Answers · Antibiotics

Viruses and resistant bacteria — answer key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. BQ1 · Practice

Model answer. Tuberculosis is caused by a bacterium, so an antibiotic can act on it. Chickenpox, mumps and cold sores are caused by viruses, and antibiotics do not affect viruses.

A full answer includes

I chose tuberculosis, which is caused by a bacterium.

Also accept

tuberculosis

Watch for

“Chickenpox.” — Chickenpox is caused by a virus. Antibiotics kill bacteria but do not affect viruses.
“Mumps.” — Mumps is caused by a virus, so an antibiotic does nothing to it.
“Cold sores.” — Cold sores are caused by a virus. An antibiotic has nothing in a virus to act on.

2. BQ2 · Practice

Model answer. An antibiotic acts on parts of a bacterial cell, such as its cell wall or ribosomes. A virus is not a cell and has none of these, so the antibiotic has nothing to act on.

A full answer includes

I chose: a virus has none of the cell parts, such as a cell wall, that antibiotics act on.

Also accept

a virus has none of the cell parts, such as a cell wall, that antibiotics act on

Watch for

“Viruses are too small for an antibiotic to reach.” — Size is not the reason. Antibiotics spread through the body to the tissues where viruses are; there is simply nothing in a virus for them to act on.
“Viruses quickly become resistant to every antibiotic.” — Resistance is something bacteria develop. Antibiotics never affected viruses in the first place.
“Antibiotics are broken down in the blood before they reach a virus.” — Antibiotics reach bacteria in the tissues, so they would reach viruses too. A virus has no cell wall or ribosomes for them to act on.

3. BQ3 · Practice

Model answer. Bacteria are resistant to an antibiotic when it does not kill them or stop them growing. Resistance belongs to the bacteria, not to the patient.

A full answer includes

I chose: the antibiotic no longer kills the bacteria or stops them growing.

Also accept

the antibiotic no longer kills the bacteria or stops them growing

Watch for

“The patient's body has got used to the antibiotic.” — Resistance belongs to the bacteria. Someone who has never taken an antibiotic can catch bacteria that are resistant to it.
“The bacteria can be killed only by heating them.” — Resistant bacteria are killed by heat like any others. Resistance means one antibiotic no longer works against them.
“The antibiotic has gone past its use-by date.” — An old antibiotic may work less well on every bacterium. Resistance is a property of particular bacteria.

4. BQ4 · Practice

Model answer. J and L. The bacteria have grown right up to them, as they have to the water disc W, so J and L do not stop these bacteria growing. K and M have clear zones, so these bacteria are not resistant to them.

A full answer includes

I chose J and L.
I did not choose W: it is the control, with no antibiotic on it.

Also accept

J and L

Watch for

“W, because the bacteria grew right up to it.” — W carries no antibiotic, so there is nothing to be resistant to. It is the control.
“M, because it has the widest zone.” — A wide zone means M stopped the bacteria growing. Resistant bacteria grow right up to the disc.
“K, because its zone is smaller than the zone of M.” — K still has a clear zone wider than the disc, so it stopped some growth. Resistance shows as growth right up to the disc.

5. BQ5 · Practice

Model answer. B. A 6 mm zone is the disc alone: the bacteria grew right up to it, so they are resistant to T, and T would not treat this infection. A 20 mm zone means T stopped the bacteria growing, so they are not resistant and T could treat it.

A full answer includes

I chose row B.
A 6 mm zone means no clear zone, so the bacteria are resistant and T would not treat the infection.

Also accept

Watch for

“Row A: 6 mm means not resistant, so T would treat the infection.” — A 6 mm zone is just the disc: T did not stop the bacteria at all. They are resistant.
“Row C: a 20 mm zone shows the bacteria are resistant.” — A wide clear zone means T stopped the bacteria growing, so they are not resistant to it.
“Row D: T would not treat the infection, whatever the zone.” — With a 20 mm zone T stops these bacteria growing, so it could treat the infection.

6. BQ6 · Practice

Model answer. Using antibiotics only when essential, for bacterial infections that need them. Every use kills the bacteria that are not resistant and leaves the resistant ones to multiply, so fewer uses mean fewer chances. Giving antibiotics for every sore throat, to healthy animals or for colds all add uses.

