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Turbulence is ubiquitous in nature yet even for the case of ordinary Newtonian fluids like water our understanding of this phenomenon is limited. Many liquids of practical importance however are more complicated (e.g. blood, polymer melts or paints), they exhibit elastic as well as viscous characteristics and the relation between stress and strain is nonlinear. We here demonstrate for a model system of such complex fluids that at high shear rates turbulence is not simply modified as previously believed but it is suppressed and replaced by a new type of disordered motion, elasto-inertial turbulence (EIT). EIT is found to occur at much lower Reynolds numbers than Newtonian turbulence and the dynamical properties differ significantly. In particular the drag is strongly reduced and the observed friction scaling resolves a longstanding puzzle in non-Newtonian fluid mechanics regarding the nature of the so-called maximum drag reduction asymptote. Theoretical considerations imply that EIT will arise in complex fluids if the extensional viscosity is sufficiently large

Uropathogenic Escherichia coli (UPEC) strain 536 (O6:K15:H31) is one of the model organisms of extraintestinal pathogenic E. coli (ExPEC). To analyze this strain's genetic basis of urovirulence, we sequenced the entire genome and compared the data with the genome sequence of UPEC strain CFT073 (O6:K2:H1) and to the available genomes of nonpathogenic E. coli strain MG1655 (K-12) and enterohemorrhagic E. coli. The genome of strain 536 is Ϸ292 kb smaller than that of strain CFT073. Genomic differences between both UPEC are mainly restricted to large pathogenicity islands, parts of which are unique to strain 536 or CFT073. Genome comparison underlines that repeated insertions and deletions in certain parts of the genome contribute to genome evolution. Furthermore, 427 and 432 genes are only present in strain 536 or in both UPEC, respectively. The majority of the latter genes is encoded within smaller horizontally acquired DNA regions scattered all over the genome. Several of these genes are involved in increasing the pathogens' fitness and adaptability. Analysis of virulence-associated traits expressed in the two UPEC O6 strains, together with genome comparison, demonstrate the marked genetic and phenotypic variability among UPEC. The ability to accumulate and express a variety of virulence-associated genes distinguishes ExPEC from many commensals and forms the basis for the individual virulence potential of ExPEC. Accordingly, instead of a common virulence mechanism, different ways exist among ExPEC to cause disease.fitness ͉ genome comparison ͉ uropathogenic Escherichia coli

We have identified in the Streptococcus pneumoniae genome sequence a two-component system (TCS13, Blp [bacteriocin-like peptide]) which is closely related to quorum-sensing systems regulating cell density-dependent phenotypes such as the development of genetic competence or the production of antimicrobial peptides in lactic acid bacteria. In this study we present evidence that TCS13 is a peptide-sensing system that controls a regulon including genes encoding Blps. Downstream of the Blp TCS (BlpH R) we identified open reading frames (blpAB) that have the potential to encode an ABC transporter that is homologous to the ComA/B export system for the competence-stimulating peptide ComC. The putative translation product of blpC, a small gene located downstream of blpAB, has a leader peptide with a Gly-Gly motif. This leader peptide is typical of precursors processed by this family of transporters. Microarray-based expression profiling showed that a synthetic oligopeptide corresponding to the processed form of BlpC (BlpC*) induces a distinct set of 16 genes. The changes in the expression profile elicited by synthetic BlpC* depend on BlpH since insertional inactivation of its corresponding gene abolishes differential gene induction. Comparison of the promoter regions of the blp genes disclosed a conserved sequence element formed by two imperfect direct repeats upstream of extended ؊10 promoter elements. We propose that BlpH is the sensor for BlpC* and the conserved sequence element is a recognition sequence for the BlpR response regulator.Signaling mechanisms controlling multicellular behavior of bacteria have attracted much attention in current research. In gram-negative bacteria, homoserine-lactone-based communication systems are prominent. Research in this area led to the term "quorum sensing" for phenomena that are controlled by cell density (12). In gram-positive bacteria, quorum sensing is accomplished by signaling systems that depend on the secretion and sensing of small peptides (11,19). At least two different mechanisms for sensing the presence of pheromone-like peptides are known (21). The first involves import of the peptide and interaction with an intracellular factor (22); the second involves binding to the extracellular portion of a membranebound histidine kinase. This leads to the autophosphorylation of the kinase and subsequent activation, e.g., phosphorylation of a cognate response regulator that mediates changes in gene expression. Quorum-sensing systems regulate a plethora of cellular functions. In Staphylococcus aureus, the AgrC-AgrAsystem is involved in the density-dependent regulation of virulence (18). In Lactobacillus strains, the production of bacteriocins is dependent on peptide-regulated two-component systems (TCS) (4, 10). In Streptococcus pneumoniae, the development of genetic competence (the natural ability to take up DNA) has been shown to be regulated by the comC-DE system (29).The com system of S. pneumoniae was the first quorumsensing system for which a biological function was defined. ...

