The distribution of phospholipids across the inner membrane (IM) of Gram-negative bacteria is unknown. We demonstrate that the IMs of Escherichia coli and Yersinia pseudotuberculosis are asymmetric, with a 75%/25% (cytoplasmic/periplasmic leaflet) distribution of phosphatidylethanolamine (PE) in rod-shaped cells and an opposite distribution in E. coli filamentous cells. In initially filamentous PE-lacking E. coli cells, nascent PE appears first in the periplasmic leaflet. As the total PE content increases from nearly zero to 75%, cells progressively adopt a rod shape and PE appears in the cytoplasmic leaflet of the IM. The redistribution of PE influences the distribution of the other lipids between the leaflets. This correlates with the tendency of PE and cardiolipin to regulate antagonistically lipid order of the bilayer. The results suggest that PE asymmetry is metabolically controlled to balance temporally the net rates of synthesis and translocation, satisfy envelope growth capacity, and adjust bilayer chemical and physical properties.

The adsorption of bovine serum albumin (BSA) on two types of silica nanoparticles (NPs), one pyrolytic (P−SiO 2 ; namely AOX50 by Evonik) and the other colloidal (lab-made by using inverse micelles microemulsion, M−SiO 2 ), is studied. Both materials are characterized in terms of size of primary particles (by transmission electron microscopy), amounts (by thermogravimetry) and distribution of silanols (IR spectroscopy in controlled atmosphere, augmented by H/D isotopic exchange and reaction with VOCl 3 , to distinguish silanols actually located at the surface of nanoparticles), water contact angle, ζ−potential and dispersion state in water, PBS buffer and BSA solutions in PBS (by dynamic light scattering, DLS). Proteins are found to act as dispersing agent toward the large aggregates formed by both types of NPs in PBS buffer, although monodispersion was not attained in the conditions investigated. The problem of the determination of the silica surface actually available in NPs agglomerates for protein adsorption is addressed, and a model based on the external area of the agglomerates determined by DLS is proposed, supported by the trend of ζ−potential in dependence on the amount of adsorbed BSA and by the UV circular dichroism spectra of adsorbed proteins. The spectra reveal the occurrence of protein-protein interactions for BSA on P−SiO 2 , where multilayers of irreversibly adsorbed BSA molecules (i.e. a so called protein hard corona) are proposed to be formed. Conversely, the model indicates the formation of a sub-monolayer protein hard corona on M−SiO 2 . The difference in protein coverage appears to be related to differences in the distribution of surface silanols, more than to differences in ζ−potential.

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