Antibiotic sensitivity tests can be carried out to choose appropriate antibiotics to use for treatment of bacterial diseases. A researcher carried out an antibiotic sensitivity test using two pathogenic bacteria, X and Y. The researcher prepared two Petri dishes containing agar. • A culture of each bacterium was spread over the surface of the agar. • Filter paper discs containing antibiotics were placed on the surface of the agar in each dish. • The Petri dishes were incubated at 25 °C for two days. The results of the test using three antibiotics, P, Q and R, are shown in Fig. 4.1. [Figure 4.1]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2020). The mark or mark type is shown in brackets.

1. [1] X – R and Y – P ;
2. [2] any two from: mp1 = resistance with ref. to P/Q/R Y, is resistant to Q / has no resistance to P ; A Y has some resistance to (antibiotic) R
3. mp2 = antibiotics used at different concentrations (antibiotic) R may, have different concentration / be less effective, compared with P ;
4. mp3 = reason for resistance ref. to gene(s) for resistance (on plasmids) or Y has, cell wall / cell membrane, that prevents entry of antibiotic Q or Y has enzyme that breaks down, Q/R ;
5. mp4 = action of antibiotics idea that antibiotics have, different / specific, target(s) / AW ; A any example of process inhibited by antibiotic, e.g. cell wall synthesis transcription translation DNA replication cell surface membrane function synthesis of folic acid
6. mp5 = AVP ; e.g. idea that P may be bacteriocidal / (antibiotic) R is bacteriostatic e.g. idea that gene for resistance to antibiotic R passed by, vertical / horizontal, transmission
7. [3] any three from: to award the MP1 to MP5 you must be able to see a difference between the two stated clearly
8. vaccines: (generally) are preventative / are not a treatment / are not a cure or use before a person, has an infection / is ill V antibiotics: are (generally) not preventative / are a treatment / are not a cure or used when a person, has an infection / is ill ;
9. vaccines: effective against bacteria and viruses (in context of different vaccines) V antibiotics: not effective against viruses / only effect against bacteria ;
10. vaccines: idea of indirect effect on pathogens V antibiotics: idea of direct effect on pathogens ; A example of effect
11. vaccines: not given as a course / give once or a few times (or with boosters) V antibiotics: given as a course / over many days / AW ;
12. vaccines: specific for particular, pathogen V antibiotics: (most) antibiotics act on a range of pathogens ;
13. vaccines: stimulate an immune response or antibiotics: do not stimulate an immune response ;
14. vaccines: detail e.g. stimulate, (B-/T-) lymphocytes / production of antibodies A stimulates production of memory cells or antibiotics: do not stimulate, (B-/ T-) lymphocytes / production of antibodies ; A no stimulation of memory cells
15. vaccines: provide long-term, protection / immunity or antibiotics: do not provide long-term, protection / immunity ;
16. vaccines: does not lead to resistance or antibiotics: may lead to resistance ;
17. vaccines: time delay before being effective / AW or antibiotics: have effect, faster / sooner / AW ;