Microorganisms - Friend and Foe

This companion follows the NCERT chapter's own path: what a microorganism is and how the four major groups plus viruses are classified; where they live; the chapter's central theme of microorganisms as friends (curd, bread, alcohol, medicines, soil fertility, cleaning the environment) and as foes (disease in humans, animals and plants, plus food poisoning); and two practical, testable closing sections — food preservation, and the nitrogen cycle. Two derivations unpack the fermentation reaction and the nitrogen cycle pathway step by step; the worked examples reuse the chapter's own activities (the sugar-and-yeast smell test, the 78% nitrogen figure) as solved problems.

What is a microorganism, and how are they grouped

A microorganism (or microbe) is a living organism too small to be seen with the unaided eye. Some, such as the fungus growing on spoiled bread, are just visible with a magnifying glass; most need a microscope — a drop of pond water or settled soil-water examined under one is teeming with tiny moving organisms, exactly what NCERT's Activities 2.1 and 2.2 ask you to observe. Microorganisms fall into four major groups: bacteria (simple single-celled organisms — some harmful like typhoid- and TB-causing types, some allies like Lactobacillus and Rhizobium), fungi (moulds such as bread mould, Penicillium and Aspergillus), protozoa (single-celled organisms such as Amoeba, Paramecium and the malaria parasite Plasmodium) and some algae (microscopic pond forms such as Chlamydomonas and Spirogyra). Viruses are also microscopic but stand apart: a virus multiplies only inside the living cells of a host — a bacterium, plant or animal — and is essentially inert outside one. Cold, influenza and most coughs are viral, and so are more serious diseases like polio and chicken pox. Microorganisms may be single-celled or multicellular, and live in every environment — ice-cold climates, hot springs, deserts, marshy lands, and inside plant and animal bodies including humans — either attached to a host or entirely free-living.

Friendly microorganisms: food, medicine and the environment

Microorganisms make curd, bread, cake and, since ancient times, alcohol. The bacterium Lactobacillus multiplies rapidly in warm milk and converts it into curd — why a spoonful of yesterday's curd stirred into fresh warm milk works as a starter culture. The same bacteria help make cheese and pickles, curd is a key ingredient of rava idli and bhatura batter, and bacteria with yeast ferment idli and dosa batter into light, bubbly batter. Yeast, a single-celled fungus, is central to baking. Added to sugared dough (Activity 2.3), it respires rapidly, converting sugar into carbon dioxide whose trapped bubbles raise the dough — the same gas-trapping that gives baked bread, pastries and cakes their light texture. When yeast instead converts sugar into alcohol in solution, the process is fermentation, discovered by Louis Pasteur in 1857 and used commercially to produce alcohol, wine and vinegar from grain or fruit sugars. Microorganisms also give us medicines and clean the environment. Antibiotics — penicillin, streptomycin, tetracycline, erythromycin — are grown from specific fungi or bacteria and kill or stop disease-causing microbes. Alexander Fleming discovered penicillin by accident in 1929, noticing that a stray mould, Penicillium, killed the bacteria around it on his culture plate. Vaccines work differently: dead or weakened microbes introduced into a healthy body trigger antibodies the body "remembers," giving lasting protection. Edward Jenner discovered the smallpox vaccine in 1798; vaccination has since eradicated smallpox and controls cholera, tuberculosis, polio and hepatitis (the Pulse Polio Programme). Meanwhile bacteria decompose organic waste into harmless, usable substances — a compost pit turns plant waste into manure while plastic in the same pit stays unchanged (Activity 2.5) — and soil bacteria and blue-green algae fix nitrogen from the atmosphere as biological nitrogen fixers, detailed below.

Harmful microorganisms: disease and food poisoning

Disease-causing microorganisms are called pathogens, entering the body through air, water, food, or contact with an infected person or carrier. Diseases spreading this way are communicable diseases — cholera, common cold, chicken pox and tuberculosis are examples; a cold sneeze spreads virus-laden droplets through the air, which is why covering your nose and mouth and keeping distance from an infected person reduces transmission. Some insects and animals act as carriers. The housefly is a mechanical carrier: it picks up pathogens from garbage and excreta and transfers them to uncovered food, so food should stay covered. The female Anopheles mosquito is a biological carrier of the malaria parasite (Plasmodium), and the female Aedes mosquito carries dengue virus; since mosquitoes breed in standing water, keeping coolers, tyres and flower pots dry directly breaks transmission.

