Microorganisms: Friend and Foe
Invisible life forms — bacteria, fungi, protozoa, some algae, and viruses apart — that are simultaneously humanity's allies in food, medicine and soil, and its foes through disease and spoilage.
A foundational biology chapter whose facts recur in Prelims as match-the-column questions linking diseases to their causative microbes, vectors and transmission, and scientists to their discoveries. For Mains it feeds GS-III science-and-technology themes — antimicrobial resistance, biofertilisers and food processing — and overlaps GS-II health governance through immunisation drives like Pulse Polio and milestones like smallpox eradication.
Understand the chapter
What Microorganisms Are — and the Four Groups (Plus Viruses)
Microorganisms (microbes) are living organisms too small to be seen by the unaided eye; some, like bread mould, are visible under a magnifying glass, but most need a microscope. The chapter classifies microbes into four major groups — bacteria, fungi, protozoa and some algae. Viruses are microscopic too but are treated separately because they reproduce only inside the cells of a host (a bacterium, plant or animal). Microbes may be single-celled (bacteria, some algae, protozoa) or multicellular (many algae and fungi).
- Four groups: Bacteria, Fungi, Protozoa, some Algae; viruses are a separate category.
- Viruses are inert outside a host and multiply only inside living host cells.
- Habitat range: ice-cold climates to hot springs, deserts to marshes, and inside human/animal bodies.
- Single-celled vs multicellular distinction cuts across the groups.
Friendly Microbes I — Food, Fermentation and Industry
Microbes are indispensable in the kitchen and in industry. The bacterium Lactobacillus converts milk into curd, and bacteria also make cheese, pickles and help ferment idli/dosa batter. Yeast (a fungus) drives fermentation — the conversion of sugar into alcohol — and the carbon dioxide it releases during respiration makes dough rise for bread and cakes. Commercially, yeast is grown on natural sugars in barley, wheat, rice and crushed fruit juices to mass-produce alcohol, wine and acetic acid (vinegar).
- Lactobacillus -> curd (a little curd added to warm milk multiplies and sets it).
- Fermentation = sugar -> alcohol, by yeast, releasing carbon dioxide.
- Yeast's CO2 raises bread/cake dough (basis of the baking industry).
- Industrial products: alcohol, wine, vinegar (acetic acid).
Friendly Microbes II — Medicine: Antibiotics and Vaccines
Antibiotics are medicines, sourced from bacteria and fungi, that kill or stop the growth of disease-causing microbes; penicillin, streptomycin, tetracycline and erythromycin are key examples. They must be taken only on medical advice and the full course completed, because misuse breeds resistance and kills beneficial gut bacteria — and they are useless against viral cold and flu. Vaccines work differently: introducing dead or weakened microbes prompts the body to make antibodies and remember the pathogen, giving lasting protection. Vaccination prevents cholera, tuberculosis, smallpox and hepatitis, and India's Pulse Polio Programme delivers polio vaccine as drops.
- Antibiotics from bacteria & fungi: penicillin, streptomycin, tetracycline, erythromycin.
- Antibiotics do NOT act on viruses (cold, flu); overuse breeds resistance (AMR).
- Vaccine principle: dead/weak microbe -> antibodies + immune memory.
- Vaccine-preventable: cholera, TB, smallpox, hepatitis, polio.
Friendly Microbes III — Soil Fertility and Cleaning the Environment
Certain bacteria fix atmospheric nitrogen and enrich the soil — these biological nitrogen fixers, along with nitrogen-fixing cyanobacteria (blue-green algae), reduce dependence on chemical fertilisers. Microbes are also nature's decomposers: they break down organic wastes (vegetable peels, dead plants and animals, faeces) into simple, reusable substances, converting plant waste into manure. Crucially, microbes cannot act on plastics, polythene and glass, which is why such items do not decompose.
- Biological nitrogen fixers enrich soil; cyanobacteria fix nitrogen too.
- Decomposers recycle organic waste into manure and nutrients.
- Microbes cannot degrade plastic, polythene or glass — basis of biodegradable vs non-biodegradable.
Harmful Microbes — Pathogens, Communicable Diseases and Carriers
Disease-causing microbes are called pathogens; they enter via the air we breathe, water we drink, food we eat, or by contact. Diseases that spread from a sick to a healthy person through air, water, food or contact are communicable diseases — e.g., cholera, common cold, chicken pox, tuberculosis. Insects and animals act as carriers (vectors): the housefly transfers pathogens onto uncovered food, the female Anopheles mosquito carries the malaria parasite Plasmodium (a protozoan), and the female Aedes mosquito carries dengue virus. Microbes also strike animals (anthrax, a bacterial disease of humans and cattle; foot-and-mouth, viral) and crops (citrus canker, rust of wheat, yellow vein mosaic of okra).
