Bacterial motility, and in particular repulsion or attraction towards specific chemicals, has been a subject of investigation for over 100 years, resulting in detailed understanding of bacterial chemotaxis and the corresponding sensory network in many bacterial species. For Escherichia coli most of the current understanding comes from the experiments with low levels of chemotactically-active ligands. However, chemotactically-inactive chemical species at concentrations found in the human gastrointestinal tract produce significant changes in E. coli's osmotic pressure, and have been shown to lead to taxis. To understand how these nonspecific physical signals influence motility, we look at the response of individual bacterial flagellar motors under step-wise changes in external osmolarity. We combine these measurements with a population swimming assay under the same conditions. Unlike for chemotactic response, a long term increase in swimming/motor speeds is observed, and in the motor rotational bias, both of which scale with the osmotic shock magnitude. We discuss how the speed changes we observe can lead to steady state bacterial accumulation.

Abstract. We report on the formation of Ge/Si quantum dots with core/shell structure that are arranged in a three-dimensional body centered tetragonal quantum dot lattice in an amorphous alumina matrix. The material is prepared by magnetron sputtering deposition of Al 2 O 3 /Ge/Si multilayer. The inversion of Ge and Si in the deposition sequence results in the formation of thin Si/Ge layers instead of the dots. Both materials show an atomically sharp interface between the Ge and Si parts of the dots and layers. They have an amorphous internal structure that can be crystallized by an annealing treatment. The light absorption properties of these complex materials are significantly different compared to films that form quantum dot lattices of the pure Ge, Si or a solid solution of GeSi. They show a strong narrow absorption peak that characterizes a type II confinement in accordance with theoretical predictions. The prepared materials are promising for application in quantum dot solar cells.

Cyanobacterial granules and aggregates can readily form in aquatic environments. The microbial communities found within and around these structures can be referred to as the cyanosphere, and can enable collective metabolic activities relevant to biogeochemical cycles. Cyanosphere communities are suggested to have different composition to that in the surrounding environment, but studies to date are mostly based on single time point samples. Here, we retrieved samples containing cyanobacterial granules from a freshwater reservoir and maintained a culture through sub-culture passages under laboratory conditions for over a year. We show that cyanobacteria-dominated granules form readily and repeatedly in this system over passages, and that this structure formation process seems to be associated with cyanobacterial motility. Performing longitudinal short-read sequencing over several culture passages, we identified a cyanosphere community comprising of 17 species with maintained population structure. Using long-read sequencing from two different time point samples, we have re-constructed full, circular genomes for 15 of these species and annotated metabolic functions within. This predicts several metabolic interactions among community members, including sulfur cycling and carbon and vitamin exchange. Using three individual species isolated from this cyanosphere, we provide experimental support for growth on carbon sources predicted to be secreted by the cyanobacterium in the system. These findings reinforce the view that the cyanosphere can recruit and maintain a specific microbial community with specific functionalities embedded in a spatially-organised microenvironment. The presented community will act as a key model system for further understanding the formation of the structured cyanosphere, its function and stability, and its metabolic contribution to biogeochemical cycles.

Bacterial Flagellar Motor is one of nature's rare rotary molecular machines. It enables bacterial swimming and it is the key part of the bacterial chemotactic network, one of the best studied chemical signalling networks in biology, which enables bacteria to direct its movement in accordance with the chemical environment. The network can sense down to nanomolar concentrations of specific chemicals on the time scale of seconds. Motor's rotational speed is linearly proportional to the electrochemical gradients of either proton or sodium driving ions, while its direction is regulated by the chemotactic network. Recently, it has been discovered that motor is also a mechanosensor. Given these properties, we discuss the motor's potential to serve as a multifunctional biosensor and a tool for characterising and studying the external environment, the bacterial physiology itself and single molecular motor biophysics.

Spatial organization is the norm rather than the exception in the microbial world. While the study of microbial physiology has been dominated by studies in well-mixed cultures, there is now increasing interest in understanding the role of spatial organization in microbial physiology, coexistence and evolution. Where studied, spatial organization has been shown to influence all three of these aspects. In this mini review and perspective article, we emphasize that the dynamics within spatially organized microbial systems (SOMS) are governed by feedbacks between local physico-chemical conditions, cell physiology and movement, and evolution. These feedbacks can give rise to emergent dynamics, which need to be studied through a combination of spatio-temporal measurements and mathematical models. We highlight the initial formation of SOMS and their emergent dynamics as two open areas of investigation for future studies. These studies will benefit from the development of model systems that can mimic natural ones in terms of species composition and spatial structure.