Abacterial cell that is resistant to a particular antibiotic may spend a large amount of energy, producing the modified proteins that are needed for resistance. in such a case, what would be the effect on this cell's fitness if no antibiotic was present?

Its fitness would lower.

Explanation:

An organism has an optimal fitness when the organisms spend their energy in performing tasks that are most appropriate to survive in the given environment.Organisms that possess the traits that are best suited to survive in a given condition is termed to be the fittest in that particular environment. In the given situation the  fitness of the bacteria would be lower than other strains that lack resistance because it is spending energy that the resistant strains can use for other purposes, such as reproduction this is because even though the bacteria are resistant to a particular antibiotic there is no point of producing those resistant proteins if the antibiotic is absent in the surroundings and hence, this is not the best trait it be possessed.