Microbes in Human Welfare

🎯 Key Points

• Curd: Lactobacillus (bacterium); Bread rise: Saccharomyces cerevisiae (yeast, CO₂ production); Swiss cheese holes: Propionibacterium sharmanii (gas production)
• Penicillin discovered by Fleming from Penicillium notatum mould; full therapeutic potential established later by Chain and Florey — discovery vs development are different people/steps, often tested separately
• Sewage treatment: Primary (physical removal) → Secondary/biological (aerobic microbes form flocs, consume organic matter) → sludge digesters (ANAEROBIC bacteria produce biogas) — aerobic in the tank, anaerobic in the digester, a common mix-up
• Biofertilisers (add nutrients: Rhizobium, Azotobacter, mycorrhiza) vs Biocontrol agents (kill pests: Bt, Trichoderma, baculoviruses) — different jobs, easy to conflate in MCQs
• Bt toxin (cry protein) kills insect larvae specifically; same mechanism used in both bio-spray form and genetically engineered Bt crops

Microbes in Food and Beverages

• Curd: Lactobacillus and other lactic acid bacteria (LAB) grow in milk and convert it to curd; also improves nutritional value by increasing vitamin B12
• Bread: Saccharomyces cerevisiae (baker's yeast) used as a leavening agent; produces CO2 that makes dough rise
• Alcoholic beverages: yeast ferments fruit/grain sugars; fermented and distilled malt produces whisky, brandy, rum; wine and beer require fermentation but no distillation
• Swiss cheese: large holes produced by the bacterium Propionibacterium sharmanii during ripening
• Roquefort cheese: ripened by growing the fungus Penicillium roqueforti on it, giving its characteristic flavour

Microbes in Medicine

• Antibiotics: chemical substances produced by microbes that can kill or retard growth of other (disease-causing) microbes; penicillin was discovered by Alexander Fleming from the mould Penicillium notatum, and its full potential was later established by Ernst Chain and Howard Florey
• Statins: produced by the yeast Monascus purpureus; lower blood cholesterol by competitively inhibiting the enzyme that catalyses cholesterol synthesis
• Vaccines: microbial products or weakened/dead pathogens used to stimulate the immune system to develop memory against a specific disease without causing the disease itself

Microbes in Sewage Treatment

• Primary treatment: physical removal of large and small particles through filtration and sedimentation
• Secondary (biological) treatment: sewage passed into large aeration tanks where it is agitated and air is pumped in, allowing aerobic microbes to grow and consume organic matter, forming flocs (masses of bacteria with fungal filaments)
• Effluent from this tank is passed into a settling tank where the flocs sediment, called activated sludge; a small part is used as inoculum and the rest is pumped into anaerobic sludge digesters, where anaerobic bacteria digest it producing biogas (methane, H2S, CO2)
• The clear effluent from the secondary treatment is gradually released into natural water bodies after BOD (Biological Oxygen Demand) is checked; lower BOD indicates lower pollution

Microbes as Biofertilisers and Biocontrol Agents

• Biofertilisers: organisms that enrich soil nutrient quality; main types are bacteria (Rhizobium, Azospirillum, Azotobacter), cyanobacteria (Anabaena, Nostoc, also Azolla-Anabaena symbiosis used in paddy fields), and fungi (mycorrhiza, e.g. Glomus, which helps absorb phosphorus from soil)
• Biopesticides/biocontrol agents: Bacillus thuringiensis (Bt) produces toxin crystals (cry proteins) lethal to insect larvae and is used as a spray or in genetically modified Bt crops; the dreaded butterfly pest of cabbage is controlled by the bacterium Trichoderma; ladybird and dragonflies are useful biocontrol agents against aphids and mosquitoes respectively
• Biocontrol reduces dependence on toxic chemical pesticides and herbicides, protecting the environment