A full answer includes

I chose: using antibiotics only for bacterial infections that need them.

Also accept

using antibiotics only for bacterial infections that need them

Watch for

“Giving antibiotics early for every sore throat.” — Most sore throats are caused by viruses. Each unneeded use still kills bacteria in the body that are not resistant and leaves the resistant ones.
“Adding antibiotics to the feed of healthy farm animals.” — Every dose given to healthy animals is another chance for resistant bacteria in them to survive while the others die.
“Using a new antibiotic for each cold, so the virus cannot resist.” — A cold is caused by a virus, which no antibiotic affects. Each antibiotic still kills the bacteria in the body that are not resistant.

7. BI1 · Practice

Model answer. The chest infection is caused by bacteria, and an antibiotic kills bacteria or stops them growing. Flu is caused by a virus. A virus has no cell wall or ribosomes of its own and copies itself inside the body's cells, so an antibiotic has nothing to act on and does not affect it.

A full answer includes

I wrote that the chest infection is caused by bacteria and the antibiotic kills them or stops them growing.
I wrote that flu is caused by a virus.
I wrote that a virus has no cell wall or ribosomes for an antibiotic to act on, so antibiotics do not affect it.
I did not write that the flu virus was resistant to the antibiotic.

Also accept

bacteria: the antibiotic kills them; flu is a virus, which antibiotics do not affect

Watch for

“The antibiotic works now because he is stronger after the flu.” — The antibiotic acts on the bacteria, not on the man. It works because the cause is now bacteria.
“The flu virus had become resistant to the antibiotic.” — Viruses are not affected by antibiotics at all, so resistance is not the reason.

8. BI2 · Practice

Model answer. 2015: 3 ÷ 60 × 100 = 5 %. 2025: 21 ÷ 60 × 100 = 35 %. A doctor treating this infection in 2025 would find the antibiotic failed about one time in three, so resistance has made it much less effective.

Checked automatically

2015: 5 % (exact value)
- Common wrong answer 20 : That is 60 ÷ 3. Divide the number resistant by the number tested.
2025: 35 % (exact value)
- Common wrong answer 21 : That is the number of samples, not the percentage. Divide by 60 and multiply by 100.

A full answer includes

I calculated 3 ÷ 60 × 100 for 2015.
I wrote 5 % for 2015.
I calculated 21 ÷ 60 × 100 for 2025.
I wrote 35 % for 2025.

Also accept

5 % in 2015; 35 % in 2025

Watch for

“Divide 60 by the number resistant.” — The percentage is the part divided by the whole: the number resistant divided by the number tested, times 100.

9. BI3 · Practice

Options as numbered on the pupil sheet: 1 — The resistant bacteria survive and multiply.; 2 — A random mutation makes a few bacteria resistant.; 3 — The next population has a larger share of resistant bacteria.; 4 — It kills the bacteria that are not resistant.; 5 — The antibiotic is used..

Model answer. A random mutation makes a few bacteria resistant; the antibiotic is used; it kills the bacteria that are not resistant; the resistant bacteria survive and multiply; the next population has a larger share of resistant bacteria.

A full answer includes

I put the random mutation first.
I put the killing of the bacteria that are not resistant before the resistant ones multiply.

Also accept

mutation, antibiotic used, non-resistant killed, resistant survive and multiply, larger share resistant

Watch for

“The antibiotic causes the mutation that makes bacteria resistant.” — The mutations happen at random, whether or not the antibiotic is there. The antibiotic only removes the bacteria that are not resistant.
“The resistant bacteria multiply first, then the others die.” — The resistant bacteria get space and nutrients to multiply once the others have been killed.

10. BI4 · Practice

Model answer. Not resistant: 1000 − 10 = 990. Survivors among them: 10 % of 990 = 99. All survivors: 99 + 10 = 109. Resistant share: 10 ÷ 109 × 100 = 9.2 % (2 significant figures), up from 1 % before.

Checked automatically

Resistant share of the survivors: 9.2 % (2 significant figures)
- Common wrong answer 1.0 : That is the share before the antibiotic: 10 ÷ 1000. Use the survivors.
- Common wrong answer 10 : That is 10 ÷ 99. Add the 10 resistant bacteria to the survivors: 109.

