This research is supported by the National Research Foundation and Ministry of Education Singapore under its Research Centre of Excellence Programme, the Start-up Grants (M4330002.C70) from Nanyang Technological University, and AcRF Tier 2 (MOE2014-T2-2-172) from Ministry of Education, Singapore. The authors thank Joey Yam Kuok Hoong for participating in the demonstration of this protocol.

1. Biofilm Cultivation
<ol>
	<li>Preparation of Bacterial Strains
	<ol>
		<li>1 day prior to biofilm cultivation, prepare planktonic bacterial cultures by inoculating 2 ml of appropriate growth medium from frozen bacterial culture. Use Luria-Broth medium (10 g L-1 NaCl, 10 g L-1 yeast extract, and 10 g L-1 tryptone) for mucoid P. aeruginosa and its &#916;pel and &#916;psl defective mutants. Incubate overnight at 37 &#176;C and 200 rpm shaking conditions. Dilut...

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Microrheology is a useful tool for local rheological measurements in heterogeneous systems, such as microbial biofilms. It is a non-destructive technique, enabling the real-time monitoring of rheological changes within the same biological sample over multiple time points. In this protocol, particle-tracking microrheology was applied to Pel and Psl exopolysaccharide mutants in order to investigate how they affect the elasticity and effective crosslinking of the biofilm matrix. Psl favors the development of elastic biofilm...

Most bacterial cells are able to employ both planktonic (free-living) and surface-attached (sessile) modes of growth 1. In the surface-attached mode of growth, bacterial cells secrete and encase themselves in large amounts of extracellular polymeric substances (EPS) to form biofilms. The EPS mainly consists of proteins, exopolysaccharide, extracellular DNA and is essential to biofilm formation 2. It serves as a physical scaffold by which bacteria can use to differentiate spatially and protects the bacteria from harmful environmental conditions and host responses. Different components of EPS have distinct roles in biofilm formation 3 and changes in the expression of EPS components can dramatically remodel biofilm structures 4. EPS components can also function as signaling molecules 5, and recent studies has shown certain EPS components interacting with microbial cells to guide their migration and biofilm differentiation 6-8.
Research on the EPS has greatly advanced based upon the morphological analyses of biofilms produced by mutants defective in a specific component of the EPS 9,10. In addition, the EPS is usually characterized at the macro-scale (bulk characterization) 11. Morphological analyses however can lack quantitative detail and bulk characterization, which returns average values, loses the detail that exists within the heterogeneity of the biofilm. There is now an increasing trend to progress to real-time characterization of the mechanic properties of EPS at the micro-scale. This protocol demonstrates how particle-tracking microrheology is able to determine the spatiotemporal effects of matrix components Pel and Psl exopolysaccharides on the viscoelasticity and effective crosslinking of Pseudomonas aeruginosa biofilms 4.
Passive microrheology is a simple and inexpensive rheology method that provides the highest throughput of spatial microrheological sampling of a material to date 12,13. In passive microrheology, probe spheres are placed in the sample and their Brownian motion, driven by thermal energies (kBT) is followed by video microscopy. Several particles can be tracked simultaneously, and the time-dependent coordinates of the particles follow a conventional random walk. Therefore, on average, the particles remain at the same position. However, the standard deviation of the displacements or the mean squared displacement (MSD) of the particles, is not zero. Since viscous fluids flow, the particle MSD in a viscous fluid grows linearly as time progresses. In contrast, the polymeric crosslinking found in viscoelastic or elastic substances help them to resist flow, and particles become limited in their displacement, leading to plateaus in the MSD curve (Figure 1A). This observation follows the relation MSD&#8733;t&#945; , where &#945; is the diffusive exponent that is related ratio of elastic and viscous contributions of the substance. For particles moving in viscous fluids &#945; = 1, in viscoelastic substances 0 &#60; &#945; &#60; 1, and in elastic substances &#945; = 0. The MSD may also be used to calculate the creep compliance, which is the tendency of the material to deform permanently over time and estimates how easily a material spreads.
The size, density and surface chemistry of the particle are critical to the correct application of microrheological experiment and are chosen with respect to the system studied (in this case the polymers of the biofilm matrix, see Figure 1B). Firstly, the particle measures the rheology of the substance with structures that are much smaller than the particle itself. If the substance&#39;s structures are of similar scale to the particle, the motion of the particle is perturbed by the shape and orientation of the individual structures. However, if the structures surrounding the particle are much smaller, this effect is small and averaged out, presenting a homogeneous environment to the particle (Figure 1B). Secondly, the density of the particle should be similar to the medium (1.05 g ml-1 for water based mediums) such that sedimentation is avoided and inertial forces are negligible. Most particles with polystyrene lattices meet the above criteria. Ideally, the particle does not interact with the polymers of the biofilm matrix as the rheological interpretation of particle MSD is only valid if motion is random, driven by thermal energy and collision with substance structures. This can be observed by checking whether the probe particle tends to bind or bounce off the surface of a pre-grown biofilm. However, despite the lack of attraction to the biofilm, the particles must be able to be incorporated into the matrix. In addition, the physiochemical heterogeneity of the biofilm may result in different particles being more suitable as probes in different regions of the biofilm. Thus, particles of different sizes and surface chemistry should be applied to the biofilm.
As such, the particle MSD is able to provide useful information on how different components contribute to the rheology and spreading of the biofilm. Furthermore, the use of different probes allows one to derive information on the spatial physiochemical heterogeneity of the biofilm. This method can be used to test the effect antimicrobial treatment on the mechanical properties of the biofilm, or applied to mixed species biofilms to investigate how the mechanical properties of the biofilm are changed from introduction of another species. Particle MSDs may also be useful for characterizing biofilm dispersal. Such studies would be helpful in our understanding of biofilms, potentially improving biofilm treatments and engineering of biofilms for useful activities.

