Abstract
SummaryFlagellated bacteria move collectively in a swirling pattern on agar surfaces immersed in a thin layer of viscous “swarm fluid,” but the role of this fluid in mediating the cooperation of the bacterial population is not well understood. Herein, we use gold nanorods (AuNRs) as single particle tracers to explore the spatiotemporal structure of the swarm fluid. Individual AuNRs are moving in a plane of ∼2 μm above swarms, traveling for long distances in high speed without interferences from bacterial movements. The particles are lifted and transported by collective mixing of small vortices around bacteria during localized clustering and de-clustering of motile cells. Their motions fit the Lévy walk model, revealing efficient fluidic flows above the swarms. These flows provide obstacle-free highways for long-range material transportations, allow swarming bacteria to perform population-level communications, and imply the essential role of the fluid phase on the emergence of large-scale synergy.
Highlights
Bacterial swarming is collective migrations of flagellated cells across agar surfaces with swirling patterns (Harshey, 2003; Copeland and Weibel, 2009; Kearns, 2010; Partridge and Harshey, 2013)
Fast-moving bacteria are trapped in a thin layer of viscous fluid called ‘‘swarm fluid’’ (Koch and Subramanian, 2011; Wu and Berg, 2012; Clement et al, 2016; Chuang et al, 2016) The layer of fluid is only micrometers thick and even thinner at the swarm edge and has Reynold number as low as 10À5
Gold Nanorods Are Lifted above the Swarming Bacteria To study bacterial swarming, we chose the wild-type Bacillus subtilis 3610 strain as the model system (Kearns and Richard, 2010)
Summary
Bacterial swarming is collective migrations of flagellated cells across agar surfaces with swirling patterns (Harshey, 2003; Copeland and Weibel, 2009; Kearns, 2010; Partridge and Harshey, 2013). The swarm fluid can trap surfactants, modify the surface tension of liquids, support the flagella operation, and carry nutrients or other signaling molecules (Kaiser, 2007; Linda et al, 2010; Wu, 2015). The bacteria react to chemical gradients to control their short-time run lengths through rotating the flagella (Burkart et al, 1998; Wadhams and Armitage, 2004). Theoretical work such as the Vicsek Model (Benjacob, 1995) is based on the assumption of collisions and alignments of a single bacterium with its neighbors in short range. To consider hydrodynamic interactions between motile cells in the context of large-scale collective dynamics (Aditi and Ramaswamy, 2001; Lega and Passot, 2003; Toner et al, 2005; Baskaran and Marchetti, 2009; Gyrya et al, 2010), some physical models treated the swarm fluid as a continuum entangling the bacterial phase and fluid together (Aditi and Ramaswamy, 2001; Gyrya et al, 2010), in which the bacterial community is treated as discrete individual self-propelled particles surrounded by an incompressible and inseparable fluid (Aditi and Ramaswamy, 2001; Wolgemuth, 2008)
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