In shallow temperature gradients, changes in temperature that bacteria experience occur over
long time-scales. Therefore slow processes, such as adaptation, metabolism, chemicals secretion,
and even gene expression become important. Since all these changes are related to cellular
activity, the cell density plays an important role. In this talk I will discuss recent findings that
show how the cell density and the chemical environment affect the behavior of the bacteria in a
temperature gradient. We find that there are mainly four density regimes, each with a distinct
behavior. At low cell-density, bacteria act as individuals and their behavior is influenced by the
chemical environment. In the intermediate cell-density regime, the temperature-dependent
consumption of nutrients becomes significant and interferes with the response to the temperature
gradient. In the high cell-density regime, cell-cell interactions initiate a collective mode of
behavior. Finally, at extremely high cell-density, a change in the physiological state of the
bacteria is observed as a result of a change in the steady-state methylation level and the
expression level of the two most abundant sensing receptors, Tsr and Tar. This change is affected
by the concentration of the different chemicals in the environment.