<table>
	<tbody>
		<tr>
			<td>Fluorspheres</td>
			<td>Invitrogen</td>
			<td>F-8821</td>
			<td>1.0 um red fluorescent (580/605) microspheres with carboxylate modification</td>
		</tr>
		<tr>
			<td>Zeiss Axio Imager M1</td>
			<td>Carl Zeiss</td>
			<td></td>
			<td>Epifluorescent Microscope</td>
		</tr>
		<tr>
			<td>Masterflex L/S Digital Drive 07523-80</td>
			<td>Cole-Parmer</td>
			<td>EW-07523-80</td>
			<td>Peristaltic pump</td>
		</tr>
		<tr>
			<td>Flow Cell Chambers</td>
			<td colspan="3">Technical University of Denmark</td>
		</tr>
		<tr>
			<td>Bubble Trap</td>
			<td colspan="3">Technical University of Denmark</td>
		</tr>
		<tr>
			<td>Silicone Tubing</td>
			<td>Dow Corning</td>
			<td></td>
			<td>3 mm outer diameter, 1 mm inner diameter</td>
		</tr>
		<tr>
			<td>Clear polypropylene plastic connectors&#160;</td>
			<td>Cole Parmer</td>
			<td>06365-83</td>
			<td>1/16 in. (1.588 mm)</td>
		</tr>
		<tr>
			<td>Binder Clips</td>
			<td></td>
			<td></td>
			<td>To clamp tubing</td>
		</tr>
		<tr>
			<td>Coverslips</td>
			<td colspan="2">Thermo Scientific&#8482; Nunc&#8482;</td>
			<td>50 x 24 mm</td>
		</tr>
		<tr>
			<td>Syringe 3 mL</td>
			<td>Terumo</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>27G Needle</td>
			<td>Terumo</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>2L Storage/Media Bottles</td>
			<td>VWR&#174;&#160;</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Trolley</td>
			<td></td>
			<td></td>
			<td>To hold biofilm setup</td>
		</tr>
	</tbody>
</table>

in situ mapping of the mechanical properties of biofilms by particle-tracking microrheology

Bacterial cells are able to form surface-attached biofilm communities known as biofilms by encasing themselves in extracellular polymeric substances (EPS). The EPS serves as a physical and protective scaffold that houses the bacterial cells and consists of a variety of materials that includes proteins, exopolysaccharides and DNA. The composition of the EPS may change, which remodels the mechanic properties of the biofilm to further develop or support alternative biofilm structures, such as streamers, as a response to environmental cues. Despite this, there are little quantitative descriptions on how EPS components contribute to the mechanical properties and function of biofilms. Rheology, the study of the flow of matter, is of particular relevance to biofilms as many biofilms grow in flow conditions and are constantly exposed to shear stress. It also provides measurement and insight on the spreading of the biofilm on a surface. Here, particle-tracking microrheology is used to examine the viscoelasticity and effective crosslinking roles of different matrix components in various parts of the biofilm during development. This approach allows researchers to measure mechanic properties of biofilms at the micro-scale, which might provide useful information for controlling and engineering biofilms.

The local viscoelastic properties of the biofilm in different regions of the biofilm, which included the voids (medium above the biofilm), plains (undifferentiated flat layer of cells) and microcolonies (see labels in Figure 2A) were investigated. The temporal changes in viscoelastic properties of the biofilm during maturation from days 3 to 5 were also determined. The MSD of the particles in the voids was used as a control and comparable to the MSD of particles in pure medium. In contrast, particles tra...

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In Situ Mapping of the Mechanical Properties of Biofilms by Particle-tracking Microrheology

Particle-tracking microrheology investigates the viscoelasticity of materials. Here, the technique is used to determine the viscoelasticity, creep compliance and effective crosslinking roles of different matrix components of a bacterial biofilm. The matrix consists of polymeric substances secreted by the bacteria and its components determine biofilm structure and mechanical properties.

In Situ Mapping of the Mechanical Properties of Biofilms by Particle-tracking Microrheology

Bacterial cells are able to form surface-attached biofilm communities known as biofilms by encasing themselves in extracellular polymeric substances ...

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9.6K vistas.  Nanyang Technological University. Particle-tracking microrheology investigates the viscoelasticity of materials. Here, the technique is used to determine the viscoelasticity, creep compliance and effective crosslinking roles of different matrix components of a bacterial biofilm. The matrix consists of polymeric substances secreted by the bacteria and its components determine biofilm structure and mechanical properties.