- Pathogen = disease-causing microbe; communicable = spreads person-to-person.
- Carriers: housefly; female Anopheles (malaria/Plasmodium); female Aedes (dengue virus).
- Animal diseases: anthrax (bacterial, Bacillus anthracis); foot-and-mouth (viral).
- Plant diseases: citrus canker (bacteria), wheat rust (fungi), okra yellow vein mosaic (virus).
Food Spoilage, Food Poisoning and Preservation
Microbes growing on food can produce toxins that cause food poisoning — vomiting, serious illness, even death — so preservation matters. Chemical preservatives such as salt, edible oils, sodium benzoate and sodium metabisulphite check microbial growth in pickles, jams and squashes. Salt preserves meat, fish, amla, raw mango and tamarind; sugar preserves jams and squashes by reducing moisture; oil and vinegar create an environment in which bacteria cannot live. Heat and cold also help: boiling kills microbes, refrigeration slows them, and pasteurisation heats milk to about 70 degrees C for 15-30 seconds and then chills it suddenly.
- Food poisoning = toxins produced by microbes on spoilt food.
- Preservatives: salt, edible oil, sodium benzoate, sodium metabisulphite.
- Sugar/salt preserve by lowering moisture; oil & vinegar make a hostile environment.
- Pasteurisation (Louis Pasteur): about 70 degrees C for 15-30 s, then sudden chilling.
Nitrogen Fixation and the Nitrogen Cycle Link
Atmospheric nitrogen is abundant but inert; plants need it fixed into usable forms. The bacterium Rhizobium lives in the root nodules of leguminous plants (pulses such as beans and peas) in a symbiotic relationship, fixing nitrogen for the plant in return for food and shelter. Nitrogen is also fixed by the action of lightning. Despite continuous use, the proportion of nitrogen in the atmosphere stays constant — pointing to the balancing nitrogen cycle.
- Rhizobium: symbiotic nitrogen fixer in legume root nodules.
- Cyanobacteria: free-living nitrogen fixers.
- Lightning also fixes atmospheric nitrogen.
- Atmospheric nitrogen stays constant due to the nitrogen cycle.
Key terms
- Microorganism (microbe)
- An organism too small to be seen by the unaided eye; needs a microscope, though some fungi show under a magnifying glass.
- Virus
- A microscopic infectious agent that reproduces only inside a living host cell; causes cold, flu, polio and chicken pox.
- Pathogen
- Any disease-causing microorganism affecting humans, animals or plants.
- Communicable disease
- An illness that spreads from an infected to a healthy person via air, water, food or contact.
- Antibiotic
- A medicine made from bacteria or fungi that kills or stops disease-causing microbes; ineffective against viruses.
- Vaccine
- A preparation of dead or weakened microbes that triggers antibodies and immune memory for lasting protection.
- Fermentation
- Microbial conversion of sugar into alcohol (with carbon dioxide), as by yeast; discovered by Louis Pasteur in 1857.
- Pasteurisation
- Heating milk to about 70 degrees C for 15-30 seconds then chilling it suddenly to kill harmful microbes.
- Preservative
- A chemical (salt, oil, sodium benzoate, sodium metabisulphite) that checks microbial growth in food.
- Nitrogen fixation
- Conversion of inert atmospheric nitrogen into usable forms by microbes (Rhizobium, cyanobacteria) or by lightning.
Must-know facts exam-ready
- Microbes are classified into four groups — bacteria, fungi, protozoa and some algae; viruses are separate and reproduce only inside host cells.
- Lactobacillus (a bacterium) sets curd; yeast (a fungus) ferments sugar into alcohol and raises dough with carbon dioxide.
- Louis Pasteur discovered fermentation in 1857 and gave his name to pasteurisation (milk at about 70 degrees C for 15-30 s).
- Alexander Fleming discovered penicillin in 1929 from a green mould (Penicillium).
- Edward Jenner discovered the smallpox vaccine in 1798.
- Robert Koch discovered Bacillus anthracis, the anthrax bacterium, in 1876.
- Common antibiotics — penicillin, streptomycin, tetracycline, erythromycin — are made from bacteria and fungi and are useless against viral cold and flu.
