Resistance to antibiotics within a population of bacteria is due to selection pressure. This can be linked to the use of antibiotics by patients. A study was carried out into the link between antibiotic use and the presence of resistant Escherichia coli (E. coli) populations in human communities. • Over 30000 patients were involved in the study. • Only patients attending large medical clinics took part in the study. • The number of prescriptions issued by each clinic was used as an estimate of antibiotic use. • Urine from patients attending the clinics was used as a possible source of antibiotic resistant E. coli. • Antibiotic resistance of E. coli in the urine samples was measured using the disc diffusion method. The disc diffusion method measures sensitivity of bacteria to an antibiotic. A bacterial population with low sensitivity to an antibiotic is resistant to that antibiotic. In the disc diffusion method a Petri dish is filled with nutrient agar and urine samples containing E. coli are spread evenly across the agar. Discs containing different antibiotics are placed on top of the agar. A lid is put on the Petri dish and the plate is incubated overnight. Fig. 2.1 shows an example of a Petri dish from the study after incubation. [Figure 2.1] Key 1 = cephalosporin 2 = trimethoprim 3 = co-amoxiclav 4 = ampicillin 5 = quinolone

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2018). The mark or mark type is shown in brackets.

1. [2] any two from 1 volume of urine ; 2 volume/concentration / composition / pH, of agar ; 3 concentration / volume, of antibiotics; 4 (incubation) temperature; 5 incubation time; 6 size/diameter / area / spacing / type / source, of discs ;
2. [2] 1 measure the, diameter / radius / area, of clear zone around the antibiotic disc (with a ruler / callipers / grid); 2 idea of the, larger / wider / bigger / AW,the clear zone is, the, more sensitive / less resistant, the bacteria are to the given antibiotic / ora or idea of no clear zone means bacteria are, resistant to / not affected by / not killed by / not sensitive to, (given) antibiotic ;
3. [2] any two from 1 idea that it shows a large, spread of data (around the mean) / difference in data / deviation from the mean / variation with the mean; 2 data is not (very) reliable / trustworthy / consistent / AW ; 3 standard deviation increases as mean increases / positive correlation between standard deviation and mean;
4. [2] any two from idea that 1 patients may not take the antibiotics prescribed ; 2 patients may not complete the course of antibiotics; 3 dose (per prescription) may differ; 4 antibiotics are also prescribed by dentists / hospitals / other practitioners ; 5 patients may buy antibiotics (without a prescription); 6 data only taken from large clinics; ora 7 patients may be prescribed, antibiotics other than those stated in table / more than one antibiotic;
5. [1] any one from 1 data, may be / is, non-linear / skewed; 2 data, not/may not be, normally distributed ; 3 data is ordinal / discontinuous / not continuous / discrete; 4 scatter, graph / diagram, shows / may show, that there is a relationship; 5 data samples are independent; 6 ref. to random selection ;
6. [1] there is no (significant) correlation between antibiotic use / (number of) prescriptions and, the percentage of urine samples containing resistant E. coli/ the presence of resistant strains of E. coli in the urine (of patients);
7. [2] 1 two from ampicillin, trimethoprim, quinolone ; 2 calculated value / r_s, is higher than the, critical / table value (of r_s at, p = 0.05/0.362) or critical value is, at p = 0.05 / 5% significance /0.362;
8. [3] description max 2 1 is higher in males than females (of the same age / at all ages) ; ora 2 is initially high in, (very) young / 3-year olds / up to 15 ; 3 decreases (steeply), in young / up to (around) 20 ; 4 resistance, more or less plateaus / fluctuates / changes little / goes up and down, between ages 20/30 and 60/80 ; 5 increases steeply from (around) 80 or increases from around, 60–80 / old age; 6 is highest at 100 years (in both); explanation (must be linked to antibiotic use) max 2 7 antibiotic / ampicillin, use is higher in the, young / old; ora 8 idea of people between 20 and 60/80 are prescribed, different antibiotics / antibiotics other than ampicillin ; 9 males, may be less likely to complete the full course of antibiotics / may take more antibiotics; ora 10 idea of young / old, have greater contact with, hospital / health care centres, and therefore greater exposure to, ampicillin / antibiotic-resistant E. coli ;