Although recessive genes do not formally exist in single genome bacteria, there are plenty of genes in bacteria that are not always expressed. It would be extremely wasteful for a bacterium to be making all the enzymes of all the metabolic pathways that it is capable of when they are not needed. So it does not. There are biological switches that turn metabolic pathways on and off depending on environmental cues.
However, a quiescent pre-existing gene was not responsible for the acquired citrate metabolism in this experiment.
