Scientists have found, to their surprise, that after two and a half years of being antibiotic free, pigs that were involved in a Canadian study carried bacteria that were still resistant to antibiotics. Scientists originally hypothesized that the antibiotic resistant mutation would be associated with some sort of fitness disadvantage, like many similar mutations. If this had been true, the bacteria would have been likely to lose their resistance when no longer in the presence of antibiotics.
One of the main researchers suspects that the reason the bacteria did not lose resistance could be due to a link between the antibiotic resistance and some other gene the bacteria need to survive. It is also possible that the antibiotic resistance has not yet been lost because the mutation that causes it to be antibiotic resistant is not harmful, or provides no evolutionary disadvantage to the bacteria. If that were true, there would be no positive or negative effect to the bacteria whether they carried the gene or not.
A main concern among the scientific community is that as long as the bacteria in the pigs hold on to this resistance, they can share it with other bacteria they encounter through horizontal gene transfer. Although no link has been proven, experts fear that this antibiotic resistance could be passed onto humans through the food supply. The meat slaughtering facilities, however, assure customers that by the time the pork is processed, there’s very few resistant bacteria remaining.
This study clearly creates a need for further research. Until more conclusive studies are conducted, it is difficult to determine whether there is cause for concern.