If low cell density = quorom sensing will not work
If high cell density = they might trigger to say there's quite a lot of us now let's stop growing, let's not take it up food. Let's stop taking up nutrients. They stop taking up oxygen because if we keep doing it, we're going to die because there's not going to be enough for us.
Bacteria need to co-ordinate the expression of virulence factors for successful pathogenicity and this is done by quorumsensing.