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Background: Bacterial chemotaxis, the ability of motile bacteria to navigate gradients of chemicals, plays key roles in the establishment of various plant-microbe associations, including those that benefit plant growth and crop productivity. The motile soil bacterium Azospirillum brasilense colonizes the rhizosphere and promotes the growth of diverse plants across a range of environments. Aerotaxis, or the ability to navigate oxygen gradients, is a widespread behavior in bacteria. It is one of the strongest behavioral responses in A. brasilense and it is essential for successful colonization of the root surface. Oxygen is one of the limiting nutrients in the rhizosphere where density and activity of organisms are greatest. The aerotaxis response of A. brasilense is also characterized by high precision with motile cells able to detect narrow regions in a gradient where the oxygen concentration is low enough to support their microaerobic lifestyle and metabolism., Results: Here, we present a mathematical model for aerotaxis band formation that captures most critical features of aerotaxis in A. brasilense. Remarkably, this model recapitulates experimental observations of the formation of a stable aerotactic band within 2 minutes of exposure to the air gradient that were not captured in previous modeling efforts. Using experimentally determined parameters, the mathematical model reproduced an aerotactic band at a distance from the meniscus and with a width that matched the experimental observation., Conclusions: Including experimentally determined parameter values allowed us to validate a mathematical model for aerotactic band formation in spatial gradients that recapitulates the spatiotemporal stability of the band and its position in the gradient as well as its overall width. This validated model also allowed us to capture the range of oxygen concentrations the bacteria prefer during aerotaxis, and to estimate the effect of parameter values (e.g. oxygen consumption rate), both of which are difficult to obtain in experiments.

Listeria monocytogenes is a food-borne pathogen that can grow as a biofilm on surfaces. Biofilm formation in food-processing environments is a big concern for food safety, as it can cause product contamination through the food-processing line. Although motile aerobic bacteria have been described to form biofilms at the air-liquid interface of cell cultures, to our knowledge, this type of biofilm has not been described in L. monocytogenes before. In this study we report L. monocytogenes biofilm formation at the air-liquid interface of aerobically grown cultures, and that this phenotype is specifically induced when the media is supplemented with glycerol as a carbon and energy source. Planktonic growth, metabolic activity assays and HPLC measurements of glycerol consumption over time showed that glycerol utilization in L. monocytogenes is restricted to growth under aerobic conditions. Gene expression analysis showed that genes encoding the glycerol transporter GlpF, the glycerol kinase GlpK and the glycerol 3-phosphate dehydrogenase GlpD were upregulated in the presence of oxygen, and downregulated in absence of oxygen. Additionally, motility assays revealed the induction of aerotaxis in the presence of glycerol. Our results demonstrate that the formation of biofilms at the air-liquid interface is dependent on glycerol-induced aerotaxis towards the surface of the culture, where L. monocytogenes has access to higher concentrations of oxygen, and is therefore able to utilize this compound as a carbon source., (Copyright © 2018 Elsevier B.V. All rights reserved.)

As the closest unicellular relatives of animals, choanoflagellates serve as useful model organisms for understanding the evolution of animal multicellularity. An important factor in animal evolution was the increasing ocean oxygen levels in the Precambrian, which are thought to have influenced the emergence of complex multicellular life. As a first step in addressing these conditions, we study here the response of the colony-forming choanoflagellate Salpingoeca rosetta to oxygen gradients. Using a microfluidic device that allows spatio-temporal variations in oxygen concentrations, we report the discovery that S. rosetta displays positive aerotaxis. Analysis of the spatial population distributions provides evidence for logarithmic sensing of oxygen, which enhances sensing in low oxygen neighborhoods. Analysis of search strategy models on the experimental colony trajectories finds that choanoflagellate aerotaxis is consistent with stochastic navigation, the statistics of which are captured using an effective continuous version based on classical run-and-tumble chemotaxis., Competing Interests: The authors declare that no competing interests exist.

