16 December 2016

Misunderstanding antibiotic resistance

Even people who try to incorporate evolutionary thinking into their research struggle with the concepts, it seems.

Thomas Wittum was interviewed on Quirks and Quarks about finding bacteria in a pig farm that were resistant to one of the “last resort” antibiotics. He repeatedly put forward the idea that these bacteria must have been introduced there from a human health care facility, like a hospital, where these particular classes of antibiotics are used the most.

This notion come perilously close to the idea that organisms evolve features that they “need.” The thinking seems to go that antibiotic resistant bacteria must come from hospitals, because why would bacteria is someplace without those antiobiotics have resistance to them?

This misses the concept of “standing variation”: that any population can have variation in a trait in the absence of any selection pressure for one version of the trait or the other. If you look at classic cases of dark coloured insects, like peppered moths, flourishing in industrial areas (industrial melanization, for those who know the fancy words), populations always had some dark insects around before industrialization. That’s standing variation. Later, the dark colour provided an advantage as the landscape got darker. But the key point is that the insects didn’t newly evolve dark colouration as regions became industrialized, because they needed it.

Similarly, it is possible that these farm bacteria came had antibiotic resistance “out of the box,” so to speak. The resistance would do very little in the farm setting, but... if those bacteria were in an environment where those antibiotics were released, they would suddenly have a huge advantage.

Having said that, it is entirely possible that the antibiotic resistance did come from a hospital or someplace similar. The manuscript itself (Mollenkopf et al., in press) doesn’t address the source of the bacteria, or the antibiotic resisting gene. But it’s not a good assumption that if we create a new antibiotic, there will be no bacteria that will be able to survive it at the start. Some bacteria might, by chance, already have a way to evade the antibiotics.


Mollenkopf DF, Stull JW, Mathys DA, Bowman AS, Feicht SM, Grooters SV, Daniels JB, Wittum TE. Carbapenemase-producing Enterobacteriaceae recovered from the environment of a swine farrow-to-finish operation in the United States. Antimicrobial Agents and Chemotherapy: in press. http://dx.doi.org/10.1128/AAC.01298-16

External links

Drug resistant "superbug" gene found on pig farm

Picture from here.

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