• Question: will there ever be a substance that could kill all bacteria. A bit like a cleaning spray

    Asked by anon-251281 to Freya on 22 Apr 2020.
    • Photo: Freya Harrison

      Freya Harrison answered on 22 Apr 2020:


      Good question! I think it’s unlikely, and even if we could make such a substance then we might not want to use it. There are a couple of reasons why it would be very hard to make something that could kill all bacteria.
      First, it would be very hard to get that substance to reach all of the bacteria you want to kill. Take bleach, for instance – that can in theory kill pretty much all bacteria, but when you spray it onto a surface there will be some bacteria that avoid getting hit by the spray droplets just by chance, or are protected by being in microscopic cracks in the surface where the bleach doesn’t get in. That’s the boring answer.
      The second reason is more interesting. And that’s that some bacteria can change their biology to protect themselves from bleach – for instance, some bacteria can change from a normal cell to a spore. That’s when they produce a hard outer covering that protects them from drying out and from things like bleach. They can change back later on and become normal, active bacteria again.
      The third reason is even more interesting, and that’s evolution. Whenever you try to kill bacteria but miss a few, the ones that survive go on to reproduce. Now, in any population of bacteria there are genetic differences between individual cells, because every time bacteria divide and copy their DNA some errors creep in – mutations. Any mutations that are in the bacteria that survive get copied into their daughter cells, and so over time those mutations get more common in the population. If those mutations make it more likely that a bacterium will avoid getting killed by bleach, then the mutants have a big evolutionary advantage and the bleach-resistant mutants will become more and more common in the population as time goes on, until most or all of the population is resistant to bleach.
      This is exactly how antibiotic resistance evolves, and there are bacteria that have become resistant to cleaning sprays and disinfectants. If you pop into the bathroom, or wherever you keep your washing machine, and look at the ingredients list on toiletries and washing powders, you’ll see that a lot of the them contain something called triclosan. This is a disinfectant, and in the last few years scientists have found that lots of bacteria have become resistant to it and don’t get killed.
      But what about why we might not want to make such a substance? Well… would you want to put bleach on your skin? Don’t try this! Things that are able to kill lots of different bacteria can do this because they attack molecules that are fundamental to all living cells… not just bacterial cells, but human cells as well. So things that kill loads of bacteria tend to be bad for us too. Antibiotics, on the other hand, very specifically targets parts of the cell, or cellular processes, that some bacteria have but animal cells don’t – that’s why we can use them as drugs. Something that could kill (almost) all bacteria would likely be something that you would not want to put on your skin or take as a drug.
      I’m going to end with a question for you, gingerninja673. If you had a bacterial infection, say a nasty stomach bug, and the doctor offered you an antibiotic that would specifically kill only the bacteria causing the bug, and one that would kill all bacteria in your body when you took it, which one would you choose and why? There’s another reason we like to find drugs that kill only specific bacteria and I think you’ll work out why 🙂

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