Microbes in Human Welfare — Study Notes

1. Introduction

Microorganisms, or microbes, are organisms that are too small to be seen with the naked eye. They include bacteria, fungi, protozoa, viruses, and certain algae. Although some microbes cause diseases, the vast majority are beneficial to humans in countless ways. This chapter explores the diverse applications of microbes in human welfare, from the production of food and beverages to the generation of industrial products, the treatment of sewage, the production of biogas, and the improvement of agricultural productivity.

The chapter demonstrates that microbes have been used by humans for thousands of years, often without understanding the underlying biology. The production of curd, bread, cheese, and alcoholic beverages relies on microbes. Modern biotechnology has further expanded the use of microbes in the production of antibiotics, vitamins, enzymes, organic acids and other valuable products. Microbes also play a vital role in environmental management, such as sewage treatment and biogas production, and in agriculture as biofertilisers and biocontrol agents. Understanding the beneficial roles of microbes helps us appreciate their importance in maintaining a sustainable and healthy environment.

2. Microbes in Household Products

Microbes are used in the production of many household products, especially in the food industry.

2.1 Curd and Cheese

• Curd: The bacterium Lactobacillus is commonly used in the production of curd from milk. It multiplies in milk and converts it into curd, and also improves its nutritional quality by adding vitamin B12. A small amount of curd containing millions of Lactobacillus bacteria is added to milk to produce more curd.
• Cheese: Cheese is produced by the action of different microorganisms. The fungus Penicillium roqueforti is used to give the roquefort cheese its characteristic flavour, while Penicillium camemberti is used for camembert cheese.

2.2 Bread and Alcoholic Beverages

• Dough: The dough used for making bread is fermented by the yeast Saccharomyces cerevisiae (baker's yeast). The yeast produces carbon dioxide, which causes the dough to rise (puff up). This makes the bread soft and fluffy.
• Alcoholic beverages: Yeast is also used for the production of wine, beer and other alcoholic drinks. The yeast Saccharomyces cerevisiae ferments sugars into alcohol and carbon dioxide in the absence of oxygen (anaerobic fermentation). This process of fermentation was discovered and exploited by ancient civilisations for brewing. The alcoholic drinks produced by fermentation include wine (from grapes), beer (from barley) and toddy (from palm sap).

3. Microbes in Industrial Products

Microbes are used on a large scale in industrial fermenters to produce valuable products such as organic acids, alcohols, antibiotics, vitamins and enzymes.

3.1 Organic Acids and Alcohols

• Acetic acid (vinegar): Produced by the bacterium Acetobacter aceti.
• Lactic acid: Produced by Lactobacillus.
• Citric acid: Produced by the fungus Aspergillus niger.
• Alcohols: Produced by Saccharomyces cerevisiae.

3.2 Enzymes

• Lipases: Used in detergent formulations for removing oily stains.
• Pectinases: Used to clarify fruit juices.
• Proteases: Used in the leather industry for dehairing and tanning, and in the food industry.
• Streptokinase: Produced by the bacterium Streptococcus, used as a clot buster for removing clots from the blood vessels of patients who have had a myocardial infarction (heart attack).

3.3 Antibiotics

Antibiotics are chemical substances produced by microorganisms that can kill or inhibit the growth of other microorganisms. The first antibiotic discovered was penicillin, produced by the mould Penicillium notatum, discovered by Alexander Fleming in 1928. Antibiotics have revolutionised medicine and saved millions of lives. Other important antibiotics include streptomycin (from Streptomyces griseus) and erythromycin. Antibiotics are now produced in large quantities in industrial fermenters.

4. Microbes in Sewage Treatment

Sewage is the municipal waste water that contains large amounts of organic matter and microbes. The treatment of sewage is essential to prevent the pollution of water bodies and the spread of disease. Microbes play a central role in sewage treatment.

4.1 Steps of Sewage Treatment

1. Primary treatment: The physical removal of large and small particles from the sewage through filtration and sedimentation. Grit and other particles are removed, and the settled solids (primary sludge) are collected, leaving behind the effluent.
2. Secondary treatment (biological treatment): The effluent is passed into a large aeration tank where it is constantly agitated and aerated. This allows the growth of aerobic microbes that consume most of the organic matter in the effluent. After a few hours, the microbes form flocs (masses of bacteria associated with fungal filaments), which settle, reducing the biochemical oxygen demand (BOD) of the effluent. The effluent is then passed into a settling tank where the bacterial flocs settle. The settled sludge is pumped into an anaerobic sludge digester.
3. Anaerobic digestion: In the anaerobic sludge digester, anaerobic bacteria digest the sludge in the absence of oxygen, producing biogas (methane) and other gases.

4.2 BOD (Biochemical Oxygen Demand)

BOD is the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria. The sewage is considered treated when the BOD of the effluent is significantly reduced, because the organic matter has been consumed by the aerobic microbes. A high BOD indicates high organic pollution; a low BOD indicates that the organic matter has been degraded.

5. Microbes in Production of Biogas

Biogas is a mixture of gases, primarily methane and carbon dioxide, produced by the microbial fermentation of organic matter in the absence of oxygen. The bacteria that produce biogas are called methanogens. The most common methanogen is Methanobacterium, which is found in the rumen (stomach) of cattle and in the anaerobic sludge of sewage treatment plants.

