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It is commonly believed that the resistance to antibiotics by micro-organisms is truly evolution at work, and that the recent surge in superbugs may very well be attributed to it.

When we refer to a bacteria's ability to "evolve" into resisting this antibiotic, I assume we mean that when we introduce antibiotics into the environment, a lot of bacteria die but some who by mere chance and luck didn't do so and that these lucky bacteria go on to survive and multiply, thus aiding in the creation of so-called superbugs, or rather, bacterial resistance to sophisticated drugs.

This whole situation already suggests to us, or at least to me, that there's much inherent complexity in these organisms and indeed a certain uniqueness to each.

I have, however, been troubled recently by the following question: is it truly impossible, for any kind of wholly new antibiotic which was not used before (no matter how complex), to succeed in killing off every single bacteria in any rich habitat/environment (no matter how populous)? And that there'll always be some bacteria on whom this shall not work apparently due to uniqueness in something of their structure?

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    $\begingroup$ If I am not completely wrong, any kind of single agent drug (this is also true for targeted therapies in cancers for example) eventually leads to resistance. The evolutionary pressure is enormous and you only need one successfull cell. $\endgroup$ – Chris Jun 18 '18 at 20:52
  • $\begingroup$ It is not bacteria surviving by pure luck that leads to resistance. Rather, they have some variation in their makeup that makes them harder to kill. They survive, pass this trait on to "offspring", and eventually outnumber the easy to kill variety. $\endgroup$ – jamesqf Jun 19 '18 at 6:01
  • $\begingroup$ @Chris this will largely be based on how aggressive an antibiotic you use, prolonged immersion in liquid salt will kill all the bacteria but would kill the host as well. $\endgroup$ – John Oct 19 '18 at 13:56
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Yes. It is down to mutations. DNA replication is not 100% accurate, there will be mistakes that are not corrected in the progeny produced.

Antibiotics works like a bit of gunk wedging itself into the machine of a bacteria cell but avoids jamming a human cell due to that very same shape. Gunk that wedges into both human and bacteria machines is just a really bad poison. So that gunk has a very special shape.

Resistance can be acquired by recognizing gunk of that particular shape and removing it (not something easy to evolve but very effective when it does) or just change the shape of the machine so the gunk cannot stick. Usually the altered machine doesn't work as efficiently as it can but in an environment polluted by fouling gunk, it works well enough.

And a rich environment means there are many bacteria.

So if your chances of mutating the right part of the bacteria machine to gain resistance is 1 in million, a rich environment with 10 million cells will have 10 such mutates.

Of course in reality we are talking about trillions (a million million) of bacteria cells per human (actually 39 trillion bacteria per human) and we are talking about billions of humans. And we are not even counting the number of bacteria in the soil and in farm animals. And it takes a few years for resistance to evolve.

When you look at the numbers involved , it is very difficult not to imagine resistances evolving. Even if the chances of that happening is 1 in hundred of trillions. It just takes a single bacteria cell to mutate and so gain resistance for it to spread and pass that resistance to other bacteria.

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The answer is no. The antibiotic will kill all the bacteria it's intended to target, unless a special resistive variant cell(s) exist. What this means is that it is possible that non-resistive cells are not present.

Also bear in mind that it's not the drug that induces the mutation, rather it just isolates and funnels the randomly generated mutant (if present). Also remember that the vast majority of mutations are deleterious or neutral, so the probability of resistant strains even existing in a population are low. [That doesn't mean they cannot exist at all.]

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