Abstract: We propose a simple but reliable technology for the generation of oxygen gradient in microfluidic devices, which is useful for cell aerotaxis study. The technology is based on single layer patterning of polydimethylsiloxane (PDMS) and integration of a thin layer polyvinylidene chloride (PVDC). The PDMS layer was patterned to define a cell chamber embedded by two micro-channels, whereas the PVDC layer acts as barrier to prevent the diffusion of gas outside into the layer, thus forcing gas to diffuse along the PDMS layer. As a result, a linear oxygen gradient in the cell chamber can be established by controlling the gas contents inside the two micro-channels. By using 3-way valves, such a linear gradient could be changed from one state to another. To demonstrate the application potential of this approach, we used this device in the study of the aerotactic behaviour of magnetospirillum sp. strain AMB-1 and observed a stable and reversible cell migration by changing the gas distribution in the cell chamber. This system should be useful for better understanding of cell aerotactic mechanism. [Copyright &y& Elsevier]

Spatiotemporal variations of oxygen concentration affect the cell behaviors that are involved in physiological and pathological events. In our previous study with Dictyostelium discoideum (Dd) as a model of cell motility, aggregations of Dd cells exhibited long-lasting and highly stable migration in a self-generated hypoxic environment, forming a ring shape that spread toward the outer higher oxygen region. However, it is still unclear what kinds of changes in the migratory properties are responsible for the observed phenomena. Here, we investigated the migration of Dd to clarify the oxygen-dependent characteristics of aerokinesis and aerotaxis. Migratory behaviors of Dd cells were analyzed under various oxygen concentration gradients and uniform oxygen conditions generated in microfluidic devices. Under hypoxic conditions below 2% O2, corresponding to less than 25 µM O2 in the culture medium, the migration of Dd cells was enhanced (aerokinesis) and the oxygen gradient guided the cells toward the oxygen-rich region (aerotaxis). The aerotaxis was attributed to the increase in the frequency of migration associated with the direction of higher O2, the acceleration of migration velocity, and the enhancement of migration straightness. Thus, aerokinesis and aerotaxis are dependent on both the oxygen level and possibly relative gradient and are essential mechanisms for the migration of Dd. [ABSTRACT FROM AUTHOR]

Abstract: Here we report the results of transient absorption and photoacoustic calorimetry studies of CO photodissociation from the heme domain of the bacterial oxygen sensor HemAT-Bs. The results indicate that CO photolysis is accompanied by an overall ΔH of −19kcalmol−1 and ΔV of +4mlmol−1 as well as a red-shifted kinetic difference spectrum all occurring in <50ns. Analysis of the ΔH/ΔV reveals that a conformational change takes place with a ΔH conf of −40kcalmol−1 and ΔV conf of −22mlmol−1. These thermodynamic changes are consistent with an increase in the solvent accessible surface area of the protein upon ligand dissociation, as observed in the X-ray structure of the ferric CN-bound and CN free forms of HemAT-Bs. [Copyright &y& Elsevier]

The regulation of aerotaxis in Pseudomonas aeruginosa is reported. P. aeruginosa possesses two aerotaxis transducers, Aer and Aer-2. The aerotactic responses of P. aeruginosa cells were induced during the transition from exponential to stationary growth phase. A deletion mutant for the anaerobic transcriptional regulator ANR showed decreased aerotaxis. The anr mutation eliminated Aer-mediated aerotaxis, but not Aer-2-mediated aerotaxis. Expression of an aer-lacZ transcriptional fusion was also induced during the transition from exponential to stationary growth phase. The anr mutant showed only background levels of aer-lacZ expression. Rapid amplification of cDNA ends (RACE) and DNA sequencing revealed that the 5′ end of the mRNA was located at an A nucleotide −67 nt upstream of aer. The aer promoter contained two putative FNR/ANR boxes at −42.5 and −93.5 bp upstream of the transcriptional start site of aer. Mutational analysis of the aer promoter region revealed that both FNR/ANR boxes were essential for the expression of the aer gene. These results indicate that ANR is required for the activation of aer expression but it is not essential for Aer-2-mediated aerotaxis in P. aeruginosa. [Copyright &y& Elsevier]