Ed Wilson asked:
The genome of a bacterium is flexible, perhaps, but it is finite. As bugs evolve resistance to new chemicals, is it possible that resistance to older drugs - particularly those no longer in the environment - may be disrupted or lost?
Martha:: Listener Ed Wilson wanted to know whether bacteria can lose resistance to a certain antibiotic when itís removed from their environment. I spoke to Julian Parkhill at the Wellcome Trust Sanger Institute who told me that antibiotic resistance can be a costly process to the bacterium. Removing the antibiotic from the bacteriumís environment means that sensitive bacteria may have an advantage as they donít have to maintain these energy costly resistance mechanisms. And so, sensitive bacteria may begin to outcompete their resistant neighbours. But as Dr. Parkhill explained, the situation isnít always this straightforward.
Julian:: The cost to the organism is often very, very low because the antibiotic resistance mechanism is kept under tight control and only used when the antibiotic is around. This means that when there's no antibiotic around, there's very little cost because of the tight control the mechanism is kept under. Also, where there is a cost, when the organism is subject to antibiotic pressure, it can often mutate and you get what's called compensatory mutations which will make the organism more fit even in the absence of antibiotic. And the final issue is that you often find that antibiotic resistances get linked together. So, if an organism is resistant to one antibiotic, it will often carry resistances to multiple different antibiotics. So, that means that even if itís not being exposed to one particular antibiotic, the resistance mechanism may still be linked to a number of other resistance mechanisms. And therefore, whatever antibiotic is used, the organism will maintain a number of other different resistance mechanisms in the population. So overall, although it can happen and sometimes antibiotic resistance is lost if an antibiotic's not being used, nature has a number of ways of maintaining these things in the population, even in the absence of the antibiotic.
Martha:: So, antibiotic resistance mechanisms may persist in bacterial populations, long after an antibiotic is removed from its environment. Thanks to Ed Wilson for his question and to Julian Parkhill from the Sanger Institute for his answer.
Yes; bacteria acquire from other microbes DNA elements that encode resistance factors. These are positively selected by the presence of antimicrobial compounds. But if this selective pressure is removed then the function of the resistance elements may be lost or discarded because it is energetically unfavourable to maintain it. Consequently, bacteria can evolve to become sensitive to a compound again. chris, Tue, 9th Apr 2013
One of the methods if combating antibiotic resistance would be to remove certain antibiotics from general use, reintroducing them decades later. As a rolling program this would help and perhaps pharmaceutical companies would not have the same pressure to develop new antibiotics. Of course the patents on these old, stored and later reintroduced antibiotics would have long expired...so not much incentive to do this is there. DaveC, Tue, 20th May 2014