How does not finishing an antibiotic course lead to a resistant strain?

My biology teacher wasn't very good at explaining this.

I know that the idea is that only the strongest bacteria survive the antibiotics, but would be killed by the rest of the antibiotic course. These survivors then go on to produce a resistant strain of bacteria.

Its the in between step that I cant grasp. How do they gain this immunity? Why is it that the descendants of the bacteria that reproduced, the ones that couldn't be killed by half a course of antibiotics, cannot now be killed by a full course of antibiotics?

When I asked my teacher to explain this, she explained it twice in the same way that she had before, and looked at me like I was stupid. I asked if it was all in one generation, and she started telling me the same story again.

Can anybody explain it to me?

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  • 7 years ago
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    Let's imagine you're infected with bacteria. Those bacteria reproduce very quickly ... in some cases they can double their numbers within half an hour!

    Now, those bacteria can reproduce by shuffling their genes with other bacteria. What that means is that if you were infected by bacteria with slightly different genes, then the next generation of bacteria can have some mix of the two. Furthermore, since they reproduce so quickly, they can quickly accumulate genetic differences (through errors) in their genome, which they then mix with others.

    Within a few generations you can have a whole host of bacteria that are genetically different to their ancestors. And some of those, eventually, will build up enough genetic difference to be immune to the antibiotic you're using to try to kill them.

    So, there are two options available to doctors. Firstly, you leave the patient alone. Our bodies have mechanisms that really don't care about the genetics of the bacteria. That's an over-simplification but it'll do for now. Macrophages, for example, simply gobble up anything foreign and dissolve them with enzymes. If you leave people alone, our bodies can handle many types of bacterial infections. The problem is, you'll feel lousy for a while.

    Secondly, you prescribe antibiotics to kill off the bacteria. The hope is that you kill them off before they have time to adapt through that genetic shuffling. If you stop taking the antibiotics early, then you leave some behind, who now continue that process of shuffling. A second dose is then required, often with a slightly different type of antibiotic.

  • 7 years ago

    Its basic bacteria evolution. What doesn't kill you makes you stronger. The stronger bacteria will take longer to be killed if you only let those live and kill the weak ones off you will be creating a much more powerful family of bacteria then you originally had.

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