Chapter 8: Microbes in Human Welfare | Summary

Industrial-scale production of microbial products requires large vessels called fermentors. Yeasts, especially Saccharomyces cerevisiae, ferment malted cereals and fruit juices to produce ethanol. Wine and beer are produced without distillation, while whisky, brandy and rum require distillation of fermented broth.

Antibiotics, one of the greatest discoveries of the 20th century, are chemical substances produced by microbes that kill or retard growth of disease-causing microbes. Penicillin, discovered by Alexander Fleming from the mould Penicillium notatum (with its full potential established by Ernest Chain and Howard Florey), was the first antibiotic. Fleming, Chain and Florey received the Nobel Prize in 1945. Antibiotics have transformed the treatment of diseases like plague, whooping cough, diphtheria, and leprosy.

Microbes are also used to produce organic acids (citric acid by Aspergillus niger, acetic acid by Acetobacter aceti, butyric acid by Clostridium butylicum, lactic acid by Lactobacillus), enzymes (lipases for detergents, pectinases and proteases for fruit juice clarification), and bioactive molecules such as Streptokinase (clot buster from Streptococcus), Cyclosporin A (immunosuppressant from Trichoderma polysporum), and Statins (cholesterol-lowering agents from Monascus purpureus).

Sewage, or municipal waste water, contains large amounts of organic matter and pathogenic microbes. It is treated in Sewage Treatment Plants (STPs) in two stages. Primary treatment involves physical removal of floating debris by filtration and of grit and solids by sedimentation. The resulting primary sludge and supernatant effluent are separated before further treatment.

Secondary (biological) treatment passes the primary effluent into aeration tanks where aerobic microbes form flocs — masses of bacteria associated with fungal filaments. These microbes consume organic matter, reducing the Biochemical Oxygen Demand (BOD). After BOD reduction, the effluent passes into a settling tank where activated sludge forms. Part of this sludge is reused as inoculum; the rest is sent to anaerobic sludge digesters where anaerobic bacteria produce biogas (methane, H2S, CO2). The treated effluent is released into rivers. Plans like Ganga Action Plan and Yamuna Action Plan aim to expand STPs to address rising urban sewage.

Biogas is a mixture of gases predominantly containing methane, produced by microbial activity on organic matter. Methanogens are bacteria that grow anaerobically on cellulosic material and produce large amounts of methane along with CO2 and H2. A common methanogen is Methanobacterium. These bacteria are found in anaerobic sludge, the rumen of cattle, and in cattle dung (gobar), which is rich in methanogens.

Biogas plants consist of a concrete tank where bio-wastes and cattle dung slurry are fed. A floating cover rises as gas is produced by microbial activity. Biogas is piped to nearby houses for cooking and lighting. Spent slurry is used as fertiliser. Biogas technology in India was developed primarily by IARI (Indian Agricultural Research Institute) and KVIC (Khadi and Village Industries Commission). Biogas plants are especially common in rural areas.

Biocontrol refers to using biological methods to control plant diseases and pests, reducing dependence on toxic chemical pesticides and insecticides. The organic farming philosophy promotes biodiversity and natural predation rather than eradication of all insects. Beneficial insects like Ladybirds (eliminate aphids) and Dragonflies (eliminate mosquitoes) are examples of natural biocontrol.

Bacillus thuringiensis (Bt) is a bacterium whose spores, when sprayed on plants and consumed by insect larvae, release a toxin that kills the larvae without harming other insects. Bt toxin genes have also been introduced into plants (e.g., Bt-cotton) via genetic engineering. Trichoderma species are free-living fungi used as effective biocontrol agents against several plant pathogens. Baculoviruses, especially in the genus Nucleopolyhedrovirus, are species-specific pathogens of insects and arthropods with no negative impact on non-target organisms, making them ideal for Integrated Pest Management (IPM).

Biofertilisers are organisms that enrich the nutrient quality of the soil, reducing the need for chemical fertilisers. The main sources are bacteria, fungi, and cyanobacteria. Rhizobium forms symbiotic associations with leguminous plant roots, fixing atmospheric nitrogen into organic forms used by the plant. Free-living nitrogen-fixing bacteria include Azospirillum and Azotobacter, which enrich soil nitrogen directly.

Fungi, particularly Glomus species, form mycorrhizal associations with plants, absorbing phosphorus from the soil and transferring it to the plant. These associations also confer resistance to root-borne pathogens, tolerance to salinity and drought, and overall improved plant growth. Cyanobacteria like Anabaena, Nostoc, and Oscillatoria are autotrophic nitrogen-fixers widely used in paddy fields as biofertilisers, also adding organic matter to soil. Many biofertilisers are now commercially available in India.

Chapter 8 demonstrates that microbes, despite their microscopic size, are indispensable contributors to human welfare across multiple domains. From converting milk to curd and producing antibiotics like Penicillin, to treating millions of gallons of sewage daily and generating renewable biogas, microbes perform tasks that neither chemical processes nor human-made technologies can fully replicate. Their roles as biocontrol agents and biofertilisers offer sustainable alternatives to toxic pesticides and chemical fertilisers, pointing towards greener agricultural practices.

For CBSE Board examinations, this chapter is highly significant because it combines concepts of microbiology, biotechnology, environmental science, and agriculture into a single applied framework. Students must be thorough with specific organisms and their products, processes like sewage treatment and biogas production, the discovery and significance of antibiotics, and the ecological benefits of biocontrol and biofertilisers. Questions from this chapter appear regularly in both short-answer and long-answer formats.