Microorganisms, also called microbes, are living organisms that are so tiny that they cannot be seen with the naked eye and require a microscope to be observed. This invisible world of life includes bacteria, fungi, protozoa and some algae, as well as viruses, which are a special category that reproduce only inside living cells. Although invisible, microorganisms are everywhere — in soil, water, air, on our skin and even inside our bodies. Some of them cause diseases, but many more are extremely useful to us and to the environment.
Because microorganisms play both helpful and harmful roles, they are described as both a friend and a foe. They help in making curd, bread, cheese, wine and antibiotics, in fixing nitrogen in the soil and in decomposing dead organic matter. At the same time, certain microbes cause diseases in humans, plants and animals, and spoil food. In this chapter we explore the variety, the friendly roles and the harmful effects of microorganisms, and the protective measures we can adopt against them.
2. Classification of Microorganisms
Microorganisms are classified into four major groups:
Bacteria: Single-celled organisms that can be found in every habitat. Examples: Lactobacillus (used to make curd), and disease-causing bacteria such as those causing typhoid and tuberculosis.
Fungi: Organisms that grow on decaying organic matter; they include yeasts, moulds and mushrooms. Yeast is used in making bread and alcohol, while some fungi cause diseases like ringworm.
Protozoa: Single-celled organisms, some of which are parasitic. Examples: Amoeba (harmless), and Plasmodium which causes malaria, and the protozoan that causes dysentery.
Some algae: Green, plant-like microorganisms. Some algae like Chlorella and Spirulina are used as food supplements.
Viruses are microscopic organisms that reproduce only inside the cells of a host organism. Diseases such as common cold, influenza, COVID-19, measles and polio are caused by viruses. Because they multiply only in living cells, viruses occupy a special position among microorganisms.
3. Where Do Microorganisms Live?
Microorganisms are found almost everywhere. They survive in soil, in water bodies like ponds, lakes and seas, in air, in the bodies of plants and animals, in hot springs, in deep oceans, in ice and even inside other organisms. A handful of soil can contain millions of bacteria, and one millilitre of pond water can harbour numerous microbes.
Some microorganisms live independently in the environment, while others live in or on the bodies of other organisms. Microbes that live on dead and decaying organic matter are called saprophytes (or saprophytic organisms), and they are vital for decomposition, which recycles nutrients back into the soil. Microbes that live on another living organism for their nutrition are called parasites, and they often cause disease in the host.
4. Microorganisms as Friends
Microorganisms are our greatest invisible helpers. Their useful roles include:
Making curd and cheese: The bacterium Lactobacillus promotes the formation of curd from milk. Bacteria are also involved in making cheese and other milk products.
Making bread, cakes and alcohol: Yeast is used in making bread and cakes because it produces carbon dioxide gas which makes the dough rise. Yeast is also used for producing alcohol by the fermentation of sugar.
Medicines and antibiotics: Antibiotics such as penicillin (discovered by Alexander Fleming) are produced from microorganisms and are used to kill or stop the growth of disease-causing bacteria. Vaccines are also prepared from weakened or killed microbes to provide immunity against diseases.
Increasing soil fertility: Certain bacteria and blue-green algae fix atmospheric nitrogen into a form that plants can use, enriching the soil. The bacteria found in the root nodules of legumes (rhizobium) convert nitrogen into nitrogenous compounds.
Cleaning the environment: Decomposing microorganisms break down dead plants and animals and other waste material, keeping the environment clean.
Food supplements: Algae such as Spirulina and Chlorella are rich in protein and are used as food supplements.
5. The Process of Fermentation
Fermentation is a chemical change brought about by microorganisms, in which complex organic substances are broken down into simpler ones. The most common example is the conversion of sugar into alcohol by yeast:
Sugar → (yeast) → alcohol + carbon dioxide
This is the basis of making alcoholic beverages, bread, idli and dosa. In idli and dosa batter, bacteria (along with yeast) ferment the batter and make it rise and turn slightly sour. Fermentation also plays a role in producing organic acids like citric acid and in tanning leather and curing tobacco.
6. Microorganisms as Foes
Despite their benefits, microorganisms also cause many problems:
Disease in humans: Bacteria cause diseases such as typhoid, tuberculosis and cholera; viruses cause common cold, influenza, COVID-19, measles and polio; protozoa cause malaria (Plasmodium) and dysentery.
Disease in animals: Anthrax is a serious disease of cattle and sheep, and foot-and-mouth disease affects livestock. These can also occasionally affect humans.
Disease in plants: Microbes cause rust of wheat, citrus canker in orange and rice blast disease, which reduce crop yields.
Food spoilage: Microorganisms grow on food, breaking it down and making it unfit for consumption. This is why we keep food in the refrigerator.
Microorganisms that cause diseases are called pathogens, and diseases that can spread from one person to another are called communicable diseases.
7. Food Preservation
Since microorganisms spoil food, we preserve food by creating conditions in which microbes cannot survive. The main methods are:
Chemical preservation: Chemicals like sodium benzoate and sodium metabisulphite are added to pickles and squashes to prevent microbial growth.
