Governments around the world are trying to heavily restrict antibiotic use in the hope that this will prevent the development of resistance which renders many medical treatments useless. However, new research demonstrates that bacterial resistance can spread regardless.

Darwin’s theory of evolution tells us that if the selection pressure of antibiotics is applied to a population of bacteria, those with genes that give them resistance will have a higher likelihood of surviving. In an environment without antibiotics, resistant bacteria appear to have no survival advantage. It therefore seems illogical that antibiotic resistance can still spread in antibiotic free environments.

Although some bacteria can survive antibiotic exposure through resistance genes, some species have alternative survival mechanisms. Known as persistors, these bacteria go into a dormant state by slowing down their metabolism when exposed to antibiotics. Once persistors have invaded a tissue they can lay dormant for months, only waking up when conditions promote their growth again and the patient is no longer receiving antibiotics.

The researchers studied a strain of dormant salmonella in the mice gut. Once the bacteria awoke from their dormancy, they were extremely quick at transferring their antibiotic resistance genes to other bacteria of the same species. The bacteria were also able to transfer the genes to other bacterial species, including E. coli. The donor bacteria directly transfer the resistance gene to the recipient bacteria through a small ring of DNA, known as a plasmid. This process is known as conjugation.

When a bacterium with the resistant plasmid was re-introduced into the gut lining, within 2-3 days 99% of observed bacteria had a resistant plasmid. These results stress that limiting antibiotic use will not completely stop the spread of resistance and alternative treatments against dormant bacteria must be urgently developed.

The researchers now want to investigate antibiotic resistant Samonella in livestock species such as pigs, where infection can impact food supply.

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