Microbes in Household Products

• Fermented foods: the dough used to make idli and dosa is fermented by bacteria, and its puffed-up (spongy) appearance is due to the CO₂ gas produced during fermentation; the dough used for bread is similarly fermented by baker's yeast (Saccharomyces cerevisiae).
• Toddy, a traditional drink of some parts of southern India, is made by fermenting the sap obtained from palms.
• Microbes are also used to ferment fish, soybean and bamboo shoots to prepare various foods; the large holes in Swiss cheese are due to CO₂ produced by the bacterium Propionibacterium sharmanii, while Roquefort cheese is ripened by growing the fungus Penicillium roqueforti on it.
• The Lactic Acid Bacteria (LAB) present in curd (such as Lactobacillus) not only improve its nutritional quality by increasing vitamin B₁₂, but in the human stomach they also play a beneficial role in checking disease-causing microbes.

Microbes in Industrial Products

• For industrial production, microbes are grown on a very large scale in vessels called fermentors (bioreactors), which can hold thousands of litres of culture; the products include beverages, antibiotics, organic acids, alcohols and enzymes.
• Fermented beverages: yeast (Saccharomyces cerevisiae, also called brewer's yeast) is used to ferment malted cereals and fruit juices to produce ethanol. Wine and beer are produced without distillation, whereas whisky, brandy and rum are produced by distillation of the fermented broth.
• Organic acids: Aspergillus niger (a fungus) produces citric acid; Acetobacter aceti (a bacterium) produces acetic acid; Clostridium butylicum (a bacterium) produces butyric acid; and Lactobacillus (a bacterium) produces lactic acid.
• Enzymes: lipases are used in detergent formulations to remove oily stains from laundry; pectinases and proteases are used to clarify bottled fruit juices; and streptokinase (produced by the bacterium Streptococcus and modified by genetic engineering) is used as a "clot buster" to remove clots from the blood vessels of heart-attack patients.
• Bioactive molecules: cyclosporin A (produced by the fungus Trichoderma polysporum) is used as an immunosuppressive agent in organ-transplant patients, and statins (produced by the yeast Monascus purpureus) lower blood cholesterol by competitively inhibiting the enzyme responsible for cholesterol synthesis.

Biogas Production and Methanogens

• Biogas is a mixture of gases (with methane as the predominant component) produced by microbial activity, and it is used as a fuel.
• Certain bacteria that grow anaerobically on cellulosic material produce large amounts of methane along with CO₂ and H₂; these bacteria are collectively called methanogens, a common example being Methanobacterium.
• Methanogens occur in the anaerobic sludge produced during sewage treatment and also in the rumen (a part of the stomach) of cattle, where they help in the breakdown of cellulose and thus play an important role in the nutrition of cattle. The dung (excreta) of cattle, called gobar, is rich in these microbes and is therefore used to generate biogas.
• A biogas plant consists of a concrete tank (digester) into which bio-wastes and cattle-dung slurry are fed; a floating cover rises as the gas is produced and stored. The gas is supplied through pipes for cooking and lighting, while the spent slurry is removed and used as manure. Biogas technology was developed in India largely by the IARI and the KVIC.

🚀 NEET Advanced Edge

Why activated sludge treatment is a two-stage oxygen strategy: The secondary treatment tank is deliberately aerated to favour AEROBIC microbes that rapidly break down organic matter into flocs; the leftover sludge is then moved to a sealed digester where ANAEROBIC bacteria take over to produce biogas — switching oxygen conditions selects for completely different microbial communities suited to each stage's goal (fast decomposition vs energy/biogas recovery).

Why Bt crops don't harm non-target insects or humans: The cry protein toxin requires an alkaline gut environment to become activated into its toxic form — present in many insect larvae guts but absent in the acidic human digestive tract, making the toxin selectively lethal rather than a general-purpose poison (the same safety logic seen in Bt cotton/biopesticides).

Worked reasoning: A water sample from a river near a sewage outflow shows a HIGH BOD value. What does this directly indicate, and why? Answer: High BOD means high organic matter content, because BOD measures the oxygen consumed by microorganisms as they decompose that organic matter — more organic pollution means more oxygen demand from decomposers, depleting dissolved oxygen and stressing aquatic life.