Synchrotron FTIR microspectroscopy of Escherichia coli at single-cell scale under silver-induced stress conditions.

The present work was focused on elucidating biochemical changes in the model bacterium Escherichia coli exposed to ionic silver mediated stress, at a single-cell scale. In order to achieve this, in situ synchrotron Fourier-transform infrared (sFTIR) microspectroscopy was performed, for the first time, on individual cells by attenuated total reflectance (ATR) combined with the use of zinc-selenide hemisphere for high spatial resolution. In a first part, the potential of the method was evaluated on bacteria subjected to a lethal 100 µM AgNO(3) concentration for 2 h compared to untreated 100 % viable cells. Differences in cell composition were assessed for the C-H stretching and protein spectral regions, indicating that the inhibitory action was targeted against both fatty acids and proteins. Transmission electron microscopy (TEM) confirmed morphological damages of the cell ultrastructure. The relevance of ATR-sFTIR microspectroscopy for highlighting the heterogeneity in Ag(+)-mediated effects within a given bacterial population was also pointed out. In a second part, cells were exposed to sub-lethal Ag(+) concentrations (<10 µM AgNO(3)) tested under "dynamic" growth mode: early addition vs. pulse in the mid-exponential phase, and compared to simultaneously batch-grown untreated bacteria or cells sampled just before the pulse, respectively. sFTIR microspectroscopy and TEM imaging were performed in close relation with growth kinetics characterization. No significant effect of the Ag(+) pulses was detected, in accordance with macrokinetics data. For early-treated cells, effects on fatty acid composition were shown, although no major alteration of protein secondary structure was noticed. These partial effects were consistent with TEM observations and growth kinetics.

Why and how membrane bioreactors with unsteady filtration conditions can improve the efficiency of biological processes.

A membrane bioreactor (MBR), an association of a bioreactor with a crossflow filtration unit, enables continuous processes with total cell retention within the reactor to be realized. Provided that high dilution rates can be applied and that inhibition processes are avoided, very high biomass concentrations can be reached, thereby improving the volumetric productivities. These membrane bioreactors have been successfully applied to various microbial bioconversion, such as alcoholic fermentation, solvents, organic acid production, starters, and wastewater treatment. On the basis of the biological reaction characteristics and bibliographic results, the potentialities and bottlenecks of this methodology are discussed. Depending on the application, it is shown how the performance of the membrane bioreactor can be enhanced by acting either on the biological reaction achievement, by controlling the balance between cell growth and death, or on the dilution rate, by increasing the permeate flux through the filtration unit. This discussion is based on results obtained in specific biological treatments applied to polluted liquid and gas.