Preservation by common salt: Salting fish and meat (and vegetables) prevents bacterial growth because microbes cannot survive in high salt concentration. This is how pickles stay good for months.
Sugar preservation: Jams and jellies use high sugar concentration, which draws water out of microbial cells and kills them.
Oil and vinegar preservation: Oil and vinegar used in pickles and salad dressings block air and kill microbes.
Heat and cold treatment: Pasteurisation kills microbes in milk by heating it to about 70°C for 15–30 seconds and then quickly cooling and sealing it. Refrigeration slows microbial growth because microbes grow slowly at low temperatures.
Drying: Removing water (as in dried fish and dehydrated vegetables) stops microbial growth.
Vacuum and nitrogen packing: Packing food in airtight packages with nitrogen prevents spoilage by keeping oxygen away.
8. Nitrogen Fixation and the Nitrogen Cycle
All living organisms need nitrogen to make proteins. The atmosphere has about 78% nitrogen, but plants and animals cannot use nitrogen directly in its gaseous form. Nitrogen is made available through:
Nitrogen fixation: Certain bacteria (Rhizobium in root nodules of legumes) and blue-green algae convert atmospheric nitrogen into nitrogenous compounds that plants can absorb.
The nitrogen cycle: Nitrogen moves from the atmosphere into soil and living things, and back. Plants take up nitrogen compounds from the soil; animals get nitrogen by eating plants; when plants and animals die, decomposing bacteria convert their nitrogen compounds back into free nitrogen gas, returning it to the atmosphere.
This cyclic movement of nitrogen between the atmosphere, soil and living organisms is called the nitrogen cycle, and it keeps nitrogen continuously available to life.
9. Microorganisms in Sewage Treatment
Microorganisms are used to clean our environment. In sewage treatment plants, decomposing bacteria are used to break down the organic waste in sewage, reducing pollution before the treated water is released. Microbes also help decompose dead bodies of animals and plants, keeping the earth clean and recycling nutrients. This beneficial use shows how microorganisms can act as our friends in waste management.
Quick Revision Tables
Table 1: Groups of Microorganisms and Their Examples
graph TD
A["Microorganisms"] --> B["Types"]
B --> B1["Bacteria"]
B --> B2["Fungi"]
B --> B3["Protozoa"]
B --> B4["Algae"]
B --> B5["Viruses"]
A --> C["Friendly Roles"]
C --> C1["Curd, bread, cheese, alcohol"]
C --> C2["Antibiotics and vaccines"]
C --> C3["Nitrogen fixation"]
C --> C4["Decomposition, cleaning"]
C --> C5["Food supplements"]
A --> D["Harmful Roles"]
D --> D1["Diseases in humans"]
D --> D2["Diseases in plants and animals"]
D --> D3["Food spoilage"]
A --> E["Prevention"]
E --> E1["Food preservation"]
E --> E2["Pasteurisation"]
E --> E3["Cleanliness and hygiene"]
Important Diagrams (SVG)
Diagram 1: The Nitrogen Cycle
Diagram 2: Common Microorganisms and Their Uses
Common Mistakes
Calling all microorganisms harmful: Most microbes are useful; only a small fraction are pathogens. Always mention both friend and foe roles.
Confusing viruses with bacteria: Viruses reproduce only inside living cells and are not affected by antibiotics, whereas bacteria are single-celled and can be killed by antibiotics.
Attributing malaria to bacteria: Malaria is caused by the protozoan Plasmodium, not by a bacterium or virus.
Thinking all fungi are harmful: Yeast is a useful fungus that makes bread rise and produces alcohol; only some fungi cause disease.
Believing nitrogen in air can be used directly by plants: Plants cannot use atmospheric nitrogen directly; it must first be fixed by Rhizobium and blue-green algae into nitrogenous compounds.
Forgetting that food spoils because of microbes: Spoilage is caused by microbial growth, which is why preservation (salt, sugar, oil, refrigeration) works.
Confusing pasteurisation with boiling: Pasteurisation heats milk to about 70°C for 15–30 seconds and quickly cools it — it does not boil the milk.
Exam Tips
Make a two-column table of friend vs foe microbes — examiners love classified answers with examples.
Remember Lactobacillus for curd and Yeast for bread/alcohol; these two examples are asked in almost every paper.
For the nitrogen cycle, always name Rhizobium and blue-green algae as the fixing agents, and decomposers for returning nitrogen to air.
List at least four preservation methods with one example each: salt (pickles), sugar (jam), oil/vinegar (pickle), pasteurisation (milk), refrigeration.
Mention fermentation in dough: Carbon dioxide produced by yeast makes dough rise — a favourite one-mark question.
Conclusion
Microorganisms are the tiniest but most numerous living things on Earth, and their influence is enormous. As friends, they help us make curd, bread, cheese and antibiotics, fix nitrogen in the soil, decompose waste and clean the environment. As foes, they cause diseases in humans, plants and animals and spoil our food. By understanding how microbes live and grow, we can use preservation methods to protect our food, practise hygiene to prevent disease, and harness useful microbes for agriculture, medicine and industry. Learning to control the harmful ones while promoting the helpful ones is the key message of this chapter — a balance that makes microorganisms truly both friend and foe.