Classifying Pathogens

The dot point's word is classifying, and classify is not in the accepted verb set. What the dot point asks for is telling pathogen groups apart, which NESA examines as Distinguish — classify → distinguish.

Plan: state the first criterion and apply it, then the second criterion and apply it, then say why the classification matters.

The first point of difference is whether the pathogen is made of cells. A cellular pathogen is a living organism with a cell membrane, its own metabolism and the ability to reproduce independently. A non-cellular pathogen has none of these — it has no cell membrane, carries out no metabolism of its own, and can only replicate using a host cell's machinery.

Bacteria, protozoans and fungi are cellular: each is a living cell that grows and divides on its own given nutrients, which is why they can be cultured on agar. Viruses and prions are non-cellular: a virus is genetic material in a protein coat that must enter a cell to replicate, and a prion is only a misfolded protein with no genetic material at all — so neither can be grown on a plate without host cells.

Within the cellular pathogens the second point of difference is whether the cell has a nucleus. Prokaryotic pathogens — the bacteria — have no nucleus and no membrane-bound organelles, carrying a single circular chromosome free in the cytoplasm, and they have distinctive cell walls containing peptidoglycan. Eukaryotic pathogens — protozoans such as Plasmodium, and fungi such as Trichophyton — have a nucleus, membrane-bound organelles, and cellular machinery much more similar to our own.

The distinction determines treatment, which is why it is more than bookkeeping. Antibiotics work by attacking structures prokaryotes have and human cells do not, peptidoglycan cell walls among them, so they are useless against viruses, which have no such structures, and dangerous to rely on against eukaryotic pathogens, whose cellular machinery resembles ours closely enough that a drug harming them often harms us. Prescribing an antibiotic for a viral infection therefore cannot work, and it accelerates resistance in the bacteria the patient does carry.

Compare requires similarities and differences against stated criteria, and the question supplies three — structure, replication and treatment. Those are the criteria; use them by name.

Plan: state the criteria and the shared ground, compare on each, then close on what the comparison predicts.

Both are transmissible agents that cause disease, both can enter through the same routes, and both provoke an immune response, so the body's initial reaction to each is similar. They will be compared on structure, on how each replicates, and on the treatments available.

A bacterium is a complete prokaryotic cell — cell membrane, cytoplasm, ribosomes, a circular chromosome and usually a peptidoglycan cell wall — and it is typically a few micrometres across. A virus is not a cell: it is genetic material, DNA or RNA, enclosed in a protein capsid, sometimes with a lipid envelope taken from a host membrane, and it is typically a hundred times smaller, small enough to pass filters that retain bacteria.

The difference in replication follows from the difference in structure. A bacterium reproduces itself by binary fission, copying its chromosome and dividing, and it can do this anywhere nutrients allow, including outside a host. A virus cannot reproduce at all on its own: it attaches to a specific receptor, inserts its genetic material, and redirects the host cell's ribosomes and enzymes to make viral components, which assemble and leave — often destroying the cell. Because attachment depends on a specific receptor, a virus's host range is usually much narrower than a bacterium's.

Treatment differs because the targets differ. Antibiotics work against bacteria by attacking prokaryote-specific structures such as the peptidoglycan wall, the bacterial ribosome or bacterial DNA replication, all of which a virus lacks entirely — so antibiotics are ineffective against viral infection. Antiviral drugs must instead target the few steps a virus performs distinctly, such as entry, reverse transcription or assembly, which is a narrower set of targets, so antivirals are more specific and harder to develop. Vaccination works against both, because it acts on the immune response rather than the pathogen.

The comparison predicts why the two behave differently in a population. A bacterium that can survive outside a host persists on surfaces and in water, so control targets sanitation, while a virus dependent on living cells spreads mainly by direct contact and short-range aerosol, so control targets contact and ventilation. Knowing which kind of pathogen is involved therefore predicts both the treatment and the control measure.