Micro-organisms

What are micro-organisms?

Micro-organisms (microbes) are living things too small to be seen with the naked eye — you need a microscope to observe them. Despite their tiny size, they are found almost everywhere: in the air, water, soil, on surfaces, and inside our bodies.

Types of micro-organisms

There are four main types studied at this level:

Harmful micro-organisms

Disease-causing micro-organisms are called pathogens. They are found on handrails, ATMs, door handles, in contaminated water and food.

Waterborne diseases like cholera and diarrhoea are major causes of child deaths globally. Effective prevention includes washing hands regularly, sterilising surfaces, and ensuring clean water supplies.

Useful micro-organisms

Not all microbes are harmful — many are essential:

• Decomposers: Bacteria and fungi break down dead plant and animal matter, recycling nutrients back into the soil. Without decomposers, nutrients would stay locked in dead material.
• Food production: Yeast (a fungus) ferments sugars to produce CO₂ (making bread rise) and ethanol. Bacteria are used to make yoghurt and cheese.
• Medicine: Penicillin — the first antibiotic — was discovered from the mould Penicillium by Alexander Fleming in 1928.
• Agriculture: Nitrogen-fixing bacteria in soil convert atmospheric nitrogen into compounds plants can use.

Scientists and micro-organisms

Louis Pasteur proved that microbes cause disease (germ theory) and developed pasteurisation — heating food to kill harmful bacteria. His work transformed medicine and food safety. Edward Jenner developed the first vaccine (smallpox) by observing that milkmaids who caught cowpox rarely got smallpox.