Microorganisms also cause disease in animals and plants. Anthrax (bacterium Bacillus anthracis, discovered by Robert Koch in 1876) threatens humans and cattle; foot-and-mouth disease of cattle is viral. In plants, citrus canker (bacterial), rust of wheat (fungal) and yellow vein mosaic of bhindi/okra (viral, insect-spread) reduce crop yield and are controlled with chemicals that kill the responsible microbes. Microorganisms growing on stored or cooked food can also produce toxic substances, causing food poisoning — serious illness and, in severe cases, death.

Food preservation: stopping microbial attack

Spoiled food develops a bad smell, taste and colour because microorganisms are growing on it. Food preservation methods all share one principle: stop microorganisms from growing. The chemical method uses salt, edible oils and preservatives such as sodium benzoate and sodium metabisulphite (common in jams and squashes). Common salt preserves meat, fish, amla, raw mango and tamarind by drawing out the moisture bacteria need; sugar preserves jams and jellies the same way; oil and vinegar preserve pickles because bacteria cannot survive in that environment. Boiling kills microorganisms directly, while refrigeration only slows their growth. Pasteurisation, devised by Louis Pasteur, heats milk to about 70 degrees Celsius for 15 to 30 seconds and chills it rapidly — killing harmful microbes without the prolonged heat of boiling, which is why pasteurised packet milk keeps for days unboiled. Sealed, air-tight packaging keeps microbes off dry fruits and vegetables entirely.

Nitrogen fixation and the nitrogen cycle

Our atmosphere is about 78% nitrogen gas by volume, essential to every living cell — yet plants and animals cannot use it directly. It must first be fixed into nitrogen compounds. The bacterium Rhizobium does this inside the root nodules of leguminous plants such as beans and peas, in a symbiotic relationship: the plant shelters and feeds the bacterium, which fixes nitrogen the plant can use. Free-living soil bacteria and blue-green algae fix nitrogen the same way without a host, and lightning fixes a small extra amount directly. Since plants and animals constantly draw fixed nitrogen out of the soil, yet atmospheric nitrogen stays roughly constant, something must return nitrogen to the air just as fast as fixation removes it — the nitrogen cycle, traced step by step in the second derivation below.

Board level - the Activity 2.4 fermentation smell test explained

Boojho sets up Activity 2.4 exactly as in the textbook: dissolving two to three teaspoons of sugar in about 500 mL of warm water, adding half a spoon of yeast powder, then keeping it covered in a warm place for four to five hours. Explain what he will observe when he smells the solution afterward, and why.

Step 1. Identify what has been added: dissolved sugar as food and living yeast as the organism, kept warm to encourage growth.

Step 2. Recall what yeast does to sugar under these conditions: it ferments the sugar, releasing carbon dioxide gas and ethyl alcohol.

Step 3. Over four to five warm hours, the growing yeast population ferments enough dissolved sugar for the alcohol produced to be smelled directly.

Answer: Boojho will smell a distinct odour of alcohol, because the yeast has fermented the dissolved sugar into ethyl alcohol and carbon dioxide gas.

Board level - calculating the nitrogen in a room's air

A closed classroom has an air volume of 200 cubic metres. Using the atmospheric composition given in the chapter, calculate the volume of nitrogen gas present in the room, and explain why plants outside cannot use any of this nitrogen directly.

Step 1. The atmosphere is about 78% nitrogen gas by volume. Volume of nitrogen gas = 78% of 200 cubic metres = 0.78 times 200 = 156 cubic metres.

Step 2. Despite this large volume, plants cannot absorb nitrogen gas directly; the two nitrogen atoms in N2 are too strongly bonded for a plant's ordinary biology to split.

Step 3. The nitrogen must first be fixed by bacteria such as Rhizobium or free-living soil bacteria and cyanobacteria, converting it into dissolved nitrogen compounds that roots can actually absorb.

