Chemotactic bacteria are known to collectively migrate towards sources of attractants. In confined convectionless geometries, concentration "waves" of swimming Escherichia coli can form and propagate through a self-organized process involving hundreds of thousands of these microorganisms. These waves are observed in particular in microcapillaries or microchannels; they result from the interaction between individual chemotactic bacteria and the macroscopic chemical gradients dynamically generated by the migrating population. By studying individual trajectories within the propagating wave, we show that, not only the mean run length is longer in the direction of propagation, but also that the directional persistence is larger compared to the opposite direction. This modulation of the reorientations significantly improves the efficiency of the collective migration. Moreover, these two quantities are spatially modulated along the concentration profile. We recover quantitatively these microscopic and macroscopic observations with a dedicated kinetic model. Suspended Escherichia coli bacteria that swim in convection-free geometries such as capillaries or microchannels, collectively migrate towards nutrient-rich regions, in the form of propagating concentration waves (1-4). In homogeneous environments, the individual trajectories of these bacteria can be described by a random walk consisting in a succession of "runs" during which they swim in straight lines, and "tumbles" characterized by random [although not complete (5)] reorientations (6). The modulation of run lengths in response to temporal variations of chemoattractant concentrations (7) biases this random walk, driving the bacteria up spatial gradients (5). The collective wavelike behaviors emerge for high enough concentrations (8) and have been so far qualitatively described by several semiempirical models based on the production of chemoattractants by the bacteria themselves (3,(8)(9)(10)(11)(12). Nevertheless, a direct validation of these ideas still requires experimental data describing individual bacteria within these populations.In the present work, we use a microfluidics-based approach that allows to simultaneously track the trajectories of hundreds of individual bacteria while measuring the global characteristics of the wave. We then derive a kinetic model that quantitatively describes the observations at both scales.The experiments were conducted with the motile and chemotactic strain RP437 (5) and its mutants, in linear silicone polydimethylsiloxane (PDMS) microchannels (500 μm × 100 μm × 1.8 cm). The channels were first filled with homogeneous suspensions of cells grown to midlog phase (∼5 · 10 8 cells∕mL) in M9 minimal medium supplemented with D-Glucose and Casamino Acids (13), and immediately sealed.As a reference, we measured the mean run [respectively (resp.) tumble] duration τ run (resp. τ tumble ) and the run velocity (V run ) before centrifugation, in the absence of gradient, for each experiment. The distributions of run and tumble durations we...

Summary Life inside ant colonies is orchestrated with diverse pheromones, but it is not clear how ants perceive these social signals. It has been proposed that pheromone perception in ants evolved via expansions in the numbers of odorant receptors (ORs) and antennal lobe glomeruli. Here we generate the first mutant lines in the clonal raider ant, Ooceraea biroi, by disrupting orco, a gene required for the function of all ORs. We find that orco mutants exhibit severe deficiencies in social behavior and fitness, suggesting they are unable to perceive pheromones. Surprisingly, unlike in Drosophila melanogaster, orco mutant ants also lack most of the approximately 500 antennal lobe glomeruli found in wild-type ants. These results illustrate that ORs are essential for ant social organization and raise the possibility that, similar to mammals, receptor function is required for the development and/or maintenance of the highly complex olfactory processing areas in the ant brain.