Flagellated bacteria, such as Escherichia coli, swim by a succession of more or less straight runs and short reorientations, the tumbles. The bacteria probe their environment while swimming through the chemotaxis pathway, which uses a short term memory to compare current concentrations of nutrients, temperature, pH, etc. with the recent past, and tunes the frequency of the reorientations in order to drift on average towards favorable conditions. On the other hand, during e.g. infection or surface colonization, the bacteria can reach very high cell densities, at which collective motions characterized by intermittent jets and swirls of large groups of cells develop, while still having to perform essential tasks like chemotaxis. During my seminar, I will show, thanks to microfluidics experiments, using new image analysis techniques and numerical simulations, how the collective motion reduces the ability to follow gradients, how physical modifications of the bacteria and of the environment partially restore it, and what are the implications for the bacteria in their natural environment.