A full answer includes

I found 990 not resistant, and 10 % of 990 = 99 survive.
I added the resistant ones: 99 + 10 = 109 survivors.
I calculated 10 ÷ 109 × 100.
I wrote 9.2 %, to 2 significant figures.

Also accept

9.2 %

Watch for

“Divide by 1000, the bacteria at the start.” — The question asks about the survivors. Most of the 1000 have been killed.
“Divide 10 by 99.” — 99 is only the survivors that are not resistant. The resistant ones survived too, so there are 109 survivors.

11. BT1 · Apply

Model answer. Every dose kills the bacteria in the chickens that are not resistant and leaves any resistant ones, which survive and multiply. Because the antibiotic is given all the time and not only when essential, resistant bacteria become common. They can pass to people, for example in undercooked meat, and an infection they cause is not treated by that antibiotic.

A full answer includes

I wrote that the antibiotic kills the bacteria in the chickens that are not resistant.
I wrote that the resistant bacteria survive and multiply, so they become common.
I wrote that resistant bacteria can pass to people, for example in food, so the antibiotic no longer treats their infections.
I did not write that the chickens themselves become resistant.

Also accept

the antibiotic selects resistant bacteria in the chickens; they spread to people and are not treated by it

Watch for

“The chickens become resistant to the antibiotic and pass that on in the meat.” — Resistance belongs to the bacteria, not to the chickens. It is resistant bacteria that can pass to people.
“The antibiotic in the meat makes people's bacteria resistant.” — The antibiotic does not change bacteria. It kills the ones that are not resistant and leaves the resistant ones to multiply.

12. BT2 · Apply

Model answer. Town P. Antibiotics are used far more often there, including for sore throats caused by viruses, where they do nothing to the cause. Every use kills bacteria in the body that are not resistant and leaves the resistant ones to survive and multiply, so resistant bacteria become common more quickly. Town Q uses antibiotics only when essential.

A full answer includes

I predicted town P.
I wrote that antibiotics are used more often there, including when a virus is the cause.
I wrote that each use kills bacteria that are not resistant and leaves resistant ones to survive and multiply.

Also accept

town P: more use gives resistant bacteria more chances to survive and multiply

Watch for

“Town Q, because the tests let bacteria survive.” — Testing does not let bacteria survive. Town Q uses fewer antibiotics, so resistant bacteria get fewer chances to take over.
“Neither, because antibiotics for a virus have no effect on bacteria.” — An antibiotic taken for a virus still acts on the bacteria living in the body, so it still kills those that are not resistant.

13. BT3 · Apply

Model answer. An antibiotic would not cure it. The cause is smaller than any bacterium and multiplies only inside living cells, so it is a virus, not a bacterium. A virus has no cell wall or ribosomes of its own, so antibiotics do not affect it.

A full answer includes

I predicted that an antibiotic would not cure it.
I wrote that the cause is smaller than a bacterium and multiplies only inside living cells, so it is a virus.
I wrote that antibiotics do not affect viruses, because a virus has no cell wall or ribosomes for them to act on.

Also accept

no: the cause is a virus, and antibiotics do not affect viruses

Watch for

“Yes, because the antibiotic would kill any germ in the leaf.” — Antibiotics act only on bacteria. This cause is too small to be a bacterium and grows only inside cells, so it is a virus.
“Yes, if the antibiotic is small enough to pass through the filter too.” — The size of the antibiotic is not the point. A virus has nothing for an antibiotic to act on, however the two meet.

14. BT4 · Apply

Model answer. Antibiotic F. Bacteria resistant to E are not killed or stopped by it, so E would fail against about 38 of every 100 of these infections, while F would fail against only about 3. Resistance has reduced the effectiveness of E.

A full answer includes

I chose antibiotic F.
I wrote that E does not kill or stop resistant bacteria, so it would fail in about 38 of every 100 such infections.
I wrote that F would fail in only about 3 of every 100.

Also accept

F: E fails against about 38 in 100 of these infections, F against about 3

Watch for

“E, because the bacteria have met it more often, so it is better tested.” — Bacteria that met E and survived are resistant to it. A higher share resistant means E fails more often.
“Either, because both antibiotics kill bacteria.” — Both kill bacteria that are not resistant, but far more of these bacteria are resistant to E.