Antibiotics and resistanceEdexcel A-Level Biology B: Mind map

Exam questions on Antibiotics and resistance

1. A microbiologist adds penicillin to a growing culture of bacteria and tetracycline to a second, identical culture. In the penicillin culture the number of living cells falls to almost zero within several hours. In the tetracycline culture the number of living cells stays roughly constant but does not increase. When the tetracycline is washed out, the cells begin to divide again.
   Explain how penicillin kills growing bacteria but does not harm human cells.2 marks
2. Researchers are testing a new compound, X, that binds to a site on bacterial ribosomes and prevents them from reading mRNA. Bacteria exposed to X do not die at once but cannot grow. Human liver cells exposed to the same dose of X are unaffected.
   Compound X is bacteriostatic. Explain why a patient's own immune system is still needed to clear the infection.2 marks
3. A patient with an infected wound is treated with an antibiotic. At first the infection responds, but after several days the bacteria in the wound are no longer killed. Tests show that the bacterial population now contains a gene for an enzyme that destroys the antibiotic. The gene is carried on a plasmid. Before treatment only about one bacterium in ten million carried it.
   Explain how treatment with the antibiotic led to a bacterial population that was mostly resistant.3 marks