5.1 Gobar Gas Plants

In gobar gas plants (cattle dung gas plants), cattle dung is mixed with water and introduced into a biogas plant. The slurry of dung provides the food for the methanogens. In the absence of oxygen, the methanogens ferment the organic matter to produce biogas. The biogas is used as a source of energy for cooking and lighting. The spent slurry is used as manure because it is rich in nitrogen and phosphorus. Biogas production is a renewable and environment-friendly source of energy and also helps in the disposal of animal waste.

6. Microbes as Biocontrol Agents and Biofertilisers

6.1 Biocontrol Agents

Biocontrol is the use of biological methods to control plant pests and diseases, as an alternative to chemical pesticides, which are harmful to the environment and human health. - Bacillus thuringiensis (Bt): This bacterium is used to control insect pests. It produces a protein (crystal toxin) that is lethal to certain insect larvae. Bt cotton is a genetically modified crop that contains the Bt toxin gene. - Trichoderma: This fungus is used as a biocontrol agent against several plant pathogens, especially soil-borne diseases. - Baculoviruses: These are viruses that attack insects and other arthropods. They are species specific and are used in biological pest management.

6.2 Biofertilisers

Biofertilisers are organisms that enrich the nutrient quality of the soil, mainly by fixing atmospheric nitrogen or solubilising phosphorus. - Rhizobium: A symbiotic nitrogen-fixing bacterium that lives in the root nodules of leguminous plants and fixes atmospheric nitrogen into a form the plant can use. - Azospirillum and Azotobacter: Free-living nitrogen-fixing bacteria that enrich the soil with nitrogen. - Blue-green algae (cyanobacteria): Like Anabaena and Nostoc, which are important nitrogen fixers, especially in paddy fields. They also add organic matter to the soil. - Mycorrhiza: A symbiotic association of fungi with plant roots. The fungus helps the plant absorb phosphorus and other nutrients from the soil, and the plant provides the fungus with carbohydrates. Mycorrhiza helps in the uptake of phosphorus and increases the tolerance of plants to stress.

Quick Revision Tables

Table 1: Microbes and Their Industrial Products

Table 2: Microbes in Food and Agriculture

Table 3: Biogas and Sewage Treatment

Mind Map

Important Diagrams (SVG)

Common Mistakes

1. Students think yeast is used only for alcohol; the same yeast (Saccharomyces cerevisiae) is also used for raising bread dough through CO2 production.
2. Penicillin is said to be produced by a bacterium; it is produced by the mould Penicillium notatum, discovered by Alexander Fleming.
3. BOD is said to measure oxygen produced; BOD measures the oxygen consumed by bacteria in oxidising the organic matter in one litre of water.
4. Sewage is said to be fully cleaned by primary treatment; primary treatment only removes particles physically, and biological (secondary) treatment is needed to degrade organic matter.
5. Biogas is said to be produced by aerobic bacteria; it is produced by anaerobic methanogens like Methanobacterium in the absence of oxygen.
6. All nitrogen fixers are said to be bacteria; blue-green algae (cyanobacteria) like Anabaena and Nostoc also fix nitrogen, especially in paddy fields.
7. Streptokinase is said to cause clotting; it is a clot buster used to remove blood clots after a heart attack.
8. Biofertilisers are said to be chemical substances; biofertilisers are living organisms that enrich the soil with nutrients.

Exam Tips

1. Learn the microorganism-product pairs: Lactobacillus (curd), Acetobacter aceti (acetic acid), Aspergillus niger (citric acid), Penicillium notatum (penicillin).
2. Remember Saccharomyces cerevisiae for both bread and alcohol, and the fact that fermentation is anaerobic.
3. For sewage treatment, remember the sequence: primary (filtration, sedimentation) to secondary (aeration, flocs) to anaerobic digester (biogas), and that BOD drops after treatment.
4. Methanogens (Methanobacterium) in the rumen of cattle and gobar gas plants are high-frequency questions.
5. For biocontrol, remember Bt (Bacillus thuringiensis), Trichoderma (fungus) and baculoviruses (species specific).
6. For biofertilisers, know Rhizobium (symbiotic in legumes), Azotobacter/Azospirillum (free living) and cyanobacteria in paddy.
7. Distinguish antibiotics (kill/inhibit microbes) from enzymes and organic acids, and note that streptokinase is a clot buster.

Conclusion

Microbes in human welfare reveal the enormous beneficial potential of organisms that are often only associated with disease. From the everyday production of curd, bread and cheese to the industrial manufacture of acids, enzymes, antibiotics and vitamins, microbes are indispensable partners of human civilisation. Their role in sewage treatment, where they degrade organic waste and reduce BOD, and in biogas production, where methanogens convert animal waste into clean energy, demonstrates their environmental importance. In agriculture, biocontrol agents and biofertilisers offer sustainable alternatives to harmful chemicals, enriching the soil and controlling pests naturally. The chapter thus shifts the perspective from microbes as pathogens to microbes as engines of food production, industry, sanitation and sustainable agriculture, laying a foundation for the biotechnology chapters that harness these microscopic organisms through genetic engineering.