- Malaria is caused by the protozoan Plasmodium carried by the female Anopheles mosquito; dengue virus is carried by the female Aedes mosquito.
- Anthrax is a bacterial disease of humans and cattle; foot-and-mouth disease of cattle is viral.
- Rhizobium fixes nitrogen symbiotically in legume root nodules; cyanobacteria (blue-green algae) are free-living nitrogen fixers.
- Common food preservatives include salt, edible oil, sodium benzoate and sodium metabisulphite.
- Smallpox has been eradicated worldwide (WHO declared eradication in 1980); polio is tackled in India through the Pulse Polio Programme.
Timeline
- 1798Edward Jenner develops the smallpox vaccine.
- 1857Louis Pasteur discovers fermentation.
- 1876Robert Koch identifies Bacillus anthracis, the cause of anthrax.
- 1929Alexander Fleming discovers penicillin from a green mould.
- 1980WHO declares smallpox eradicated worldwide.
- 1995India launches the Pulse Polio Immunisation Programme.
- 2014WHO certifies India polio-free.
Memory tricks remember it for good
Traps to avoid
- Viruses are NOT one of the four microbe groups — they are separate and multiply only inside a host cell.
- Antibiotics do NOT cure viral cold and flu; using them there is wrong and fuels resistance.
- Malaria is caused by a protozoan (Plasmodium), not a bacterium; only the FEMALE Anopheles transmits it.
- Anopheles vs Aedes: Anopheles = malaria, Aedes = dengue — easy to swap; both vectors are female mosquitoes.
- Lactobacillus is a BACTERIUM (curd) while yeast is a FUNGUS (bread/alcohol) — do not lump them together.
- Anthrax is bacterial (Koch's Bacillus anthracis) but foot-and-mouth disease of cattle is viral; Rhizobium is symbiotic whereas cyanobacteria are free-living fixers.
Exam focus
🧠 Prelims angles
- Match-the-following: disease to causative microbe (bacterium/virus/protozoan) to mode of transmission (Table 2.1 style).
- Scientist-discovery-year pairs: Jenner, Pasteur, Koch, Fleming.
- Vector-disease-pathogen triads: Anopheles-malaria-Plasmodium; Aedes-dengue virus; housefly as a mechanical carrier.
- Nitrogen-fixing microbes (Rhizobium, cyanobacteria) and the symbiotic legume association.
- Food preservatives (sodium benzoate, sodium metabisulphite) and pasteurisation parameters (about 70 degrees C, 15-30 s).
- Vaccine-preventable diseases and eradication facts (smallpox eradicated; Pulse Polio).
✍️ Mains angles GS-III
- Antimicrobial Resistance (AMR) as a public-health emergencyUse the chapter's caution on completing antibiotic courses and antibiotics in livestock feed to argue for a One Health approach and India's National Action Plan on AMR (GS-III).
- Vaccination and disease eradication as cost-effective public healthCite smallpox eradication and Pulse Polio; discuss cold chain, Mission Indradhanush and equitable coverage (GS-II health governance).
- Biofertilisers for sustainable agricultureLink biological nitrogen fixation (Rhizobium, cyanobacteria) to lower chemical-fertiliser dependence, soil health and reduced subsidy burden (GS-III).
- Microbes in food processing and the circular bioeconomyConnect fermentation industries and waste-to-manure decomposition to food-processing potential and biodegradable-waste management (GS-III).
Last-minute revision tick as you recall
- Four groups: Bacteria, Fungi, Protozoa, some Algae; viruses separate (need a host to reproduce).
- Lactobacillus -> curd; yeast -> bread & alcohol (fermentation = sugar to alcohol + CO2).
- Fleming 1929 -> penicillin; antibiotics from bacteria & fungi; useless vs cold/flu.
- Jenner 1798 -> smallpox vaccine; vaccines train antibodies and immune memory.
- Malaria = Plasmodium (female Anopheles); Dengue virus (female Aedes).
- Anthrax = Bacillus anthracis (Koch 1876, bacterial); foot-and-mouth = viral.
- Pasteurisation: milk about 70 degrees C, 15-30 s, then chill (Louis Pasteur).
- Preservatives: salt, sugar, oil/vinegar, sodium benzoate, sodium metabisulphite; plus heat/cold.
- Rhizobium: symbiotic nitrogen fixer in legume root nodules; cyanobacteria are free-living fixers.
Distilled from NCERT Class 8 · Science (Class 8) for UPSC. Always cross-check facts with the original NCERT.