Effect of Quantity and Configuration of Attached Bacteria on Bacterial Propulsion of Micro-Beads

Supplementary Information Effect of Quantity and Configuration of Attached Bacteria on Bacterial Propulsion of Micro-Beads Bahareh Behkam and Metin Sitti NanoRobotics Laboratory, Department of Mechanical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA Fabrication of Unpatterned Polystyrene Micro-Beads The 10 µm pure polystyrene (PS) micro-beads (72986, Sigma-Aldrich, St Louis, MO) used in this research work were electrostatically stabilized with an alkyl sulfonate It was observed that the surfactant interfered with the particle-protein binding leading to weak adhesion of bacteria at very small densities (approximately bacterium/100 µm2) Unpatterned beads were prepared by first removing their surfactant coating 10% polystyrene beads were diluted in deionized (DI) water times The suspension was then centrifuged at 800g and resuspened in 1:1 DI water/Isopropyl alcohol (IPA) The wash procedure was repeated five times to ensure removal of substantial amount of the adsorbed surfactant, thus, causing the bacteria to uniformly coat the surface of the unpatterned beads at an adhesion density of approximately bacterium/7 µm (Fig S1(a)) 2 Fabrication of Patterned Polystyrene Micro-Beads A plasma-based patterning technique was developed to prepare the patterned micro-beads Depending on plasma power, process time, and the type of gas used, exposure of polystyrene to gas plasma leads to etching of up to few nanometers from the surface.S1,S2 Moreover, plasma treatment provides a rich variety of chemical functionalities at the surface, leading to attraction of proteins to the surface by electrostatic or dipolar interactions 10% polystyrene bead suspension was diluted in DI water 10 times and the diluted suspension was washed times For plasma etching, a monolayer of PS beads, approximately mm in diameter was prepared on a glass coverslip, using a previously developed particle self-assembly method.S3 Briefly, a mm thick layer of polydimethylsiloxane (PDMS) (Sylgard 184, Dow Corning, Midland, MI) was prepared according to the manufacturer's instruction A circle, approximately mm in diameter was cut out of the PDMS layer The perimeter of the circle was cut slantwise to allow wetting of the walls and to provide a concave air-water interface The PDMS ring was then placed on a glass slide and an aliquot portion of the 1% PS bead suspension (the volume was calculated to provide a dense monolayer of the beads after evaporation of water) was deposited within the circle The assembly was placed in a 10 cm Petri dish and covered to reduce the evaporation rate, thus allowing for the formation of the monolayer After 24 hours, the glass slide was taken out and the PDMS ring was removed To pattern the beads, the monolayer bead sample was placed in an air plasma cleaner (PDC-32G, Harrick Plasma, Ithaca, NY) at 18 W RF coil power for 2.5 minutes This process etched the portion of the micro-bead that was not masked by the underlying glass slide or the adjacent beads, revealing a functionalized hydrophilic surface Afterwards, the sample was immediately placed in motility medium (0.01 M potassium phosphate, 0.067 M sodium chloride, 10-4 M EDTA, 0.01 M glucose, and 0.002% Tween20, pH 7.0) S4 and sonicated for 2.5 minutes to release the beads from the glass slide Bacteria attach to the plasma treated regions of the patterned beads at a density of approximately bacterium/12 µm2; while very few or none adhere to the untreated area (Fig S1(b)) REFERENCES S1 E Bonaccurso, B Cappella, K Graf, J Phys Chem 110, 17918 (2006) S2 X M Zhao, Y N Xia, O Schueller, D Qin, G M Whitesides, Sensor Actuat APhys 65, 209 (1998) S3 N Denkov, O Velev, P Kralchevsky, I Ivanov, H Yoshimura, K Nagayama, Langmuir 8, 3183 (1992) S4 N Darnton, L Turner, K Breuer, H Berg, Biophyis J 86, 1863 (2004) Fig S1 Optical microscopy images and schematics of 10 µm polystyrene beads with S marcescens bacteria attached in different configurations: (a) Bacteria uniformly adhere to the unpatterned micro-bead; (b) Bacteria only adhere to the plasma treated region of the patterned micro-bead  ... slantwise to allow wetting of the walls and to provide a concave air-water interface The PDMS ring was then placed on a glass slide and an aliquot portion of the 1% PS bead suspension (the volume was... Fabrication of Patterned Polystyrene Micro-Beads A plasma-based patterning technique was developed to prepare the patterned micro-beads Depending on plasma power, process time, and the type of gas... glucose, and 0.002% Tween20, pH 7.0) S4 and sonicated for 2.5 minutes to release the beads from the glass slide Bacteria attach to the plasma treated regions of the patterned beads at a density of