i) Many strains of bacteria, including MRSA, have developed resistance to antibiotics as a result of natural selection. To prevent further resistance arising it is important to avoid over-use of antibiotics.

j) Mutations of pathogens produce new strains. Antibiotics and vaccinations may no longer be effective against a new resistant strain of the pathogen. The new strain will then spread rapidly because people are not immune to it and there is no effective treatment.
Higher Tier candidates should understand that:
■ antibiotics kill individual pathogens of the non-resistant strain
■ individual resistant pathogens survive and reproduce, so the population of the resistant strain increases
■ now, antibiotics are not used to treat non-serious infections, such as mild throat infections, so that the rate of development of resistant strains is slowed down.

k) The development of antibiotic-resistant strains of bacteria necessitates the development of new antibiotics.

l) People can be immunised against a disease by introducing small quantities of dead or inactive forms of the pathogen into the body (vaccination). Vaccines stimulate the white blood cells to produce antibodies that destroy the pathogens. This makes the person immune to future infections by the microorganism. The body can respond by rapidly making the correct antibody, in the same way as if the person had previously had the disease.
MMR vaccine is used to protect children against measles, mumps and rubella.

m) Uncontaminated cultures of microorganisms are required for investigating the action of disinfectants and antibiotics.
For this:
■ Petri dishes and culture media must be sterilised before use to kill unwanted microorganisms
■ inoculating loops used to transfer microorganisms to the media must be sterilised by passing them through a flame
■ the lid of the Petri dish should be secured with adhesive tape to prevent microorganisms from the air contaminating the culture.

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