title: Osmotic pressure in a bacterial swarm.
abstract: 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.
authors.affiliation: Rowland Institute at Harvard, Cambridge, Massachusetts.
authors.forename: Liyan
authors.initials: L
authors.lastname: Ping
authors.affiliation: Department of Physics, Chinese University of Hong Kong, Hong Kong, P. R. China.
authors.forename: Yilin
authors.initials: Y
authors.lastname: Wu
authors.affiliation: Rowland Institute at Harvard, Cambridge, Massachusetts; Department of Molecular and Cellular Biology, Harvard University, Cambridge, Massachusetts.
authors.forename: Basarab G
authors.initials: BG
authors.lastname: Hosu
authors.affiliation: Rowland Institute at Harvard, Cambridge, Massachusetts; Physics Department, Brown University, Providence, Rhode Island.
authors.forename: Jay X
authors.initials: JX
authors.lastname: Tang
authors.affiliation: Rowland Institute at Harvard, Cambridge, Massachusetts; Department of Molecular and Cellular Biology, Harvard University, Cambridge, Massachusetts. Electronic address: hberg@mcb.harvard.edu.
authors.forename: Howard C
authors.initials: HC
authors.lastname: Berg
mesh_terms: D000362:Agar / Q000737:chemistry; D002138:Calibration; D003470:Culture Media / Q000737:chemistry; D004926:Escherichia coli / Q000502:physiology*; D005456:Fluorescent Dyes; D008081:Liposomes / Q000378:metabolism; D008856:Microscopy, Fluorescence; D008954:Models, Biological; D009997:Osmotic Pressure / Q000502:physiology*; D014867:Water / Q000378:metabolism*
chemical_list: D003470:Culture Media; D005456:Fluorescent Dyes; D008081:Liposomes; D014867:Water; D000362:Agar