Chapter 10: Microbes in Human Welfare

Distribution and diversity

• Distribution: Microbes occur in soil, water, air, and inside plants and animals; some inhabit extreme environments.
• Extremes: They can live in thermal vents reaching about \(100\,^{\circ}\mathrm{C}\), deep soil, beneath thick snow, and in highly acidic environments.
• Groups: Bacteria, fungi, protozoa, and microscopic plants are microbes; viruses, viroids, and prions are also included. Prions are proteinaceous infectious agents.
• Cultures: Bacteria and many fungi grow on nutritive culture media, forming colonies visible to the naked eye.

Health and nutritional effects

• Nutrition: LAB increase the vitamin \(B_{12}\) content of curd.
• Gut health: LAB in curd inhibit disease-causing microbes in the stomach and support healthy intestinal microbial flora.
• Dairy products: LAB contribute to the production of curd, yoghurt, butter, and cheese.

Butter flavour

• Diacetyl: Leuconostoc species attack citrate in milk and produce diacetyl, which gives butter its characteristic flavour and aroma.

Raw cheese and water-based types

• Raw cheese: Prepared by curdling milk with LAB.
• Soft cheese: Contains 50–80% water.
• Semihard cheese: Contains about 45% water.
• Hard cheese: Contains less than 40% water.

Fermentation in foods

• Idli and dosa: Bacteria ferment the dough; carbon dioxide (\(CO_2\)) produced during fermentation makes it puff up.
• Bread: Baker’s yeast, Saccharomyces cerevisiae, ferments the dough.
• Toddy: A traditional drink made by fermenting sap from palms, especially in parts of southern India.
• Other foods: Microbes ferment fish, soybean, and bamboo shoots.

SCP: examples and importance

• Examples: Spirulina, yeasts, and Fusarium graminearum.
• Environmental benefit: SCP production can help manage industrial and agricultural wastes, reducing environmental pollution.

How probiotics benefit health

• Actions: Probiotics compete with pathogens, inactivate bacterial toxins, produce inhibitors of bacterial growth, and stimulate non-specific immunity.
• Examples: Lactobacilli and Streptococci are used as friendly bacteria.

Alcoholic fermentation

• Fermentation: Yeast and certain bacteria break down carbohydrates to form ethanol and carbon dioxide.
• Yeast species: *Saccharomyces cerevisiae* is brewer’s yeast; *S. ellipsoideus* is wine yeast; *S. sake* is sake yeast; *S. pyriformis* is used for ginger beer.

Meaning and action

• Word roots: “Anti” means against and “bio” means life; antibiotics act against disease-causing organisms but are pro-life in relation to humans.

Penicillin: discovery and importance

• Discovery: Alexander Fleming noticed that *Staphylococci* did not grow around a mould contaminating an unwashed culture plate.
• Source and name: The mould, *Penicillium notatum*, produced the inhibitory chemical Fleming named penicillin.
• Commercial extraction: Ernst Chain and Howard Florey contributed to its commercial extraction.
• Recognition and use: Fleming, Chain and Florey received the Nobel Prize in 1945; penicillin was widely used for wounded American soldiers during World War II.
• Other applications: Antibiotics are used against infections such as plague, whooping cough, leprosy and diphtheria.

Qualities of a good antibiotic

• Safe for the host: Harmless, with no side effects, and does not harm normal microflora of the alimentary canal.
• Effective: Acts quickly, destroys the pathogen and works against all its strains.

Key fact

• First microbial organic acid: Lactic acid; Scheele produced it through microbial fermentation in 1789.

Gas composition and formation

• Composition: Biogas contains 50–70% methane, 30–40% carbon dioxide and 1–5% hydrogen, with traces of oxygen, nitrogen and hydrogen sulphide.
• Variable products: Microbial gaseous end-products depend on the microbes and the organic substrates they use.
• Methanogens: Anaerobic bacteria that grow on cellulosic material and produce methane, carbon dioxide and hydrogen; examples include *Methanobacterium* and *Methanococcus*.

Methanogens in cattle and dung

• Habitats: Methanogens occur in anaerobic sewage sludge and in the rumen of cattle.
• Role in cattle: In the rumen, they help break down cellulose and contribute to cattle nutrition.
• Raw material: Cattle dung (gobar) is rich in these bacteria and is used to make biogas, also called gobar gas.

Plant outputs and importance

• Biogas outlet: A pipe supplies gas for cooking and lighting in nearby rural homes.
• Spent slurry: Removed through a separate outlet and used as manure in agricultural fields.
• Advantage: Biogas is an eco-friendly, pollution-free source of energy.
• Indian development: Biogas technology in India was developed mainly through the efforts of the Indian Agricultural Research Institute (IARI) and the Khadi and Village Industries Commission (KVIC).

• Chemical fertilisers: Overuse to meet rising agricultural demand pollutes water bodies and groundwater; fertiliser residues may also accumulate in crop plants.
• Organic farming: Raising crops using biologically derived manures, fertilisers and pesticides, along with practices such as resistant varieties, crop rotation and intercropping.

• Rhizobium: Forms root nodules on leguminous plants and fixes atmospheric nitrogen into organic forms that the plant can use.
• Free-living nitrogen fixers: Azospirillum and Azotobacter fix atmospheric nitrogen in soil, enriching its nitrogen content.

• Nutrient transfer: The fungal symbiont absorbs phosphorus from the soil and passes it to the plant.
• Other benefits: Mycorrhizal plants show resistance to root-borne pathogens, greater tolerance to salinity and drought, and improved growth and development.

• Cyanobacteria: Autotrophic microbes found in aquatic and terrestrial habitats; many can fix atmospheric nitrogen.
• Examples: Anabaena, Nostoc and Oscillatoria; cyanobacteria are especially useful as biofertilisers in paddy fields.
• Soil fertility: Blue-green algae add organic matter to soil; cyanobacterial biofertilisers are extremely low-cost.
• Rice fields: Aulosira fertilissima is considered the most active nitrogen fixer in Indian rice fields.

• Availability and use: Biofertilisers are commercially available and used by farmers to replenish soil nutrients and reduce dependence on chemical fertilisers.
• Yield: They can increase plant yield by 15–35% and remain effective under semi-arid conditions.
• Soil and plant health: They improve soil texture, help prevent pathogens from flourishing, and produce vitamins and growth-promoting biochemicals.
• Environmental benefit: Biofertilisers are non-polluting compared with excessive use of chemical fertilisers.