A microfluidic platform for characterizing the structure and rheology of biofilm streamers
Authors: Savorana Giovanni , Rusconi Roberto , Stocker Roman , Secchi Eleonora 
Published: July 2020

&lt;p&gt;In many environmental or medical settings, biofilm formation is the most successful strategy for bacterial colonization&lt;sup&gt;1,2&lt;/sup&gt;. In the biofilm lifestyle, bacteria embed themselves in a self-secreted matrix of extracellular polymeric substances (EPS), acting as a shield against mechanical and chemical insults&lt;sup&gt;3&lt;/sup&gt;. When ambient flow is present, this viscoelastic EPS scaffold can take a streamlined shape, forming biofilm threads suspended in flow, called streamers&lt;sup&gt;4&lt;/sup&gt;. In many situations, the streamer architecture can enhance the harmful effects of biofilms, bridging the spaces between obstacles in the flow path&lt;sup&gt;5&lt;/sup&gt;. Despite their importance for biofilm survival, little is known about the material properties of the matrix. In particular, these are really hard to probe with traditional rheological techniques when the biofilm grows into the thread-like streamer shape.&lt;/p&gt;
&lt;p&gt;In this work we present a microfluidic platform that allows to reproducibly grow biofilm streamers in controlled chemical and flow conditions and to characterize their structure and mechanical properties in situ&lt;sup&gt;6&lt;/sup&gt;.This platform overcomes the main sources of error and variability of the experiments performed with traditional flow-chambers: the randomness in the location and shape of the streamers. Our device consists of a straight channel with isolated micropillars, where a bacterial suspension is injected at a constant flow rate. The micropillars act as nucleation points for the growth of a pair of biofilm filaments, developing on the midplane of the channel under the action of secondary flows. The microfluidic technology allows to control the chemical and flow conditions and to perform live imaging of the growth process. By controlling the flow rate, we are also able to perform in situ stress tests on the streamers by inducing controlled variations of the fluid shear stress exerted on them. We developed a theoretical framework to estimate the material properties of biofilm streamers from the flow-induced deformation measured in our experiment. Thanks to this platform, we are able to investigate the role of the different EPS components&lt;sup&gt;7&lt;/sup&gt;and the physico-chemical microenvironment in determining biofilm structure and rheology.&lt;/p&gt;
&lt;p&gt;References&lt;/p&gt;
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&lt;p&gt;&lt;sup&gt;4&lt;/sup&gt;Rusconi et al., J. R. Soc. Interface 7, 1293 (2010).&lt;/p&gt;
&lt;p&gt;&lt;sup&gt;5&lt;/sup&gt;Drescher et al., Proc. Natl. Acad. Sci. 112, 11353 (2015).&lt;/p&gt;
&lt;p&gt;&lt;sup&gt;6&lt;/sup&gt;Savorana et al., paper in preparation&lt;/p&gt;
&lt;p&gt;&lt;sup&gt;7&lt;/sup&gt;Secchi et al., paper in preparation&lt;/p&gt;