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