Bacterial floc mediated rapid streamer formation in creeping flows

Abstract

One of the central puzzles concerning the interaction of low Reynolds number fluid transport with bacterial biomass is the formation of filamentous structures called streamers. In this manuscript, we report our discovery of a new kind of low Re bacterial streamers, which appear from pre-formed bacterial flocs. In sharp contrast to the biofilm-mediated streamers, these streamers form over extremely small timescales (less than a second). Our experiments, carried out in a microchannel with micropillars rely on fluorescence microscopy techniques to illustrate that floc-mediated streamers form when a freely-moving floc adheres to the micropillar wall and gets rapidly sheared by the background flow. We also show that at their inception the deformation of the flocs is dominated by recoverable large strains indicating significant elasticity. These strains subsequently increase tremendously to produce filamentous streamers. Interestingly, we find that these fully formed streamers are not static structures and show viscous response at time scales larger than their formation time scales. Finally, we show that such novel streamer formation can lead to rapid clogging of microfluidic devices.

Nonlinear deformation and localized failure of bacterial streamers in creeping flows

“…Bacterial flocs laden flow in the device led to the rapid formation of bacterial streamers. Such a mode of streamer formation has already been studied by Hassanpourfard et al 12. which showed that bacterial flocs could adhere to micropillar walls and get rapidly sheared by background flow to form streamers.…”

“…which showed that bacterial flocs could adhere to micropillar walls and get rapidly sheared by background flow to form streamers. In an earlier publication, Hassanpourfard et al 12. had studied the inception of streamers from flocs and found nucleating streamers to be dominated by large recoverable strains indicating significant elasticity.…”

Failure through expanding voids in bacterial streamers

“…Like biofilms, streamers consist of bacterial cells embedded in a matrix of self-secreted extracellular polymeric substances (EPS) and are excellent examples of soft materials of biological origin. Due to their morphology, streamers can colonize closed channels significantly faster than surface-hugging biofilms; recently streamers forming in very low Reynolds number conditions ( ≲ 1) have been implicated for their role in rapid fouling of biomedical devices [2][3][4] , filtration units 5,6 and even colonization of porous media 1,7,8 . In many of these applications, a better understanding of deformation, fracture and failure of streamers is crucial 5 .…”

“…These include creep behavior of the polymeric EPS, fluid-structure interaction, moving interfaces and life processes, all of which cannot be independently controlled easily. In addition, time scale of streamer formation can vary considerably 3,7,8,11 and very long-time scales can let significant changes in the background conditions affecting these nonlinear behaviors.…”

Pore-scale hydrodynamics influence the spatial evolution of bacterial biofilms in a microfluidic porous network

“….& Some& bacterial& species& can& produce& extracellular& polymeric& substances&(EPS),&which&improve&a&soil's&moisture&retention&and&act&to&aggregate&soil&particles& together,& further& altering& underground& hydrodynamics 12,13 .& Thus,& bacterial& characteristics& are& tightly& coupled& with& the& dynamics& of& soil& conditions.& This& bacteria=soil& interplay& has& implications&for&bioremediation,&water&quality,&nutrient&cycling,&and&underground&ecology.&It&is& therefore& necessary& to& study& soil& bacteria& within& the& structural& and& hydrodynamic& context& of& their& natural& environment& and& on& length& scales& appropriate& to& cellular& functions& (i.e.& the& pore& scale)&to&elicit&emergent&behaviours.&Unfortunately,&the&opacity&of&soil&presents&a&challenge&for& the&direct&visualization&and&measurement&of&bacterial&traits&at&the&pore=scale&in#situ.& Experiments& in& sand& columns& and& micromodels& have& enabled& measurements& of& bulk& bacterial&transport&through&porous&media&and&have&even&allowed&some&preliminary&imaging&of& bacteria& in& pore=spaces [14][15][16][17] .& Nafion,& a& transparent& fluoropolymer& that& is& sometimes& used& as& a& sand& substitute,& can& further& increase& the& imaging& compatibility& of& bacteria& in& flow& cells 18,19 .& However,&these&systems&do&not&have&a&defined&structure&and&are&often&treated&as&'black&boxes',& making&it&impossible&to&correlate&pore=scale&hydrodynamics&with&bacterial&biofilm&distribution.&& In& other& synthetic& systems,& microfluidic& platforms& have& been& used& to& visualize& bacterial& behaviour& in& flow& through& narrow& channels& and& around& tight& corners 20,21,22 .& These& platforms& reduce& the& physicochemical& complexity& of& natural& porous& media& while& testing& bacterial& characteristics&in&highly&parameterized&and&fully&defined&systems 23 .&&Microfluidic&systems&have& the& added& benefit& of& retaining& the& same& physical& structure& for& each& experimental& replicate,& allowing& flow& in& the& channels& to& be& computationally& simulated 24,25 .&a...…”