A world too small to see
Some living things are so small that they can be seen only through a microscope. These are microorganisms or microbes. They live almost everywhere: in soil, water, air, inside our bodies, and even in hot springs and ice.
The five groups
Group | Features | Examples |
|---|---|---|
Bacteria | single-celled; shapes include rods, spheres and spirals | Lactobacillus (curd), Rhizobium |
Fungi | many are multicellular; grow as threads or as single cells | bread mould, yeast, Penicillium, mushrooms (large fungi) |
Protozoa | single-celled, animal-like | Amoeba, Paramecium, Plasmodium |
Algae | contain chlorophyll; make their own food | Chlamydomonas, Spirogyra |
Viruses | reproduce only inside the cells of a host; much smaller than bacteria | viruses causing the common cold, influenza, polio, measles |
Viruses are a special case: outside a host cell they show no signs of life.
Friendly microbes
Curd: the bacterium Lactobacillus multiplies in milk and turns it into curd.
Bread and idli: yeast respires and releases carbon dioxide, whose bubbles make dough rise. This is a kind of fermentation, discovered by Louis Pasteur in 1857. Yeast is also used to make alcohol from sugar.
Medicines: antibiotics are medicines, often made from bacteria and fungi, that kill or stop the growth of disease-causing bacteria. Penicillin was discovered by Alexander Fleming in 1928 from the mould Penicillium. Antibiotics do not work against viruses, so they should not be taken for colds or flu, and only as a doctor prescribes.
Vaccines: a vaccine contains dead or weakened microbes. When introduced into the body, it trains the body to make antibodies, so it can fight the real microbe later. Edward Jenner developed the vaccine for smallpox in 1796. Smallpox has since been wiped out worldwide.
Soil fertility: Rhizobium in the root nodules of pulses, and some blue-green algae, fix nitrogen from the air into the soil.
Cleaning the environment: microbes decompose dead plants, animals and wastes into simple substances, returning nutrients to the soil.
Harmful microbes
Microbes that cause disease are pathogens. A disease that spreads from one person to another is a communicable disease.
Disease | Microbe | How it spreads | Prevention |
|---|---|---|---|
Tuberculosis | bacterium | air | BCG vaccine, isolation, treatment |
Cholera | bacterium | water and food | clean water, hygiene, vaccine |
Typhoid | bacterium | water | clean water, hygiene |
Malaria | protozoan (Plasmodium) | female Anopheles mosquito | stop mosquito breeding, nets |
Measles | virus | air | vaccine |
Polio | virus | air and water | polio drops (vaccine) |
Hepatitis A | virus | water | clean water, vaccine |
Insects that carry pathogens are called carriers. Houseflies carry germs from garbage to food. Mosquitoes breed in stagnant water, so coolers, tyres and pots should not be left holding water.
Microbes also harm animals (anthrax in cattle, foot-and-mouth disease) and plants (citrus canker, rust of wheat, yellow vein mosaic of bhindi), reducing crop yields.
Food poisoning and preservation
Microbes growing in food can produce poisons that cause food poisoning. Preservation stops their growth:
Method | Example |
|---|---|
Chemicals | sodium benzoate and sodium metabisulphite in jams and squashes |
Salt | pickles, salted fish |
Sugar | jams, jellies (sugar draws water out, so microbes can't grow) |
Oil and vinegar | pickles |
Heating and cooling | boiling; refrigeration slows microbes |
Pasteurisation | milk heated to about 70 °C for 15–30 seconds, then quickly chilled |
Airtight packing | dry fruits, snacks |
The nitrogen cycle
Nitrogen makes up about 78% of air, but plants can't use the gas directly. Nitrogen-fixing bacteria and blue-green algae, along with lightning, convert it into compounds that plants absorb. Animals get nitrogen by eating plants. When plants and animals die, decomposers return nitrogen compounds to the soil, and other bacteria release nitrogen gas back into the air. The amount of nitrogen in the air stays roughly constant.
Summary
Microbes include bacteria, fungi, protozoa, algae and viruses.
They make curd, bread and antibiotics, fix nitrogen and decompose waste.
Some cause diseases; vaccines, hygiene and controlling carriers prevent them.
Salt, sugar, oil, heating, cooling and pasteurisation preserve food.