Microorganisms

What Are Microorganisms?

Microorganisms (microbes) are living things too small to see with the naked eye — most are smaller than 0.1 mm. They are found everywhere: in soil, water, air, and inside other living organisms. Some are essential for life on Earth; others cause disease.

The four main groups are: bacteria, viruses, fungi, and protists.

Bacteria

Structure:

• Prokaryotic — no membrane-bound nucleus; DNA floats freely in the cytoplasm
• Single-celled with a cell wall (made of peptidoglycan, not cellulose)
• May have a flagellum (tail) for movement
• Some have a protective capsule

Reproduction: binary fission — the cell copies its DNA and splits in two. Under ideal conditions, bacteria can double every 20 minutes, meaning one cell can become millions in just a few hours.

Beneficial bacteria:

• Gut bacteria aid digestion and produce vitamin K
• Decomposers break down dead matter, recycling nutrients
• Nitrogen-fixing bacteria (e.g., Rhizobium) in root nodules convert N2 gas into nitrates for plants
• Used to make yoghurt, cheese, and vinegar

Harmful bacteria:

• Tuberculosis (TB) — caused by Mycobacterium tuberculosis
• Salmonella — food poisoning
• Streptococcus — throat infections

Viruses

Structure:

• NOT living cells — they have no cell membrane, cytoplasm, or ribosomes
• Consist of genetic material (DNA or RNA) enclosed in a protein coat (capsid)
• Some have an outer lipid envelope

Reproduction: viruses cannot reproduce independently. They inject their genetic material into a host cell, hijack its machinery to copy the virus, then burst the cell to release new viruses (lytic cycle).

Diseases caused by viruses:

• Influenza (flu)
• COVID-19
• Measles, mumps, rubella
• HIV/AIDS
• Common cold

Because viruses replicate inside host cells using the host's own machinery, they are very difficult to treat without harming the host. Antibiotics have NO effect on viruses.

Fungi

Structure:

• Eukaryotic — have a nucleus and membrane-bound organelles
• Cell walls made of chitin (not cellulose like plants)
• Include yeasts (single-celled) and moulds (multicellular, with thread-like hyphae)

Beneficial fungi:

• Yeast (Saccharomyces cerevisiae): used in bread-making (CO2 causes dough to rise) and alcohol fermentation
• Penicillium: produces the antibiotic penicillin
• Decomposers: fungi break down dead organic material

Harmful fungi:

• Athlete's foot (Tinea pedis)
• Thrush (Candida albicans)
• Some moulds produce toxins (mycotoxins)

Protists

• Eukaryotic — have a nucleus
• Mostly single-celled
• A diverse group — not all closely related

Examples:

• Amoeba — moves by extending pseudopods; lives in water and soil
• Plasmodium — causes malaria; transmitted by Anopheles mosquitoes
• Euglena — can photosynthesise and move; has features of both plants and animals

Useful Microorganisms — Summary

Use	Microorganism
Yoghurt and cheese	Lactobacillus bacteria
Bread and beer	Yeast (fungus)
Antibiotics (penicillin)	Penicillium mould
Decomposition	Bacteria and fungi
Nitrogen fixation	Rhizobium bacteria
Sewage treatment	Bacteria

Treating Microbial Diseases

Antibiotics: kill bacteria or stop them reproducing. They work by targeting bacterial cell walls or ribosomes — structures not found in human cells. They do NOT work on viruses.

Antivirals: interfere with viral replication. Harder to develop than antibiotics because viruses use host cell machinery.

Vaccines: prevent infection by training the immune system before exposure to the real pathogen.

Antibiotic resistance: when bacteria evolve to survive antibiotic treatment. Caused by overuse and misuse of antibiotics. A major global health concern — some bacteria are now resistant to nearly all available antibiotics (superbugs, e.g., MRSA).

Key Terms

• Microorganism — a living thing too small to see without a microscope
• Prokaryote — cell without a membrane-bound nucleus (bacteria)
• Eukaryote — cell with a membrane-bound nucleus (fungi, protists, plant and animal cells)
• Binary fission — asexual reproduction in bacteria; the cell splits into two
• Pathogen — a microorganism that causes disease
• Antibiotic resistance — the ability of bacteria to survive antibiotic treatment through evolution

Common Mistakes

• Saying viruses are alive — they cannot carry out life processes independently; they need a host cell
• Thinking antibiotics treat viral infections such as flu or colds — they do not
• Confusing fungi cell walls (chitin) with plant cell walls (cellulose)
• Assuming all bacteria are harmful — the vast majority are harmless or beneficial

Tips and Tricks

• Remember: Viruses are Very small and Viral (not living cells) — antibiotics are useless against them
• Binary fission doubles the population — after n divisions, there are 2 to the power n bacteria
• Prokaryote = Produced Kare-free (no nucleus); Eukaryote = Eukaryote has a Enclosed nucleus
• The mnemonic BVFP covers the four groups: Bacteria, Viruses, Fungi, Protists