Microbes are microscopic organisms — bacteria, fungi, protozoans, viruses and microscopic algae. CBSE Class 12 has a chapter on Microbes in Human Welfare. NEET asks specific names, products and applications almost every year.
Core Concepts
Microbes in household
Lactobacillus in curd. Yeast (Saccharomyces cerevisiae) in bread, idli and beer. Propionibacterium in Swiss cheese. Penicillium roqueforti in blue cheese.
The process behind curd making is simple but elegant: when we add a small amount of old curd to warm milk, the Lactobacillus bacteria multiply rapidly at 37-40°C, fermenting lactose into lactic acid. The acid lowers pH, causing casein (milk protein) to coagulate. This is why warm temperature is essential — too cold and the bacteria grow slowly, too hot and they die.
Microbes in industry
Fermented beverages (wine, beer, whisky). Antibiotics (penicillin from Penicillium, streptomycin from Streptomyces). Organic acids (citric from Aspergillus niger, acetic from Acetobacter). Enzymes (lipase, protease).
Here is a table of the most NEET-relevant industrial products:
| Product | Microbe | Type of Microbe |
|---|---|---|
| Penicillin | Penicillium notatum | Fungus |
| Streptomycin | Streptomyces griseus | Bacterium (actinomycete) |
| Citric acid | Aspergillus niger | Fungus |
| Acetic acid | Acetobacter aceti | Bacterium |
| Ethanol | Saccharomyces cerevisiae | Fungus (yeast) |
| Lactic acid | Lactobacillus | Bacterium |
| Butyric acid | Clostridium butylicum | Bacterium |
| Cyclosporin A | Trichoderma polysporum | Fungus |
| Statin (lovastatin) | Monascus purpureus | Fungus (yeast) |
NEET loves matching microbe names to products. Make a table like the one above and revise it weekly. One question from this table appears almost every year.
Microbes in sewage treatment
Primary — physical removal. Secondary — aeration tanks with floc-forming microbes that reduce BOD. Tertiary — chemical or advanced treatment. Anaerobic digesters produce biogas (methane).
Let us walk through the secondary treatment in detail because NEET asks about it:
After large solids are removed physically, the liquid enters large tanks where vigorous aeration is provided. This encourages the growth of aerobic bacteria and fungi.
Aerobic microbes form mesh-like masses called flocs. These flocs consume the bulk of organic matter in the sewage, reducing BOD (Biochemical Oxygen Demand) from around 300 mg/L to under 30 mg/L.
The effluent from the aeration tank goes to a settling tank. The flocs settle as activated sludge. A small portion of this sludge is recycled back to the aeration tank as inoculum.
The remaining sludge goes to an anaerobic digester where methanogens (like Methanobacterium) break it down, producing a mixture of gases — mainly methane (CH4) and CO2. This biogas can be used as fuel.
Microbes in agriculture
Rhizobium in legume nodules fixes nitrogen. Azotobacter free in soil also fixes nitrogen. Mycorrhiza (Glomus) helps phosphorus uptake. Blue-green algae fix nitrogen in paddy fields.
Biofertilisers vs chemical fertilisers: Biofertilisers are organisms that enrich the soil. They do not pollute groundwater, do not cause eutrophication, and improve soil health over time. Chemical fertilisers give quick results but degrade soil structure and contaminate water bodies.
Key associations to remember:
- Rhizobium — symbiotic, only in legume root nodules, fixes to
- Azotobacter and Azospirillum — free-living in soil, fix atmospheric nitrogen
- Glomus (mycorrhiza) — fungal association with plant roots, improves phosphorus uptake
- Anabaena and Nostoc — cyanobacteria (blue-green algae), fix nitrogen in paddy fields
Microbes as biocontrol
Bacillus thuringiensis (Bt) produces toxin against insects. Trichoderma controls fungal pathogens. Baculovirus targets specific insect species without harming others.
