6.6.1 Bacteriostatic and bactericidal antibiotics Quiz: Pearson Edexcel Biology A (Salters-Nuffield), Unit 6

The 20 questions

1. A bacteriostatic antibiotic is one that:

   • Kills bacteria directly by destroying their cell walls or membranes, leaving no survivors
   • Kills viruses that have entered bacterial cells, so that the bacteria are then free of infection
   • Stimulates the immune system to produce more antibodies against bacteria
   • Stops bacteria growing and reproducing, so the immune system can then clear them

2. An antibiotic that kills bacteria outright, rather than only stopping their growth, is described as:

   • Bacteriostatic, because it halts growth and leaves the bacteria alive for the immune system
   • Bactericidal, because it kills bacteria directly, for example by disrupting cell wall synthesis
   • Prophylactic, because it prevents infection before any bacteria have entered the body
   • Antigenic, because it binds to bacterial surface antigens and tags them for destruction

3. Penicillin is classed as bactericidal because it:

   • Blocks the 70S ribosome from making proteins, so the bacterium stops growing but survives
   • Inhibits the synthesis of the peptidoglycan cell wall, causing the bacterium to lyse
   • Activates the immune system to produce histamine, which breaks down the bacterial membrane
   • Blocks the replication of viral DNA inside the bacterium, which stops its reproduction

4. Tetracycline is classed as bacteriostatic because it:

   • Dissolves the capsid of the bacteria, so their genetic material leaks into the cell
   • Destroys the peptidoglycan wall, causing the bacteria to burst and die
   • Blocks protein synthesis on the bacterial 70S ribosome
   • Stimulates phagocytes to engulf and destroy the bacteria in the tissue

5. In core practical 15, a bacterial lawn is spread on agar and antibiotic discs are placed on it. What does a clear zone of inhibition indicate?

   • The antibiotic has prevented bacterial growth in that area, so the bacteria are sensitive to it
   • The bacteria have become resistant to the antibiotic, so the disc has no effect on them
   • The antibiotic has increased bacterial growth around the disc, which shows the drug is effective
   • The disc has killed the fungi on the agar, which is the only effect of antibiotics on bacteria

6. Which factor should be controlled to make a fair comparison of antibiotic effects in a disc diffusion investigation?

   • The concentration of the bacterial suspension and the volume of antibiotic applied to each disc
   • The colour of the agar, which must be different for each antibiotic to show the effect clearly
   • The age of the investigator, who must be the same person for all of the discs in the trial
   • The time of day at which the investigation is carried out, as bacteria grow at fixed rates only in the morning

7. A zone of inhibition is 20 mm for antibiotic A and 8 mm for antibiotic B under the same conditions. Which conclusion is best supported?

   • Antibiotic A is a virus, so it cannot be compared with the bacterial antibiotic B at all
   • The two antibiotics are equally effective, since both produce zones of inhibition on the agar
   • Antibiotic A is more effective against this bacterium than antibiotic B under these conditions
   • Antibiotic B is more effective, since a smaller zone shows that more bacteria have been killed

8. Why is the difference between bacteriostatic and bactericidal action important in treatment?

   • Bacteriostatic drugs are always more effective, because they remove the need for an immune response
   • The difference is irrelevant to treatment because all antibiotics act in exactly the same way
   • Bacteriostatic drugs may be enough in patients with strong immune systems, while bactericidal drugs may be needed when defences are weak
   • Bactericidal drugs are never used because they always kill the patient's own immune cells as well

9. Which group of antibiotics targets the bacterial cell wall?

   • Beta-lactams such as penicillin, which inhibit cross-linking of peptidoglycan
   • Macrolides, which bind to the 50S subunit and stop protein synthesis without killing bacteria
   • Tetracyclines, which bind to the 30S subunit of the ribosome and stop translation
   • Quinolones, which target the nucleus of human cells and so are used against viruses

10. Which statement about bacterial resistance is accurate?

    • Resistance arises only in viruses, which are able to alter their structure in response to drugs
    • Resistance arises when the immune system produces antibodies that protect bacteria from antibiotics
    • Resistance arises when bacteria deliberately change their cell walls in response to a drug in the body
    • Resistance can arise by natural selection when mutant bacteria survive exposure to an antibiotic and reproduce

