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I would imagine the bacterial genome is highly conserved and limited in its space, but maybe I am wrong.

If you were to take a strain of antibiotic resistant bacteria and kept them isolated, but fed well and so forth, how long would it take for them to lose their resistance? A year? A decade? 100 years? 1000 years? At some point it seems like that trait would disappear, but I have no feeling for how long. Please support your answer with a relevant citation.

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My purpose is simple: I am thinking about a strategy for dealing with antibiotic resistance. If we were to ban them across the entire world (could be impossible) how long would we need to wait before they would be usable again. If it was a matter of years, then we could almost do a rotation of existing antibiotics (if we had enough) because I would rather not live in post-antiobitic world.

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Antibiotic resistances in bacteria is commonly encoded by extrachromosomal DNA, the plasmids. These are circular pieces of DNA, which are much smaller than the hosts genome and which replicate independently from it. See the image from the Wikipedia:

enter image description here

These plasmids can be transfered between different bacterial cells, which then also get resistant. Plasmids are divided between daughter cells, when the parent cell divides. One of the few exceptions seems to be Mycobacterium tuberculosis, which does not seem to carry plasmids but also develops resistances. It has been hypothesized that they contain extrachromosomal single-stranded DNA ("Does Mycobacterium tuberculosis have plasmids?")

Regarding your question: Plasmids which carry antibiotic resistances will only disappear, when the antibiotic is not seen for a while, since the cells, which don't carry it, have a growth advantage over cells who are still carriers (since they save the energy of forming the plasmid). However, these resistance plasmids are nothing new, evolutionary speaking. They appeared as a countermeasure against fungal toxins.

In the lab, bacterial strains loose plasmids within a few days, when not kept under selection pressure according to my experience. There are a few paper who looked into it:

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Lab strains of E. coli have been in use for many decades now. They have all retained a large number of genes encoding subunits of the flagellar apparatus and the chemotaxis system which confer absolutely no advantage under normal culture conditions. I conclude from this that the selective advantage conferred by losing "unused" genes must be very weak.

Also, most antibiotic resistance is encoded on plasmids. As @Chris points out, these can be lost easily.

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  • $\begingroup$ I would expect bacterial DNA to have some natural decay rate over generations, so I find it very interesting they do maintain these "vestigial" genes; is it possible they are actively conserved? $\endgroup$
    – sciencenewbie
    Mar 18, 2014 at 23:19
  • $\begingroup$ No. DNA doesn't "decay". It changes. $\endgroup$
    – swbarnes2
    Mar 18, 2014 at 23:56
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    $\begingroup$ In general terms entropy is the decay of order. So call it what you will, the question remains of its rate. If you wrote a message that wasn't vital for the organism to survive, over time it will become corrupted unless every DNA replication is 100% efficient. $\endgroup$
    – sciencenewbie
    Mar 19, 2014 at 16:35
  • $\begingroup$ @sciencenewbie Under standard culture conditions (37C rich medium) they aren't expressed, and it is difficult to conceive of a selective advantage under conditions in which it is unnecessary to move towards nutrients. But some weak selective pressure can't be ruled out I guess. $\endgroup$
    – Alan Boyd
    Mar 19, 2014 at 16:49
  • $\begingroup$ @swbarnes2 It is called information theory and can be applied anywhere. You are not going to win me over by coming across in a hostile tone. $\endgroup$
    – sciencenewbie
    Mar 19, 2014 at 22:56
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If you were to take antibiotic resistant bacteria and kept them isolated, but fed well and so forth, how long would it take for them "forget" their resistance?

It's not a matter of "forgetting". Bacteria are resistant if their DNA is such that it gives them a biology that renders the antibiotic non-lethal.

You put a resistant bacteria in glycerol stocks, you can freeze them for years, maybe longer, and when you revive the population, and get it growing again, the resulting population will still be resistant.

Also, when we talk about bacteria, or any kind of evolution, single organisms don't change their DNA. Populations change their proportion of different alleles. Any allele might drift out of a population, and if the resistance-granting mutation is deleterious, natural selection might speed that along, but not all resistance-granting alleles are deleterious enough for that. If a resistance-granting allele is survival neutral in the absence of the antibiotic, or another mutation somewhere else restores the bacterias' survivability absent antibiotic to what the sensitive genotype's fitness is, the resistance-granting allele might not drift out of the population for a long time.

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  • $\begingroup$ I'm not talking about putting resistant bacteria in glycerol stocks and freezing them, I'm curious about keeping them multiplying in a petri dish. Finally, I use "forgetting" not in a technical sense, but a figurative. The allele must eventually be lost, but I would like some sense of how long until it is lost. $\endgroup$
    – sciencenewbie
    Mar 18, 2014 at 23:13
  • $\begingroup$ " I'm curious about keeping them multiplying in a petri dish." So you are talking about a population, not an individual. If you abandon that distinction, nothing makes any sense. "The allele must eventually be lost" You can get strep resistance in TB through GidB deletions. You want to believe that a population descended from a deletion mutant will forget a deletion? $\endgroup$
    – swbarnes2
    Mar 18, 2014 at 23:52
  • $\begingroup$ So in a trillion years without the presence of any antibiotic, will that allele still be there in a population? I would be surprised if so. I don't care if it is a day, or a billion years, but I want a sense of how long. Saying it will never be lost makes no sense to me. $\endgroup$
    – sciencenewbie
    Mar 19, 2014 at 4:50
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    $\begingroup$ swbarnes2 and @sciencenewbie I'll remind you both of our community guidelines: biology.stackexchange.com/help/behavior $\endgroup$
    – kmm
    Mar 20, 2014 at 0:52
  • $\begingroup$ @swbarnes2 You assume too much. I am not a creationist. I believe in the theory of evolution. I don't understand why you believe it is okay to treat people like this. $\endgroup$
    – sciencenewbie
    Mar 20, 2014 at 4:01

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