The Bt toxin story is worth knowing in detail: Bt produces a crystal protein (Cry protein) that is inactive in the bacteria. When an insect larva eats it, the alkaline gut pH activates the toxin. It creates pores in the gut wall, causing the larva to die. This specificity is why Bt is safe for humans — our gut is acidic, not alkaline.
Worked Examples
Lactobacillus converts lactose to lactic acid, lowering pH and causing milk proteins to coagulate. The lactic acid also inhibits spoilage microbes, preserving the curd.
Fleming left a Petri dish of Staphylococcus near an open window. A Penicillium mould contaminated it. Around the mould, bacterial colonies died. Fleming isolated the active compound in 1928.
BOD measures how much oxygen microbes need to break down organic matter. High BOD means heavily polluted water. Effective sewage treatment reduces BOD, meaning the organic load has been consumed. A BOD below 30 mg/L is considered safe for discharge.
Rhizobium needs the low-oxygen environment inside root nodules. The plant provides leghemoglobin to keep oxygen levels low (because nitrogenase, the nitrogen-fixing enzyme, is destroyed by oxygen). Outside nodules, Rhizobium lives as a free bacterium but cannot fix nitrogen.
Common Mistakes
Calling yeast a bacterium. It is a fungus.
Saying Rhizobium fixes nitrogen while free in soil. It does only in root nodule symbiosis.
Confusing Bacillus thuringiensis (biocontrol) with Bacillus anthracis (anthrax pathogen).
Writing that antibiotics kill viruses. Antibiotics work against bacteria, not viruses. Penicillin inhibits bacterial cell wall synthesis — viruses have no cell wall.
Saying primary treatment of sewage involves microbes. Primary treatment is purely physical — filtration and sedimentation. Microbes come in during secondary treatment.
Exam Weightage and Revision
Microbes in Human Welfare is one of the most scoring chapters in NEET biology. It carries 2-3 questions per year, and they are almost always factual recall — no tricky application needed. CBSE boards give 5-8 marks from this chapter. The PYQ pattern is simple: name the microbe, name the product, state the use.
| Question Type | Frequency in NEET | Example |
|---|---|---|
| Microbe-product matching | Every year | Which organism produces cyclosporin A? |
| Sewage treatment process | Most years | What is the role of flocs in secondary treatment? |
| Biofertiliser identification | Every 2 years | Name a free-living nitrogen fixer |
| Biocontrol agent | Every 2 years | How does Bt toxin work? |
The highest-yield revision for this chapter is a two-column table: microbe name in one column, product or function in the other. Ten rows cover the entire chapter. NEET will pick one row and ask about it.
Practice Questions
Q1. Name the organism used in the production of Swiss cheese. What gives Swiss cheese its characteristic holes?
Propionibacterium shermanii (or Propionibacterium freudenreichii). The holes are caused by large amounts of CO2 produced during fermentation by this bacterium. The CO2 gets trapped in the cheese as it solidifies, creating the characteristic holes (called “eyes”).
Q2. Why can Azotobacter be used as a biofertiliser in non-leguminous crops while Rhizobium cannot?
Azotobacter is a free-living nitrogen fixer — it fixes atmospheric nitrogen independently in the soil without needing a plant host. Rhizobium is symbiotic and requires specific legume root nodules to fix nitrogen. Since non-leguminous crops (like wheat, rice) do not form root nodules with Rhizobium, it is ineffective for them.
Q3. What is the difference between primary and secondary sewage treatment?
Primary treatment is physical — sedimentation and filtration remove large solid debris. It does not significantly reduce BOD. Secondary treatment is biological — aerobic microbes in aeration tanks consume dissolved organic matter, drastically reducing BOD (from ~300 to under 30 mg/L). The activated sludge from secondary treatment is then sent to anaerobic digesters.
Q4. Cyclosporin A is produced by which organism? What is its medical significance?
