Osmotic pressure in a bacterial swarm.

Using Escherichia coli as a model organism, we studied how water is recruited by a bacterial swarm. A previous analysis of trajectories of small air bubbles revealed a stream of fluid flowing in a clockwise direction ahead of the swarm. A companion study suggested that water moves out of the agar into the swarm in a narrow region centered ∼ 30 μm from the leading edge of the swarm and then back into the agar (at a smaller rate) in a region centered ∼ 120 μm back from the leading edge. Presumably, these flows are driven by changes in osmolarity. Here, we utilized green/red fluorescent liposomes as reporters of osmolarity to verify this hypothesis. The stream of fluid that flows in front of the swarm contains osmolytes. Two distinct regions are observed inside the swarm near its leading edge: an outer high-osmolarity band (∼ 30 mOsm higher than the agar baseline) and an inner low-osmolarity band (isotonic or slightly hypotonic to the agar baseline). This profile supports the fluid-flow model derived from the drift of air bubbles and provides new (to our knowledge) insights into water maintenance in bacterial swarms. High osmotic pressure at the leading edge of the swarm extracts water from the underlying agar and promotes motility. The osmolyte is of high molecular weight and probably is lipopolysaccharide.

[1]  D. B. Kearns,et al.  A field guide to bacterial swarming motility , 2010, Nature Reviews Microbiology.

[2]  R. Harshey,et al.  Swarming: Flexible Roaming Plans , 2012, Journal of bacteriology.

[3]  H. Berg,et al.  Growth of Flagellar Filaments of Escherichia coli Is Independent of Filament Length , 2012, Journal of bacteriology.

[4]  R. Harshey,et al.  A mechanical role for the chemotaxis system in swarming motility , 2006, Molecular microbiology.

[5]  J. Hartley,et al.  Chemical and Physical Properties of Lipopolysaccharide of Yersinia pestis , 1974, Journal of bacteriology.

[6]  J. Alonso,et al.  Overexpression of the recA Gene Decreases Oral but Not Intraperitoneal Fitness of Salmonella enterica , 2010, Infection and Immunity.

[7]  Howard C. Berg,et al.  Visualization of Flagella during Bacterial Swarming , 2010, Journal of bacteriology.

[8]  Howard C. Berg,et al.  Microbubbles reveal chiral fluid flows in bacterial swarms , 2011, Proceedings of the National Academy of Sciences.

[9]  H. Berg,et al.  Dynamics of bacterial swarming. , 2010, Biophysical journal.

[10]  J B Armstrong,et al.  Nonchemotactic Mutants of Escherichia coli , 1967, Journal of bacteriology.

[11]  H. Berg,et al.  Swarming Motility: It Better Be Wet , 2005, Current Biology.

[12]  T. Murray,et al.  Swarming motility, secretion of type 3 effectors and biofilm formation phenotypes exhibited within a large cohort of Pseudomonas aeruginosa clinical isolates. , 2010, Journal of medical microbiology.

[13]  Howard C. Berg,et al.  E. coli in Motion , 2003 .

[14]  T. Matsuyama,et al.  Dimorphic transition in Escherichia coli and Salmonella typhimurium: surface-induced differentiation into hyperflagellate swarmer cells. , 1994, Proceedings of the National Academy of Sciences of the United States of America.

[15]  S. Porwollik,et al.  Sensing wetness: a new role for the bacterial flagellum , 2005, The EMBO journal.

[16]  V. Holá,et al.  Virulence factors in Proteus bacteria from biofilm communities of catheter-associated urinary tract infections. , 2012, FEMS immunology and medical microbiology.

[17]  R. Harshey,et al.  Bacterial motility on a surface: many ways to a common goal. , 2003, Annual review of microbiology.

[18]  Michael A. Siano,et al.  Genetics of Swarming Motility in Salmonella enterica Serovar Typhimurium: Critical Role for Lipopolysaccharide , 2000, Journal of bacteriology.

[19]  A. Verkman,et al.  Airway Surface Liquid Osmolality Measured Using Fluorophore-Encapsulated Liposomes , 2001, The Journal of general physiology.

[20]  Yilin Wu,et al.  Water reservoir maintained by cell growth fuels the spreading of a bacterial swarm , 2012, Proceedings of the National Academy of Sciences.

[21]  Role of swarming migration in the pathogenesis of bacillus endophthalmitis. , 2006, Investigative ophthalmology & visual science.