Bacteria, Viruses and Pathogens

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 1.3 — Common features of prokaryotic organisms such as bacteria, with named examples
• 1.4 — The term pathogen, and that pathogens may be fungi, bacteria, protoctists or viruses

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

A bacterium is a cell without a nucleus. Its DNA is not locked in a library — it floats in the cytoplasm as one circular chromosome, with small extra rings called plasmids alongside. That is what 'prokaryote' means: before the nucleus.

Think of it like an office with no filing room

A eukaryotic cell is an office with separate rooms: the plans live in a locked file room (the nucleus), the power plant is a separate unit (mitochondria). A bacterial cell is one open-plan room. The plans lie on the floor where anyone can reach them, and the work happens all around them. It is smaller, simpler and much faster to copy — which is exactly why bacteria can divide every twenty minutes and you cannot.

Viruses are stranger. A virus is not a cell at all. It is a package: a protein coat with one type of nucleic acid inside — either DNA or RNA, never both. It has no cytoplasm, no membrane of its own machinery, no way of releasing energy and no way of building anything. It can only reproduce by getting inside a living cell and making that cell build copies of it. On the eight-characteristic checklist it fails almost everything, which is why biology says viruses are not living organisms.

Why 'is a virus alive?' has an answer

1. Nutrition — no. It takes nothing in.
2. Respiration — no. It has no metabolism at all.
3. Excretion — no. It makes no waste, because it runs no reactions.
4. Growth — no. It is assembled at full size.
5. Reproduction — only inside a host cell, using the HOST's machinery, not its own.
6. So: not a living organism. Biologists usually call it a particle.

Finally, 'pathogen' is not a group of organisms — it is a job description. A pathogen is any organism that causes disease, and the specification wants you to know that pathogens are found in four different categories: fungi, bacteria, protoctists and viruses. Athlete's foot is a fungus. Pneumonia is a bacterium. Malaria is a protoctist. Influenza is a virus.

That distinction between a bacterium and a virus has one consequence you meet constantly outside the classroom, and it follows from the structures above rather than from anything new. ANTIBIOTICS work by attacking things a bacterium has and a human cell does not — above all the bacterial cell wall, and the bacterium's own machinery for making proteins. Look back at what a virus is: a strand of genetic material in a protein coat, with no cell wall, no cytoplasm and no machinery of its own, reproducing inside your cells using YOUR machinery. There is nothing there for an antibiotic to attack, and anything that did attack the process would have to damage the human cell doing the work. That is why antibiotics cure a bacterial chest infection and do nothing whatever for influenza.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Antibiotics — why the bacterium/virus distinction decides whether they work

What an antibiotic attacks

Structures and processes a bacterium has and a human cell does not — chiefly the bacterial cell wall, and the bacterium's own protein-making machinery.

Why a virus is unaffected

A virus has no cell wall, no cytoplasm and no metabolism of its own, so it possesses none of those targets.

Why no antibiotic could be designed for it

A virus reproduces inside a host cell using the HOST'S machinery, so a drug stopping that process would have to damage the human cell too.

Who becomes resistant

Bacteria, not viruses. A virus was never affected in the first place, so there is nothing for it to become resistant to.

Bacteria — the features to state

• Microscopic and single-celled
• Have a cell wall, cell membrane, cytoplasm and plasmids
• No nucleus — instead a single circular chromosome of DNA
• Some can photosynthesise, but most feed off other living or dead organisms
• Named examples: Lactobacillus bulgaricus, a rod-shaped bacterium used to make yoghurt from milk; Pneumococcus, a spherical bacterium that is the pathogen causing pneumonia

Viruses — the features to state

• Not living organisms — they are particles
• Smaller than bacteria
• Parasitic: can only reproduce inside living cells
• Infect every type of living organism
• No cellular structure; a protein coat containing one type of nucleic acid, either DNA or RNA
• Named examples: tobacco mosaic virus (discolours tobacco leaves by preventing chloroplast formation); influenza virus (flu); HIV (causes AIDS)

Learn this definition · Pathogen

An organism that causes disease. Pathogens may be fungi, bacteria, protoctists or viruses.

Prokaryote vs eukaryote

A. Prokaryote (bacteria): no nucleus, DNA as a circular chromosome plus plasmids, no membrane-bound organelles.

B. Eukaryote (plants, animals, fungi, protoctists): DNA inside a nucleus, membrane-bound organelles such as mitochondria.

Not this: 'Viruses are the smallest living things.'

This: Viruses are not classed as living at all. They carry out none of the characteristics of life independently — reproduction only happens using a host cell's machinery.

One named pathogen from each category — learn all four

Pneumococcus

bacterium — causes pneumonia

Plasmodium

protoctist — causes malaria

Influenza virus

virus — causes flu

HIV

virus — causes AIDS

Learn this definition · Prokaryote

An organism whose cell has no nucleus. Its DNA is a single circular chromosome lying in the cytoplasm, with plasmids alongside, and it has no membrane-bound organelles.

