Vaccination and Antibiotics

Having defences against pathogens is good; having medicine to prevent and treat disease is even better. This lesson covers two of the great triumphs of modern biology. Vaccination uses the body's own memory cells to make us immune before we ever meet a dangerous pathogen. Antibiotics kill bacteria once they are inside us — though crucially they do not work on viruses, and antibiotic resistance is now a serious global problem. We finish with the B6 required practical: culturing microorganisms using aseptic technique and measuring the zone of inhibition around antibiotic or antiseptic discs, including the $\pi r^2$πr2 area calculation.

By the end of this lesson you should be able to explain how a vaccine produces immunity, describe herd immunity and weigh the benefits and risks of vaccination, explain what antibiotics do and why they do not affect viruses, explain antibiotic resistance and how to slow it, distinguish painkillers from antibiotics, and carry out and interpret the microbiology required practical.

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How Vaccination Works

A vaccine contains a small, safe amount of a dead or inactivated pathogen (or just its antigens). It cannot cause the disease, but it carries the pathogen's antigens. When the vaccine is injected:

1. The antigens are recognised by the white blood cells (lymphocytes).
2. The lymphocytes produce the correct antibodies against those antigens.
3. Crucially, the body also makes memory cells.
4. If the real, live pathogen later enters the body, the memory cells recognise it at once and produce the right antibody quickly and in large amounts, destroying the pathogen before it can make you ill.

In effect, a vaccine gives the immune system a harmless "practice run" so that it is ready for the real thing.

Herd immunity

If a large proportion of the population is vaccinated, the pathogen finds it very hard to spread, because there are too few susceptible people to pass it between. This protects even the unvaccinated — for example babies too young for the vaccine, or people who cannot have it for medical reasons. This indirect protection is called herd immunity. If vaccination rates fall too low, herd immunity breaks down and outbreaks can return.

The idea of a threshold makes this clearer. For herd immunity to work, the proportion of immune people has to be above a certain level — and that level is higher for diseases that spread very easily. The reasoning is straightforward: if almost everyone an infected person meets is already immune, the pathogen runs out of new hosts and each infection leads, on average, to fewer than one new infection, so the outbreak dies out. If vaccination drops below the threshold, the infected person meets enough susceptible people that each case causes more than one further case, and the disease spreads again. This is exactly why public-health campaigns aim for very high uptake rather than just "most people": a measles-type pathogen, which is extremely infectious, needs a much larger fraction of the population vaccinated than a less contagious one to keep the chain of transmission broken.

Benefits and risks of vaccination

Benefits	Risks / drawbacks
Protects the vaccinated individual from a serious disease	A few people have mild side effects (e.g. sore arm, brief raised temperature)
Provides herd immunity, protecting the wider community	Very rarely, a more serious reaction occurs
Has controlled, and in the case of smallpox eradicated, diseases that once killed or disabled many	A vaccine does not always give 100% protection to everyone
Cheaper than treating widespread illness	Some people are unable to be vaccinated for medical reasons

Exam Tip: A vaccine does not contain a live, fully active pathogen, so it cannot give you the disease — this is a very common misconception. Describe vaccine contents as a dead or inactivated pathogen, or its antigens.

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Antibiotics

Antibiotics are medicines that kill bacteria (or stop them reproducing) inside the body without harming the body's own cells. Penicillin, discovered by Alexander Fleming, was the first widely used antibiotic. Antibiotics have transformed medicine: infections that were once often fatal can now be cured.

Antibiotics do not affect viruses

This is one of the most heavily examined facts in B6: antibiotics kill bacteria but have NO effect on viruses. Viruses reproduce inside the body's own cells, so a drug that destroyed them would be likely to damage the body's cells too. This is why your doctor will not give you antibiotics for a cold or for flu — these are viral illnesses, and the antibiotic simply would not work.

Exam Tip: If a question describes a patient given antibiotics for "flu" or "a cold" and asks why they did not get better, the answer is that flu and colds are caused by viruses, and antibiotics do not affect viruses.

The problem of antibiotic resistance

Bacteria reproduce quickly, and random mutations sometimes produce a bacterium that the antibiotic cannot kill — a resistant bacterium. When an antibiotic is used, the non-resistant bacteria die but any resistant ones survive and reproduce, passing on their resistance. Over time, resistant strains spread. MRSA is a well-known example of a bacterium resistant to many antibiotics. This is natural selection in action (linked to B5).

It is worth being clear about why this happens so fast and how the resistance spreads. Bacteria can divide roughly every 20 minutes in good conditions, so a single resistant cell can become millions within a day — natural selection that would take many generations in a large animal happens in bacteria in hours. The resistant gene then spreads in two ways: down the generations, as each resistant bacterium passes the gene to its offspring; and sideways between bacteria, because bacteria can pass small rings of DNA to their neighbours, so resistance can move even between different bacterial species. Resistance also spreads between people — for example in hospitals, where many vulnerable patients, frequent antibiotic use and close contact let resistant strains such as MRSA pass from person to person. This is why good hygiene (hand-washing, cleaning) in hospitals is part of controlling resistance, alongside more careful prescribing.

Resistant bacteria are dangerous because the infections they cause are very hard to treat. To slow the development of resistance:

• doctors should not prescribe antibiotics for non-serious or viral infections;
• patients should always complete the whole course, so that all the bacteria (including any partly resistant ones) are killed;
• the use of antibiotics in farming should be restricted;
• the development of new antibiotics should be supported (though this is slow and expensive).

Painkillers and other medicines

Some medicines, such as painkillers (e.g. paracetamol), relieve the symptoms of a disease — they reduce pain or fever so you feel better — but they do not kill the pathogen. The body's immune system still has to clear the infection. Understanding this distinction is important: painkillers treat how you feel, antibiotics treat the bacterial cause.

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Required Practical: Culturing Microorganisms and the Zone of Inhibition

To investigate which antibiotics or antiseptics are most effective, we grow bacteria on an agar plate and add discs soaked in different substances. Where a substance kills or stops the bacteria, a clear ring with no growth appears around the disc — the zone of inhibition. The larger the clear zone, the more effective the substance.

Aseptic technique

Microorganisms must be cultured aseptically — keeping out unwanted microbes that could contaminate the culture or be harmful. Key steps:

• Sterilise the inoculating loop by passing it through a flame until red hot, then let it cool.
• Sterilise the agar, Petri dish and nutrient broth before use.
• Work near a Bunsen flame, which creates a rising air current that carries stray microbes away.
• Lift the lid of the Petri dish only slightly and for as short a time as possible.
• After inoculating, tape the lid on (but not all the way round, so air can enter) and incubate.
• In schools, cultures are incubated at no more than 25 °C so that pathogens harmful to humans (which grow best at body temperature, 37 °C) are unlikely to thrive.