Cyclosporin A is produced by the fungus Trichoderma polysporum. It is used as an immunosuppressant in organ transplant patients. It suppresses the immune system’s T-cell response, preventing rejection of the transplanted organ.
Q5. Why is Bt toxin safe for humans but lethal for certain insect larvae?
Bt produces a crystal protein (Cry protein) that is inactive at neutral or acidic pH. The insect larva’s gut is highly alkaline (pH 9-10), which activates the toxin, creating pores in the gut lining and killing the larva. Human gut pH is acidic (pH 1-3 in the stomach), so the Cry protein is never activated and passes through harmlessly.
FAQs
Why are microbes considered useful despite causing diseases?
The vast majority of microbes are non-pathogenic. Only a tiny fraction cause disease. The rest decompose organic matter, produce antibiotics, fix nitrogen, help in sewage treatment, and produce food products. Without microbes, life on Earth would collapse within weeks.
What is the difference between antibiotics and vaccines?
Antibiotics are chemicals that kill or inhibit bacteria. They are therapeutic — used after infection. Vaccines are preparations of weakened or killed pathogens that stimulate the immune system to produce antibodies. They are preventive — used before infection.
Can we use biogas from sewage treatment as cooking fuel?
Yes. The methane-rich biogas from anaerobic digesters is used as fuel in many Indian villages and urban treatment plants. The technology is called a biogas plant or gobar gas plant when using cattle dung.
Why do we add curd to warm milk, not cold milk?
Lactobacillus grows optimally at 37-40°C. Cold milk slows bacterial growth, so curd takes much longer to set or may not set at all. Boiling milk first kills competing bacteria, then cooling to warm temperature gives Lactobacillus an uncontested field.
Make a table of microbe and product — six rows. NEET picks one question from that table each year.
Biogas Production — A Closer Look
Biogas technology is important for both NEET and Indian rural development. The process uses anaerobic bacteria to break down organic waste.
The biogas plant (gobar gas plant):
- Input: Cow dung mixed with water (slurry), or agricultural waste, kitchen scraps
- Process: Anaerobic digestion by methanogens (Methanobacterium, Methanococcus) in a sealed tank
- Products: Biogas (60% CH4, 35% CO2, 5% other gases) and nutrient-rich slurry (used as fertiliser)
Why biogas matters in India: India has over 300 million cattle — an enormous source of dung. Biogas plants convert this waste into cooking fuel and fertiliser. The government has installed over 5 million biogas plants across rural India. A single-family plant can produce enough gas for cooking needs and reduce dependence on firewood (saving forests) and LPG (saving money).
Antibiotics — Discovery and Resistance
Fleming’s discovery (1928): Alexander Fleming noticed that a Penicillium mould contaminating a Staphylococcus culture had killed bacteria around it. He isolated the active compound — penicillin — but could not produce it in quantity. Howard Florey and Ernst Chain scaled up production during World War II, saving millions of soldiers’ lives. All three shared the 1945 Nobel Prize.
How antibiotics work:
- Penicillin: Inhibits bacterial cell wall synthesis (blocks transpeptidase enzyme). Without cell wall, bacteria burst due to osmotic pressure.
- Streptomycin: Binds bacterial ribosome (30S subunit), blocking protein synthesis
- Tetracycline: Also blocks protein synthesis at the 30S subunit but by a different mechanism
Antibiotic resistance — a growing crisis: Overuse and misuse of antibiotics has led to resistant bacteria (MRSA, MDR-TB). Bacteria develop resistance through mutations and horizontal gene transfer. India has some of the world’s highest rates of antibiotic resistance due to over-the-counter availability and use of antibiotics in animal farming. This is why antibiotics should be taken only when prescribed and always completed for the full course.
Make a table of microbe and product — six rows. NEET picks one question from that table each year.
Microbes punch above their weight. Learn the famous names and their one signature product, and this chapter is pure marks.