11. Why might a bacteriostatic drug fail to clear an infection in a patient with a severely weakened immune system?

    • The drug increases the number of phagocytes, which then destroy the drug instead of the bacteria
    • The drug actively protects the bacteria from the immune system, so they can multiply unchecked
    • The drug only holds the bacteria in check
    • The drug is destroyed by the immune system before it can reach the bacteria at all

12. Which measurement best quantifies the effectiveness of an antibiotic in core practical 15?

    • The pH of the agar at the end of the investigation, measured with indicator paper
    • The number of colonies that grow on the disc surface, counted under a microscope
    • The mass of the agar plate before and after the investigation, measured on a balance
    • The diameter of the clear zone around the disc, measured in millimetres

13. Which statement evaluates the claim that bactericidal antibiotics are always better than bacteriostatic ones?

    • The claim is incorrect, since bactericidal drugs are never effective against any type of bacterium
    • The claim is correct, since killing a pathogen is always better than merely slowing its growth
    • The claim is correct, since bacteriostatic drugs never affect the immune response to any infection
    • The claim is oversimplified, since the better choice depends on the infection, the site and the patient's immune status

14. Which change would most likely make an antibiotic less effective against a bacterium?

    • A mutation that increases the thickness of the peptidoglycan wall, which the drug can easily penetrate
    • A mutation that removes all ribosomes from the cell, which makes protein synthesis faster
    • A mutation that makes the bacterium more sensitive to stomach acid, which then destroys it
    • A mutation that alters the target enzyme so the drug can no longer bind to it

15. Which group of pathogens is not affected by most antibiotics?

    • Gram-positive bacteria, which have a thick peptidoglycan wall that is easily penetrated
    • Viruses, which lack the bacterial structures that antibiotics target
    • Mycobacteria, which have a cell wall of chitin that is broken down by most antibiotics
    • Gram-negative bacteria, which have an outer membrane that is easily destroyed by penicillin

16. Macrolide antibiotics act by:

    • Cross-linking the peptidoglycan of the cell wall, which causes the bacterium to burst
    • Inhibiting DNA gyrase, which prevents the bacterial chromosome from being copied during division
    • Binding to the 50S subunit of the bacterial 70S ribosome, which blocks protein synthesis
    • Dissolving the outer membrane lipopolysaccharide, which exposes the bacterium to lysozyme

17. A disc of diameter 6 mm is placed on an agar lawn and the total diameter of the clear zone is 18 mm. What is the radius of the zone of inhibition measured from the disc edge?

    • 12 mm, since 18 minus 6 gives the diameter of the zone of inhibition directly
    • 3 mm, since 6 divided by 2 gives the disc radius, which is subtracted from 18 mm
    • 9 mm, since 18 divided by 2 gives the radius without subtracting the disc
    • 6 mm, since (18 - 6)/2 = 6

18. Which observation would most strongly suggest that an antibiotic is bactericidal rather than bacteriostatic?

    • The viable count stays constant while the antibiotic is present, with no decrease over time
    • The bacterial count rises more slowly, but never falls, in the presence of the antibiotic
    • The viable count of bacteria falls over time while the antibiotic is present
    • The bacteria change colour but their number is unaffected by the antibiotic over the whole test

19. Why should patients complete the full prescribed course of an antibiotic?

    • To ensure the infection is fully controlled, reducing the chance that surviving resistant bacteria cause a relapse
    • Because stopping early always causes the bacteria to die in the blood of the patient
    • Because the course length is set to match the time the immune response takes to fully disappear
    • Because the drug becomes stronger the longer it is taken, so the last doses are the most effective

20. Aminoglycoside antibiotics, such as streptomycin, are bactericidal because they:

    • Dissolve the outer membrane, so the bacterium is exposed to lysozyme in the tissue
    • Cross-link the peptidoglycan of the cell wall, so the wall weakens and the bacterium bursts
    • Bind the 30S ribosomal subunit and cause misreading of mRNA, so faulty proteins are made
    • Block DNA gyrase, so the bacterial chromosome cannot be copied during cell division

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