Answering 'why do antibiotics not work here?' — the four-step chain

1. State what antibiotics attack: structures and processes a bacterium has and a human cell does not — chiefly the bacterial cell wall and the bacterium's own protein-making machinery.
2. State that the pathogen in the question is a virus.
3. State that a virus has none of those structures: no cell wall, no cytoplasm, no machinery of its own.
4. State that it reproduces inside the host's cells using the host's machinery, so anything stopping it would have to damage the patient's own cells.

Bacterium vs virus — the comparison every question on this page is built from

Model answer [4 marks]

Describe the structure of a bacterial cell. [4]

A bacterium is microscopic and single-celled. It has a cell wall, a cell membrane and cytoplasm. It has no nucleus: its DNA is a single circular chromosome lying in the cytoplasm. It also carries plasmids, which are small extra rings of DNA.

Model answer [3 marks]

Explain why a virus cannot be grown in a flask of nutrient broth, but a bacterium can. [3]

A bacterium is a complete cell with its own machinery, so given nutrients it can respire, build proteins and divide on its own. A virus has no cytoplasm and no machinery of its own, so it can build nothing from the nutrients. It can only be copied inside a living host cell, using that cell's machinery, so it needs living cells rather than broth.

Model answer [4 marks]

A doctor refuses to prescribe antibiotics for a patient with influenza. Explain the biology behind that decision. [4]

Antibiotics attack structures and processes that a bacterium has and a human cell does not — chiefly the bacterial cell wall and the bacterium's own protein-making machinery. Influenza is caused by a virus, which has no cell wall, no cytoplasm and no machinery of its own, so there is no such target. The virus reproduces inside the patient's own cells using their machinery, so a drug that stopped it would have to damage those cells. Taking antibiotics unnecessarily also selects for resistant bacteria.

Mark-losing trap. 'No nucleus' does not mean 'no DNA'. A bacterium's DNA is a circular chromosome lying loose in the cytoplasm.

Mark-losing trap. The main DNA is the circular chromosome; plasmids are the SMALL extra rings alongside it.

Mark-losing trap. Bacteria have cell walls. Only the material differs from a plant's — it is not cellulose.

Mark-losing trap. Viruses are not the smallest living things. They are not classed as living at all — call one a particle.

Mark-losing trap. It is BACTERIA that become antibiotic-resistant. A virus was never affected, so there is nothing for it to resist.

Mark-losing trap. Pathogen is a job, not a group. Say which of fungi, bacteria, protoctists or viruses the pathogen is.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — In a bacterial cell, what structure holds the main DNA, given that there is no nucleus?
2. Grade 7 · Explain [4 marks] — Explain why a virus is not classified as a living organism, referring to its structure and its reproduction.
3. Grade 8 · Compare [3 marks] — Which statement correctly compares a bacterial cell with a plant cell?
4. Grade 9 · Explain [4 marks] — Antibiotics kill bacteria but have no effect on viral infections such as influenza. Explain why, using what you know about the structure of each.
5. 9+ · Evaluate [4 marks] — A student writes: 'Pathogens are a fifth kingdom of organisms, alongside plants, animals, fungi and protoctists.' Evaluate this statement.

The people behind this science

Two ways into the same idea — the one who proved germs cause disease, and the one who found what kills bacteria — and what does not. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Louis Pasteur — the one who proved germs cause disease

Pasteur established germ theory — that specific microorganisms cause specific diseases — against an establishment convinced illness rose from bad air. Everything this lesson says about pathogens rests on that argument being won.

• “How did you prove microorganisms cause disease rather than appearing because of it?”
• “Why was the idea that bad air caused illness so hard to displace?”
• “How do you show that one particular microbe causes one particular disease?”
• “What is the difference between a bacterium and a virus, as you would explain it?”
• “Why does heating milk make it safe without boiling it?”

Alexander Fleming — the one who found what kills bacteria — and what does not

This lesson's hardest idea is that bacteria and viruses are fundamentally different kinds of thing. Fleming's penicillin is the proof in practice: it attacks the bacterial cell wall, which a virus does not have — and he warned about resistance decades before it became a crisis.

• “Why does penicillin destroy bacteria but do nothing at all to a virus?”
• “What did you actually see on the contaminated plate, and why did you look twice?”
• “You warned about resistance very early. What did you foresee?”
• “What makes a bacterial cell a target when a human cell is not?”
• “Why can a virus only reproduce inside another living cell?”

Then defend it

On Incandio a lesson is not finished when the questions come out right. You teach the idea back to Ember, an AI apprentice who asks the awkward question, and then you argue it against Martinus Beijerinck in a structured debate marked against descriptors you can read before you enter. Learn it, teach it, then defend it — all three happen on this page once the app loads.

Carry on through the course

• Previous lesson: The Four Eukaryote Groups
• Next lesson: Levels of Organisation
• What Makes Something Alive — The eight processes every living organism carries out — and why a fire, which seems to do several of them, is not alive.
• The Four Eukaryote Groups — Plants, animals, fungi and protoctists — how to tell them apart from their features, and the named examples the specification expects.
• All of Biology · Incandio Science