Final answer:
Quorum Sensing is a bacterial communication system that allows them to adjust their behavior according to their population density.
Step-by-step explanation:
The bacterial communication system that allows bacteria to adjust their behavior according to their population density is called Quorum Sensing. This mechanism involves bacteria producing chemical signals, called autoinducers, which detect population density and regulate gene expression. When the cell population reaches a critical threshold, these autoinducers trigger a cascade of reactions that activate genes associated with cellular functions.