Methicillin-resistant Staphylococcus aureus (MRSA) is a bacterium that causes difficult-to-treat infections in humans. MRSA developed from a similar strain called Methicillin-sensitive Staphylococcus aureus (MSSA) that was treatable with antibiotics. Through natural selection, some MSSA bacteria gained mutations that made them resistant to antibiotics. Those resistant bacteria survived exposure to antibiotics and passed on the resistance genes through asexual reproduction, leading to the emergence of the MRSA strain.

1. Antibiotic Resistance! ! ! ! ! 1
1 Methicillin-resistant Staphylococcus aureus (MRSA) is a bacterium responsible for
several difficult-to-treat infections in humans.
! A person who gets infected with MRSA does not respond to antibiotics that were
previously effective at killing the bacteria.
! 1 (a) (i) Which of the following is a reason why the MRSA bacteria are dangerous to people?
! Tick the correct answer. [1 mark]
!
The MRSA bacteria are more powerful than before.
The MRSA bacteria produce more toxins.
! New treatments are needed to kill the MRSA bacteria.
!
1 (a) (ii) The MRSA bacteria developed from a strain of bacteria called Methicillin-sensitive
Staphylococcus aureus (MSSA).
! MSSA could be killed with antibiotics whereas MRSA cannot.
! Explain how natural selection could have caused the development of MRSA. [5 marks]
Mutation in DNA or description of mutation (gives resistance to antibiotics) [1]
! Some survive [1]
! (survivors) reproduce or multiply [1]
asexual reproduction [1]
gene for resistance or the mutation is passed on (to offspring) [1]
!
1 (b) Doctors will not prescribe antibiotics for non-serious illnesses, such as a mild throat
infection.
! Suggest why. [1 mark]
! Bacteria in general Reduce chances of developing antibiotic resistant bacteria [1]
! End
(Total 7 marks)
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This is one of those questions that is reasonably easy to
understand, but difficult to explain! The key point is that a
mutation causes the resistance. Some students talk about
the bacteria ‘getting used to’ the antibiotics. Use the video
and this answer to make sure you explain it well.