Flagellum expression and swimming activity by the zoonotic pathogen Escherichia albertii.

Flagella are the well-known structural appendages used by bacteria for motility. Although generally reported to be non-motile, the enteropathogenic bacterial species Escherichia albertii produces flagella intermittently. We found that E. albertii expressed flagella under specific environmental conditions. After several generations (involving 4 to 12-h incubations), six of the twelve strains we investigated displayed swimming motility in various aquatic environments, including pond water containing nutrients from pigeon droppings (10% suspension) as well as in 20 × -diluted tryptic soy broth. The most significant motility determinant was a temperature between 15 and 30 °C. At 20 °C in the 10% pigeon-dropping suspension, microscopic observations revealed that some cells (1%-95% of six strains) showed swimming motility. Electron microscopy showed that the E. albertii cells expressed flagella. Lower concentrations of some substrates (including nutrients) may be of secondary importance for E. albertii flagella expression. Interestingly, the non-motile strains (n = 6/12) contained pseudogenes corresponding to essential flagella structural proteins. After being released from its host into surface water, E. albertii may express flagella to move toward nutrient sources or new hosts.

Facile recycling of Escherichia coli and Saccharomyces cerevisiae cells from suspensions using magnetic modification method and mechanism analysis.

Cell recycling for catalysis using whole cell significantly reduced the cost of the catalyst, therefore a simple and rapid method for cell recovery from the reaction system was very important. A facile method for cell modification and recycling was developed by simply mixing the carboxyl-functionalized Fe3O4 magnetic nanoparticles (MNPs) and cell suspension. The mode microbes Escherichia coli and Saccharomyces cerevisiae were easily modified by directly adsorbing MNPs on their surface, which facilitated rapid separation and recycling of cells from suspensions with the aid of magnetic field. It was found that the pH value and ionic strength of cell suspensions played a major role on the modification. Reducing pH value or raising ionic strength facilitated the aggregation of MNPs onto cell surface due to the compression of electrical double layer of MNPs and cells. Interestingly, E. coli might exhibit distinct mechanism of MNP-magnetic modification from S. cerevisiae according to FESEM images, and explained as the different properties of surface zeta potential of the two microbes. The mechanism of modification and the interaction between MNPs and the cell wall structure of E. coli and S. cerevisiae were analyzed. The binding was mainly contributed as the high specific surface area and high surface energy of MNPs, and the decrease of electrostatic repulsive force between MNPs and cells caused by the compression of the interfacial electrical double layer around MNPs and cells. In addition, the hydrogen bonding interaction between the hydroxyl groups of cell wall polysaccharides and the carboxyl groups of MNPs might be supplementary in the binding of MNPs and cells. This work provided a detailed understanding of binding mode in the level of cell structure.

High sensitive mesoporous TiO2-coated love wave device for heavy metal detection.

This work deals with the design of a highly sensitive whole cell-based biosensor for heavy metal detection in liquid medium. The biosensor is constituted of a Love wave sensor coated with a polyelectrolyte multilayer (PEM). Escherichia coli bacteria are used as bioreceptors as their viscoelastic properties are influenced by toxic heavy metals. The acoustic sensor is constituted of a quartz substrate with interdigitated transducers and a SiO2 guiding layer. However, SiO2 shows some degradation when used in a saline medium. Mesoporous TiO2 presents good mechanical and chemical stability and offers a high active surface area. Then, the addition of a thin titania layer dip-coated onto the acoustic path of the sensor is proposed to overcome the silica degradation and to improve the mass effect sensitivity of the acoustic device. PEM and bacteria deposition, and heavy metal influence, are real time monitored through the resonance frequency variations of the acoustic device. The first polyelectrolyte layer is inserted through the titania mesoporosity, favouring rigid link of the PEM on the sensor and improving the device sensitivity. Also, the mesoporosity of surface increases the specific surface area which can be occupied and favors the formation of homogeneous PEM. It was found a frequency shift near -20±1 kHz for bacteria immobilization with titania film instead of -7±3 kHz with bare silica surface. The sensitivity is highlighted towards cadmium detection. Moreover, in this paper, particular attention is given to the immobilization of bacteria and to biosensor lifetime. Atomic Force Microscopy characterizations of the biosurface have been done for several weeks. They showed significant morphological differences depending on the bacterial life time. We noticed that the lifetime of the biosensor is longer in the case of using a mesoporous TiO2 layer.

Live cell lithography: using optical tweezers to create synthetic tissue.

We demonstrate a new method for creating synthetic tissue that has the potential to capture the three-dimensional (3D) complexity of a multi-cellular organism with submicron precision. Using multiple laminar fluid flows in a microfluidic network, we convey cells to an assembly area where multiple, time-shared optical tweezers are used to organize them into a complex array. The cells are then encapsulated in a 30 microm x 30 microm x 45 microm volume of photopolymerizable hydrogel that mimicks an extra-cellular matrix. To extend the size, shape and constituency of the array without loss of viability, we then step to an adjacent location while maintaining registration with the reference array, and repeat the process. Using this step-and-repeat method, we formed a heterogeneous array of E. coli genetically engineered with a lac switch that is functionally linked to fluorescence reporters. We then induced the array using ligands through a microfluidic network and followed the space-time development of the fluorescence to evaluate viability and metabolic activity.

Evaluation of a chromogenic medium supplemented with glucose for detecting Enterobacter sakazakii.

A commercial chromogenic agar medium (DFI) was supplemented with glucose (mDFI) to enhance the specificity of Enterobacter sakazakii (E. sakazakii) detection. Escherichia vulneris (E. vulneris), a putative false-positive strain on the DFI medium, produces alpha-glucosidase. The enzyme alpha- glucosidase hydrolyzes a substrate, 5-bromo-4-chloro-3- indolyl-alpha,D-glucopyranoside (XalphaGlc), producing green colonies. E. sakazakii strains produced green colonies on both DFI and mDFI agar, whereas E. vulneris produced green colonies on DFI agar but small white colonies on mDFI agar. E. sakazakii and E. vulneris were also readily differentiated by colony color when the mixed culture of the two strains was plated on mDFI agar and incubated for 24 h at 37 degrees C. The results indicate that the selectivity of the commercial chromogenic agar medium could be improved by a simple supplementation with glucose.