• Question: Does MRSA mutate?

    Asked by missc to Enda, Jean, Kate, Kev, Tim on 19 Nov 2012.
    • Photo: Jean Bourke

      Jean Bourke answered on 19 Nov 2012:

      Yes. All living things that reproduce mutate, it is a natural part of reproduction and what causes evolution. Mutations occur when the DNA in a cell get’s accidentally altered a bit.

      MRSA can mutate in various ways, as can we all.
      There are 3 principal kinds of mutations: advantageous (there give the organism a competitive advantage in the current environment and so allow it outcompete its rivals and reproduce more), disadvantageous (do the opposite and allow the organism to be outcompeted), neutral (neither advantageous or disadvantageous).

      Most are neutral but you have to remember that a mutation that is advantageous in one environment may be disadvantageous in another. Advantageous mutations tend to be passed on more as the organism that has them reproduces more, this is how evolution works. Disadvantageous mutations may still be passed on if they are not that bad.

      MRSA is still mutating, in fact there are many strains of MRSA that have slightly different characteristics and arose to due to mutation and different environmental pressures.

    • Photo: Enda O'Connell

      Enda O'Connell answered on 19 Nov 2012:

      Hi Missc,

      Short answer: yes it has already mutated and still does constantly.

      MRSA stands for Methicillin Resistant Stapholococcus aureus. Stapholococcus aureus is a bacterium that normally lives quite happily on your skin and nasal passages, without causing you any trouble. Occasionally though, it can cause skin infections like boils, or sinus problems and even food poisoning.

      If you had a small cut in your hand that became infected with just one Stapholococcus aureus, the bacteria could divide and produce over one million Stapholococcus aureus in about 10 hours (if your body’s defences did nothing to stop it). During the dividing process, the daughter cells need to receive DNA, which is copied from the mother cell’s DNA. This copying process isn’t always 100% accurate and these copying mistakes are what we call mutations. In that 10 hours of one Stapholococcus aureus dividing to give you one million Stapholococcus aureus, there would be around 300 mutations. If you left the Stapholococcus aureus to divide for 30 hours then every single letter (and there are about 3,000,000 letters) in its DNA code would have mutated a massive 30 times…

      These mutations could do a number of things: do nothing, i.e. have no effect; do something to the Stapholococcus aureus that would cause it to divide even faster and be even more successful; do something that would make it less successful; or even kill it.

      One thing that would make it more successful, would be to not be affected by the antibiotic Methicillin, which normally kills Stapholococcus aureus. If you were to take Methicillin and it killed all of the Stapholococcus aureus bugs then you would know that none of the mutations gave it methicillin resistance. But if just one Stapholococcus aureus cell survived, then it is possible that it has become methicillin resistant.

      In that case, or in the case that the one original Stapholococcus aureus that got into your wound was already methicillin resistant, then doctors would prescribe another antibiotic called Vancomycin, which normally kills the remaining bacteria. However, some Stapholococcus aureus bacteria are resistant to vancomycin as well, which can be bad news for the patient.

      So you see, MRSA and all other cells are constantly mutating as they divide. Most of the time it does nothing at all, but sometimes it can confer an advantage to the cell. This is how evolution happens, as the cells, or individual, has some advantage that allows it to be more successful, and this can be passed on, allowing more offspring to survive and reproduce with that beneficial mutation.

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