What is the meaning of "fitness cost" in microbiology?

Hi,

The cost to the ‘fitness’ of an organism is it’s ability to replicate and survive in a competitive environment. 

For instance if antibiotic resistance could be acquired by bacteria without any "fitness cost" all the human bacteria (as well as all the environmental ones) would be pan-resistant already. 

As you know bacteria are masters of survival. They are also hyper-efficient, and need to carefully budget their energy quota not only on defense, but also on attack and redeployment, not to mention communication (fitness cost). If the bacterial population does not need to be resistant in a particular environment, it will not waste its precious energy (and time!) on resistance genes for a “Just in case” scenario.

Dear  Ditthawat Nonghanphithak,

The evolution of antibiotic resistance is typically the result of small changes allowing for survival in a microbe or other organism under special circumstances where the organism faces extremely strong selection pressure due to the presence of some antibiotic drug. In other cases, it is the result of the transfer of pre-existing antibiotic resistance genes from one microbe to another, and the selection of such microbes in an environment containing antibiotics. Even in the first example, evolution does not produce a truly new function. In fact the change produced often makes the microbe less fit when the antibiotic is removed--it reproduces slower than it did before it was changed. This effect is widely recognized, and is called the fitness cost of antibiotic resistance.

Let's say you have an antibiotic which targets an important biological pathway. Mutations that confer antibiotic resistance often involve modification of the target enzyme to prevent antibiotic binding. These mutations often make enzyme suboptimal compared to evolutionary optimized "wild-type" version. This can reduce fitness, manifesting as decreased virulence, transmission, and growth rate. However, despite being less fit under normal growth conditions, this mutant can survive under conditions of antibiotic treatment. So this a trade off also known as fitness cost. 

A fly strain of D. melanogaster, C4 is resistant to the mushroom toxin, alpha-amanitin. Alpha-amanitin blocks the activity of RNA pol II. This strain has a mutation that alters the structure of RNA pol II and that is how it is resistant to the toxin. It has an advantage in the presence of the toxin. However, in the absence of the toxin it has poor survival as compared to its wild type counterpart that does not have the mutation. The mutation has an advantage in the presence of the toxin but a disadvantage in the absence of the toxin. In other words, the mutation has a fitness cost.