No. The numbers expressing it drop, but there are often enough advantages in competition situations that the trait stays around and can rapidly recolonize the population. Carrying the genetic material is a slight cost. Expressing it costs more, but that could be made conditional.

Furthermore, note that in this case, the point is that the trait also confers resistance to the same type of molecule occurring naturally as part of the innate immune system of animals, meaning that for bacterial species with a large pathogenic component to their lifestyle the resistance retains a fitness advantage in their niche.