Density gradient centrifugation is a technique that involves separating material in a solution on the basis of buoyant density. For our experiments, a sample of interest is layered on top of a medium of known density (density gradient medium) and centrifuged. Any material present in the sample with a density greater than that of the density gradient medium should migrate through the density gradient medium under the centrifugal force, forming a pellet at the bottom of the experimental vessel, as long as sufficient sedimentation time is provided. Any material less dense than the density gradient medium should not be able to pass through the medium, no matter how long centrifugal force is applied. This separates the sample into two fractions: one with material less dense than the density gradient medium, and one with material more dense than the density gradient medium (Figure , steps 1-2). Our density gradient centrifugation method was loosely based on the method developed by for the separation of soil particles and soil bacteria. First, 5 mL of OptiPrep™ density gradient medium (STEMCELL Technologies, Cat #07820) was placed in the bottom of a 15 mL centrifuge tube. OptiPrep™ is a non-ionic, non-toxic to cells, isosmotic, and metabolically inert medium consisting of 60% w/v iodixanol (molecular weight = 1550 g/mol). OptiPrep™ has a density of 1.320 ± 0.001 g cm -3 as specified by the manufacturer. Next, a 500 μL layer of ultrapure water was carefully placed on top of the density gradient medium followed by 750 μL of INS suspension, taking great care to avoid any mixing between layers (Figure , step 1). This water layer was placed between the density gradient medium and the INS suspension to prevent mixing between the two when pipetting. Ultrapure water is defined herein as distilled water purified by a Milllipore system to a resistivity of 18.2 M Ω cm at 25 °C. The tube was then centrifuged at 3,214 × g for 2 hours in an Eppendorf S-4-104 swing-bucket rotor (Figure , step 2).