title: Bacterial surface appendages strongly impact nanomechanical and electrokinetic properties of Escherichia coli cells subjected to osmotic stress.
abstract: The physicochemical properties and dynamics of bacterial envelope, play a major role in bacterial activity. In this study, the morphological, nanomechanical and electrohydrodynamic properties of Escherichia coli K-12 mutant cells were thoroughly investigated as a function of bulk medium ionic strength using atomic force microscopy (AFM) and electrokinetics (electrophoresis). Bacteria were differing according to genetic alterations controlling the production of different surface appendages (short and rigid Ag43 adhesins, longer and more flexible type 1 fimbriae and F pilus). From the analysis of the spatially resolved force curves, it is shown that cells elasticity and turgor pressure are not only depending on bulk salt concentration but also on the presence/absence and nature of surface appendage. In 1 mM KNO(3), cells without appendages or cells surrounded by Ag43 exhibit large Young moduli and turgor pressures (∼700-900 kPa and ∼100-300 kPa respectively). Under similar ionic strength condition, a dramatic ∼50% to ∼70% decrease of these nanomechanical parameters was evidenced for cells with appendages. Qualitatively, such dependence of nanomechanical behavior on surface organization remains when increasing medium salt content to 100 mM, even though, quantitatively, differences are marked to a much smaller extent. Additionally, for a given surface appendage, the magnitude of the nanomechanical parameters decreases significantly when increasing bulk salt concentration. This effect is ascribed to a bacterial exoosmotic water loss resulting in a combined contraction of bacterial cytoplasm together with an electrostatically-driven shrinkage of the surface appendages. The former process is demonstrated upon AFM analysis, while the latter, inaccessible upon AFM imaging, is inferred from electrophoretic data interpreted according to advanced soft particle electrokinetic theory. Altogether, AFM and electrokinetic results clearly demonstrate the intimate relationship between structure/flexibility and charge of bacterial envelope and propensity of bacterium and surface appendages to contract under hypertonic conditions.
authors.affiliation: Laboratoire de Chimie Physique et Microbiologie pour l'Environnement, Nancy Université, CNRS UMR7564, Villers-lès-Nancy, France. gregory.francius@lcpme.cnrs-nancy.fr
authors.forename: Grégory
authors.initials: G
authors.lastname: Francius
authors.forename: Pavel
authors.initials: P
authors.lastname: Polyakov
authors.forename: Jenny
authors.initials: J
authors.lastname: Merlin
authors.forename: Yumiko
authors.initials: Y
authors.lastname: Abe
authors.forename: Jean-Marc
authors.initials: JM
authors.lastname: Ghigo
authors.forename: Christophe
authors.initials: C
authors.lastname: Merlin
authors.forename: Christophe
authors.initials: C
authors.lastname: Beloin
authors.forename: Jérôme F L
authors.initials: JF
authors.lastname: Duval
mesh_terms: D001696:Biomechanical Phenomena; D004586:Electrophoresis; D004926:Escherichia coli / Q000502:physiology*; D018625:Microscopy, Atomic Force; D009995:Osmosis / Q000502:physiology