4At high cell density, swimming bacteria exhibit collective motility patterns, self-organized through physical 5 interactions of a however still debated nature. Although high-density behaviours are frequent in natural 6 situations, it remained unknown how collective motion affects chemotaxis, the main physiological function 7 of motility, which enables bacteria to follow environmental gradients in their habitats. Here, we systemati-8 cally investigate this question in the model organism Escherichia coli, varying cell density, cell length, and 9 suspension confinement. The characteristics of the collective motion indicate that hydrodynamic interac-10 tions between swimmers made the primary contribution to its emergence. We observe that the chemotactic 11 drift is moderately enhanced at intermediate cell densities, peaks, and is then strongly suppressed at higher 12 densities. Numerical simulations reveal that this suppression occurs because the collective motion disturbs 13 the choreography necessary for chemotactic sensing. We suggest that this physical hindrance imposes a fun-14 damental constraint on high-density behaviours of motile bacteria, including swarming and the formation 15 of multicellular aggregates and biofilms.Indroduction 17 When the cell density of a suspension of swimming bacteria increases, collective motion emerges, char-18 acterized by intermittent jets and swirls of groups of cells [1][2][3]. This behaviour is observed for many 19 microorganisms not only in artificial but also in natural situations, often at an interface, e.g. when bac-20 teria swarm on a moist surface in the lab [4][5][6][7][8] or during infection [9], or at an air-water interface during 21 formation of pellicle biofilms [1, 10]. Bacterial collective motion has been extensively studied experimentally 22 [11][12][13][14] and theoretically [15][16][17][18][19][20], and it is known to emerge from the alignment between the self-propelled 23 cells [21]. Two alignment mechanisms have been proposed, based either on steric interactions between the 24 rod-like bacteria [22][23][24] or on the hydrodynamics of the flow they create as they swim [15, 17], which 25 displays a pusher force dipole flow symmetry [3, 25, 26]. However, the relative importance of these two 26 mechanisms has not been clearly established so far [27]. 27Bacterial collective motion contrasts to the behaviour of individual motile cells in dilute suspension, 28 when bacteria swim in relatively straight second-long runs interrupted by short reorientations (tumbles), 29 resulting at long times in a random walk by which they explore their environment [28]. Bacteria can 30 furthermore navigate in environmental gradients by biasing this motion pattern: they lengthen (resp. 31 shorten) their runs when swimming toward attractive (resp. repulsive) environment [28]. The biochemical 32 signaling pathway controlling this chemotactic behaviour is well understood in E. coli [29, 30] and it is 33 one of the best modeled biological signaling systems [31]. Bacteria monitor -via...