw materials and whether the primary goal is leavening or alcohol production (with or without distillation).

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