In the old days. If you wanted to know what a gene did, you made a mutation in it and then tested its growth characteristics in some way to see what it could not do. In some cases, this would tell you something, but sometimes, it would not. If you did not have the growth conditions right, or your gene did something subtle, its phenotype would remain a mystery. Often, you needed to know something about your gene before you could learn anything else. A chicken or the egg conundrum that kept geneticists tearing their hair out.
In today's world, this is no longer the case. With the use of a microarray that can look at the gene expression of an entire genome of a microbe in one experiment, it becomes relatively easy to learn a large amount about a gene of interest. Merighi et al. [5] describe experiments to determine the role of preA and preB in virulence and find that this two-component regulatory system does in fact turn on genes that change the behavior of cells. However, they do not find overt virulence factor influences, but they do trace down what genes are affected by PreA the response regulator of the system.