Answer: The room contains 156 cubic metres of nitrogen gas, but none of it is usable by plants until soil bacteria fix it into nitrogen compounds.

Higher-order thinking (HOTS / olympiad-style) - crop rotation with a legume

A farmer grows wheat every season and finds his yield drops each year unless he buys nitrogen fertiliser. His neighbour instead rotates the field with a season of moong (a legume) every third year, uses far less fertiliser, and still gets a healthy wheat yield. Using Rhizobium and the nitrogen cycle, explain why crop rotation with a legume works, and predict what would happen to soil fertility if the neighbour ploughed the moong plant's roots and stalks back into the soil instead of removing them after harvest.

Step 1. Wheat has no root nodules, so it only draws nitrogen compounds out of the soil to build its own proteins; season after season of wheat alone steadily depletes fixed soil nitrogen, exactly why the first farmer's yield falls without added fertiliser.

Step 2. Moong hosts Rhizobium in root nodules, in a symbiotic relationship: the plant shelters and feeds the bacteria, which fix atmospheric nitrogen into compounds around the roots, adding fixed nitrogen to the soil rather than only removing it.

Step 3. Wheat planted after moong grows in freshly enriched soil, so it needs less purchased fertiliser for the same yield — the neighbour's advantage from crop rotation.

Step 4. If the moong's roots and stalks are ploughed back in rather than removed, decomposer bacteria and fungi break this organic matter down, releasing further nitrogen compounds into the soil — enriching it beyond what standing Rhizobium fixation alone provides.

Answer: Crop rotation with moong works because Rhizobium in its root nodules fixes atmospheric nitrogen into the soil, unlike wheat which only depletes it; ploughing the moong's roots and stalks back in would enrich the soil further, since decomposers would release the nitrogen locked in that buried plant matter. Timing note: allow about three minutes. The chain has four short links — wheat depletes, moong's Rhizobium fixes, rotation transfers the benefit, ploughing-in adds decomposition on top — so write one sentence per link. The trap here is saying "legumes add nitrogen to soil" without naming Rhizobium and the nodules; it is the bacteria, not the plant itself, that fix the nitrogen.

Students consistently mix up a handful of facts here, and examiners know it. Watch for: treating viruses as just another "fifth type of bacteria" rather than a case that reproduces only inside a host cell; assuming every microorganism is harmful, when most of the ones you rely on daily (curd, bread, cheese, soil fertility) are friends; confusing a carrier like the housefly, which merely transports pathogens on its body, with the female Anopheles mosquito, inside which the malaria parasite actually develops — different kinds of "carrying." Students often say "boiling and pasteurisation are the same," when pasteurisation is a brief, controlled heat treatment (about 70 degrees Celsius for 15 to 30 seconds, then rapid chilling) rather than full boiling. Another slip is crediting legumes themselves with fixing nitrogen, rather than naming Rhizobium, the bacterium in their root nodules, as the actual fixer. Finally, students forget the nitrogen cycle needs a return path: fixation alone would drain the atmosphere, and denitrification (bacteria converting soil compounds back to nitrogen gas) closes the loop and keeps the 78% roughly constant.

• Name the four major groups of microorganisms and explain why viruses are treated separately.
• Describe three ways microorganisms help humans (food, medicine, environment, soil) with an example organism for each.
• Explain the difference between an antibiotic and a vaccine, and why antibiotics fail against viral illnesses.
• Define pathogen and communicable disease, with two examples and their mode of transmission.
• Distinguish a mechanical carrier (housefly) from a biological carrier (female Anopheles mosquito).
• List four methods of food preservation and how each stops microbial growth.
• Explain pasteurisation and how it differs from simply boiling milk.
• Describe the Rhizobium-legume symbiosis and name one other way nitrogen gets fixed.
• Trace the nitrogen cycle from atmospheric gas through fixation, plant and animal use, decomposition and denitrification.

Ask the tutor: Ask your tutor to walk through one more disease from Table 2.1 or 2.2 that was not covered in the worked examples here and quiz you on its causative organism, transmission and prevention.