Tag Archives: fitness cost

“Defying Nature”


I occasionally hear people/experts saying that antibiotic resistance can be acquired by bacteria without any cost to the ‘fitness’ of the organism (i.e. it’s ability to replicate and survive in a competitive environment)

I could not disagree more…

If antibiotic resistance could be acquired without any cost to fitness, I think all the human bacteria (as well as all the environmental ones) would be pan-resistant already. It just doesn’t make sense to me.

Bacteria are masters of survival. They are also hyper-efficient, and need to carefully budget their energy quota not only on defence, but also on attack and redeployment, not to mention communication. If the bacterial population does not need to be resistant in a particular environment, it will not waste its precious energy on resistance genes for a “Just in case” scenario.

Most in-vitro studies show that acquisition of antibiotic resistance does indeed have a fitness cost. A few don’t. For those exceptions I would offer the following explanations:

  • Just like anti-microbial susceptibility testing, I suspect that in-vitro bacterial fitness correlates only roughly with in-vivo fitness.
  • The fitness cost may be so miniscule, that it is impossible to demonstrate in the laboratory setting.

So in an antibiotic free environment, the susceptible strain will eventually win out over the resistant one. This might take days, or it might take decades, depending on the relative fitness difference. The fitness gap between susceptible and resistant strains might also be narrowed by compensatory mutations, but it will never be zero. It is all just a matter of time, evolutionary time…

If a bacterium doesn’t need to be resistant, then it won’t be, and it will eventually dispose of the means to be resistant.

Bacteria are lean, mean, replicating machines. They are also highly obedient to the Laws of Evolution…


For a nice article on the above concept, with a few references attached, click here. About a 5 minute read.