Public Reporting Burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Manuscript published: Nature 439, 303-306 (2006) Report Title ABSTRACT This is a report of a publication supported by the research grant:"Artificial 'spin ice' in a geometrically frustrated lattice of nanoscale ferromagnetic islands", R.

Biofilms are communities of sessile microbes that are phenotypically distinct from their genetically identical, free-swimming counterparts. Biofilms initiate when bacteria attach to a solid surface. Attachment triggers intracellular signaling to change gene expression from the planktonic to the biofilm phenotype. For , it has long been known that intracellular levels of the signal cyclic-di-GMP increase upon surface adhesion and that this is required to begin biofilm development. However, what cue is sensed to notify bacteria that they are attached to the surface has not been known. Here, we show that mechanical shear acts as a cue for surface adhesion and activates cyclic-di-GMP signaling. The magnitude of the shear force, and thereby the corresponding activation of cyclic-di-GMP signaling, can be adjusted both by varying the strength of the adhesion that binds bacteria to the surface and by varying the rate of fluid flow over surface-bound bacteria. We show that the envelope protein PilY1 and functional type IV pili are required mechanosensory elements. An analytic model that accounts for the feedback between mechanosensors, cyclic-di-GMP signaling, and production of adhesive polysaccharides describes our data well.

The Vps/VacJ ABC transporter system is proposed to function in maintaining the lipid asymmetry of the outer membrane. Mutations in vps or vacJ in Shigella flexneri resulted in increased sensitivity to lysis by the detergent sodium dodecyl sulfate (SDS), and the vpsC mutant showed minor differences in its phospholipid profile compared to the wild type. vpsC mutants were unable to form plaques in cultured epithelial cells, but this was not due to a failure to invade, to replicate intracellularly, or to polymerize actin via IcsA for movement within epithelial cells. The addition of the outer membrane phospholipase gene pldA on a multicopy plasmid in a vpsC or vacJ mutant restored its resistance to SDS, suggesting a restoration of lipid asymmetry to the outer membrane. However, the pldA plasmid did not restore the mutant's ability to form plaques in tissue culture cells. Increased PldA levels also failed to restore the mutant's phospholipid profile to that of the wild type. We propose a dual function of the Vps/VacJ ABC transporter system in S. flexneri in both the maintenance of lipid asymmetry in the outer membrane and the intercellular spread of the bacteria between adjacent epithelial cells. Shigella flexneri is the causative agent of bacillary dysentery in humans. The bacteria invade the colonic epithelium, replicate intracellularly, and spread cell to cell, provoking the acute inflammatory response that is characteristic of the disease (1). While much is known about the initial invasion of epithelial cells by Shigella, less is understood about the requirements for intracellular replication and intercellular spread.Many of the genes required for pathogenesis of S. flexneri are carried on a large plasmid (2, 3). Plasmid-encoded virulence factors include the invasion plasmid antigen (Ipa) effector proteins required for S. flexneri to enter the host cell and escape from the vacuole (4-6). These effector proteins are secreted by a type III secretion system (TTSS) that is also encoded on the virulence plasmid. When the effectors contact epithelial cells, they induce cytoskeletal changes leading to internalization of the bacteria (7,8). Once engulfed by the epithelial cell, the bacteria lyse the vacuole and replicate inside the cytosol of the host cell (6).Within the host cell cytoplasm, S. flexneri uses another virulence plasmid-encoded protein, IcsA, to polymerize actin at one pole of the cell, resulting in the propulsion of the bacteria throughout the host cell (9, 10). This actin-based motility also promotes movement of the bacteria into adjacent epithelial cells. Intercellular movement results in S. flexneri cells being surrounded by a double membrane, one from the cell which the bacteria are exiting and another from the cell which they are entering (11). The secreted effector proteins are required for the bacteria to escape this double-membrane-bound vacuole, and once free in the cytoplasm, the bacteria repeat the cycle of replication and intercellular spread (12, 13).An understanding of the mechanisms of invas...

Biofilms are surface-mounted, multicellular communities of microbes. Biofilms are often associated with chronic infections that resist treatment, evade the immune system, and damage host tissue. An essential characteristic of the biofilm state is that constituent organisms are bound in a polymeric matrix. This matrix gives the system spatial structure and clusters bacteria near each other, facilitating intercellular interactions. The Pseudomonas aeruginosa strain PAO1 is widely studied as a model biofilm-forming organism. The polymeric matrix of PAO1 biofilms is dominated by two bacteria-produced extracellular polymers, Pel and Psl. We use a combination of optical and atomic force microscopy to examine the roles of these polymers in very early biofilm development. In agreement with other researchers, we find that Psl mediates strong attachment to a glass surface. We find that Pel alone can mediate some attachment, but not as permanent as that mediated by Psl. Unexpectedly, we find that Pel promotes symmetric attachment, in the form of rod-shaped bacteria lying down flat on the surface, and that the presence of Pel makes attachment forces more short-ranged than they are with Psl alone. We suggest that these effects may result from synergistic interactions of Pel with the Psl polymeric matrix.