Biology: Class 12 — Chapter 8: Microbes in Human Welfare

Besides macroscopic plants and animals, microbes are the major components of biological systems on Earth. They are present everywhere – in soil, water, air, inside our bodies, and in extreme environments like thermal vents (geysers) where temperatures exceed 100°C, deep in soil, under thick snow layers, and in highly acidic environments.

• Diversity of Microbes: Includes protozoa, bacteria, fungi, microscopic animal and plant viruses, viroids, and proteinaceous infectious agents called prions.
• Culturability: Bacteria and many fungi can be grown on nutritive media to form visible colonies, which are highly useful in scientific studies.

Microbes and their products are used daily in various household activities:

• Curd Production: Micro-organisms like Lactobacillus and others, collectively called Lactic Acid Bacteria (LAB), grow in milk and convert it to curd. During growth, LAB produce acids that coagulate and partially digest milk proteins. A small amount of curd added as an inoculum (starter) contains millions of LAB, which multiply at suitable temperatures. LAB also improves nutritional quality by increasing Vitamin $B_{12}$ and plays a beneficial role in checking disease-causing microbes in the stomach.
• Dough Fermentation: Dough used for making foods like dosa and idli is fermented by bacteria. The puffed-up appearance of the dough is due to the production of $CO_2$ gas. Dough for making bread is fermented using baker's yeast (Saccharomyces cerevisiae).
• Traditional Drinks and Foods: Toddy, a traditional drink of southern India, is made by fermenting sap from palms. Microbes are also used to ferment fish, soyabean, and bamboo shoots.
• Cheese: One of the oldest food items utilizing microbes. Different varieties are known by their characteristic texture, flavor, and taste:
  • Swiss Cheese: Characterized by large holes due to the production of a large amount of $CO_2$ by the bacterium Propionibacterium sharmanii.
  • Roquefort Cheese: Ripened by growing a specific fungus on them, which imparts a particular flavor.

Industrial production requires growing microbes in very large vessels called fermentors to synthesize valuable products like beverages and antibiotics.

Municipal waste-water (sewage) contains large amounts of organic matter and pathogenic microbes. It must be treated in Sewage Treatment Plants (STPs) before disposal to make it less polluting. Treatment is carried out in two distinct stages:

Biogas is a mixture of gases (predominantly methane) produced by microbial activity, used as fuel.

• Methanogens: Anaerobic bacteria that grow on cellulosic material and produce large amounts of methane along with $CO_2$ and $H_2$. A common example is Methanobacterium.
• Occurrence: These bacteria are found in anaerobic sludge during sewage treatment and in the rumen (a part of the stomach) of cattle, where they help break down cellulose.
• Gobar Gas: The excreta (dung) of cattle, commonly called gobar, is rich in these bacteria and is used to generate biogas (gobar gas).
• Biogas Plant Structure: Consists of a concrete tank (10-15 feet deep) where bio-wastes and a slurry of dung are fed. A floating cover is placed over the slurry, which rises as gas is produced. The plant has an outlet pipe to supply biogas to nearby houses, and another outlet to remove spent slurry, which is used as fertilizer.
• Development in India: The technology was developed mainly due to the efforts of the Indian Agricultural Research Institute (IARI) and the Khadi and Village Industries Commission (KVIC).

Biocontrol refers to the use of biological methods for controlling plant diseases and pests, reducing reliance on toxic chemical pesticides and insecticides.

• Natural Predation: Organic farming relies on natural predation and maintaining biodiversity rather than eradication. Examples:
  • Ladybird (beetle with red and black markings) is used to control aphids.
  • Dragonflies are used to get rid of mosquitoes.
• Microbial Biocontrol Agents:
  • Bacillus thuringiensis (Bt): Used to control butterfly caterpillars. Dried spores are mixed with water and sprayed onto vulnerable plants. In the gut of the insect larvae, the toxin is released, killing them while leaving other insects unharmed. Genetic engineering has introduced Bt-toxin genes directly into plants, creating pest-resistant varieties like Bt-cotton.
  • Trichoderma: Free-living fungi common in root ecosystems that act as effective biocontrol agents against several plant pathogens.
  • Baculoviruses (Genus Nucleopolyhedrovirus): Pathogens that attack insects and other arthropods. They are excellent candidates for species-specific, narrow-spectrum insecticidal applications, having no negative impacts on plants, mammals, birds, fish, or non-target insects. They are highly useful in Integrated Pest Management (IPM) programs.

Biofertilisers are organisms that enrich the nutrient quality of the soil. The main sources are bacteria, fungi, and cyanobacteria:

• Bacteria:
  • Rhizobium: Forms symbiotic associations in the root nodules of leguminous plants, fixing atmospheric nitrogen into organic forms.
  • Azospirillum and Azotobacter: Free-living bacteria in the soil that fix atmospheric nitrogen, enriching soil nitrogen content.
• Fungi (Mycorrhiza): Fungi form symbiotic associations with plants called mycorrhiza. Many members of the genus Glomus form mycorrhiza. The fungal symbiont absorbs phosphorus from the soil and passes it to the plant. These plants also show resistance to root-borne pathogens, tolerance to salinity and drought, and overall increased growth.
• Cyanobacteria: Autotrophic microbes widely distributed in aquatic and terrestrial environments that can fix atmospheric nitrogen. Examples include Anabaena, Nostoc, and Oscillatoria. In paddy fields, cyanobacteria serve as an important biofertiliser. Blue-green algae also add organic matter to the soil, increasing its fertility.