Bacterial chemotaxis is a paradigm for how environmental signals modulate cellular behavior. Although the network underlying this process has been studied extensively, we do not yet have an end-to-end understanding of chemotaxis. Specifically, how the rotational states of a cell’s flagella cooperatively determine whether the cell ‘runs’ or ‘tumbles’ remains poorly characterized. Here, we measure the swimming behavior of individual E. coli cells while simultaneously detecting the rotational states of each flagellum. We find that a simple mathematical expression relates the cell’s run/tumble bias to the number and average rotational state of its flagella. However, due to inter-flagellar correlations, an ‘effective number’ of flagella—smaller than the actual number—enters into this relation. Data from a chemotaxis mutant and stochastic modeling suggest that fluctuations of the regulator CheY-P are the source of flagellar correlations. A consequence of inter-flagellar correlations is that run/tumble behavior is only weakly dependent on number of flagella.DOI: http://dx.doi.org/10.7554/eLife.01916.001

Many bacteria are motile only when nutrients are scarce. In contrast, Salmonella enterica serovar Typhimurium is motile only when nutrients are plentiful, suggesting that this bacterium uses motility for purposes other than foraging, most likely for host colonization. In this study, we investigated how nutrients affect motility in S. enterica and found that they tune the fraction of motile cells. In particular, we observed coexisting populations of motile and nonmotile cells, with the distribution being determined by the concentration of nutrients in the growth medium. Interestingly, S. enterica responds not to a single nutrient but apparently to a complex mixture of them. Using a combination of experimentation and mathematical modeling, we investigated the mechanism governing this behavior and found that it results from two antagonizing regulatory proteins, FliZ and YdiV. We also found that a positive feedback loop involving the alternate sigma factor FliA is required, although its role appears solely to amplify FliZ expression. We further demonstrate that the response is bistable: that is, genetically identical cells can exhibit different phenotypes under identical growth conditions. Together, these results uncover a new facet of the regulation of the flagellar genes in S. enterica and further demonstrate how bacteria employ phenotypic diversity as a general mechanism for adapting to change in their environment.

Layered materials, such as graphene, transition metal dichalcogenides and black phosphorene, have been established rapidly as intriguing building blocks for optoelectronic devices. Here, we introduce highly polarization sensitive, broadband, and high-temperature-operation photodetectors based on multilayer germanium sulfide (GeS). The GeS photodetector shows a high photoresponsivity of about 6.8 × 10 A W, an extremely high specific detectivity of 5.6 × 10 Jones, and broad spectral response in the wavelength range of 300-800 nm. More importantly, the GeS photodetector has high polarization sensitivity to incident linearly polarized light, which provides another degree of freedom for photodetectors. Tremendously enhanced photoresponsivity is observed with a temperature increase, and high responsivity is achievable at least up to 423 K. The establishment of larger photoinduced reduction of the Schottky barrier height will be significant for the investigation of the photoresponse mechanism of 2D layered material-based photodetectors. These attributes of high photocurrent generation in a wide temperature range, broad spectral response, and polarization sensitivity coupled with environmental stability indicate that the proposed GeS photodetector is very suitable for optoelectronic applications.

Artificial van der Waals heterostructures of 2D layered materials are attractive from the viewpoint of the possible discovery of new physics together with improved functionalities. Stacking various combinations of atomically thin semiconducting transition metal dichalcogenides, MX 2 (M = Mo, W; X = S, Se, Te) with a hexagonal crystal structure, typically leads to the formation of a staggered Type II band alignment in the heterostructure, where electrons and holes are confined in different layers. Here, the comprehensive studies are performed on heterostructures prepared from monolayers of WSe 2 and MoTe 2 using differential reflectance, photoluminescence (PL), and PL excitation spectroscopy. The MoTe 2 /WSe 2 heterostructure shows strong PL from the MoTe 2 layer at ≈1.1 eV, which is different from the quenched PL from the WSe 2 layer. Moreover, enhancement of PL intensity from the MoTe 2 layer is observed because of the near-unity highly efficient photocarrier transfer from WSe 2 to MoTe 2 . These experimental results suggest that the MoTe 2 /WSe 2 heterostructure has a Type I band alignment where electrons and holes are confined in the MoTe 2 layer. The findings extend the diversity and usefulness of ultrathin layered heterostructures based on transition metal dichalcogenides, leading to possibilities toward future optoelectronic applications.

Spectroscopic studies of mechanically exfoliated monolayer MoTe 2 was performed over a wide temperature range from 4.2 to 300 K. At a low temperature, the photoluminescence (PL) spectra for monolayer MoTe 2 showed two sharp peaks for excitons and charged excitons (trions). The homogeneous linewidth of the exciton peak broadened linearly as the temperature increased. This linear linewidth broadening was caused by acoustic-phonon scattering of the exciton, i.e., shortening of exciton dephasing. The broadening factor due to exciton-acoustic-phonon interactions was found to be 0.11 meV/K. This small value for the exciton-phonon coupling coefficient and the lack of a Stokes shift suggest that exciton-phonon interactions in monolayer MoTe 2 are in the weak coupling regime.