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It is presently believed that flows of viscoelastic polymer solutions in geometries such as a straight pipe or channel are linearly stable. Here we present experimental evidence that such flows can be nonlinearly unstable and can exhibit a subcritical bifurcation. Velocimetry measurements are performed in a long, straight micro-channel; flow disturbances are introduced at the entrance of the channel system by placing a variable number of obstacles. Above a critical flow rate and a critical size of the perturbation, a sudden onset of large velocity fluctuations indicates presence of a nonlinear subcritical instability. Together with the previous observations of hydrodynamic instabilities in curved geometries, our results suggest that any flow of polymer solutions becomes unstable at sufficiently high flow rates.PACS numbers: 47.20.Gv, 61.25.he Solutions containing polymer molecules do not flow like water. Even when flowing slowly, these fluids can exhibit hydrodynamic instabilities [1][2][3][4][5][6][7][8] and a new type of turbulence -the so-called purely elastic turbulence [9,10] even at low Reynolds numbers (Re). These phenomena, driven by the anisotropic elasticity of the fluid, were experimentally observed only in geometries with sufficient curvature, like rotational flows between two cylinders [1,11,12] and plates [13], in curved channels [10,14], and around obstacles [15]. Most of the nonlinear flow behavior observed in these studies arises from the extra elastic stresses due to the presence of polymer molecules in the fluid. These elastic stresses are history dependent and evolve on the time-scale λ that in dilute solutions is proportional to the time needed for a polymer molecule to relax to its equilibrium state [16].A common feature of the above-mentioned geometries is the presence of curved streamlines in the base flow with a sufficient velocity gradient across the streamlines. It has been argued that this is a necessary condition for infinitesimal perturbations to be amplified by the normal stress imbalances in viscoelastic flows [1,8,13]. This condition can be written as (λ U N 1 )/(R Σ) ≥ M [8, 13,17], where M is a constant that only depends on the type of flow geometry, U is a typical velocity along the streamlines, R is the radius of streamline curvature, and N 1 and Σ are the first normal stress difference and the shear stress, correspondingly. According to this condition, purely elastic linear instabilities are not possible when the curvature of the flow geometry is zero, and infinitesimal perturbations decay at a rate proportional to 1/λ [8,18,19].Nevertheless, the absence of a linear instability does not imply absolute stability. Indeed, recent theoretical [17,[20][21][22][23] and indirect experimental [24,25] evidence points towards a finite-amplitude transition in viscoelastic flows with parallel streamlines even at low Reynolds number, where viscous and elastic forces dominate over inertial forces. An earlier study of the flow of a polymeric melt extruded out of a thin cylindrical capi...

We investigate the rheological characteristics of human blood plasma in shear and elongational flows. While we can confirm a Newtonian behavior in shear flow within experimental resolution, we find a viscoelastic behavior of blood plasma in the pure extensional flow of a capillary breakup rheometer. The influence of the viscoelasticity of blood plasma on capillary blood flow is tested in a microfluidic device with a contraction-expansion geometry. Differential pressure measurements revealed that the plasma has a pronounced flow resistance compared to that of pure water. Supplementary measurements indicate that the viscoelasticity of the plasma might even lead to viscoelastic instabilities under certain conditions. Our findings show that the viscoelastic properties of plasma should not be ignored in future studies on blood flow.

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