Bacterial Capsule Thickness Quantification

Capsule thickness was measured using the FITC-dextran zone of exclusion method, as previously described, with minor modifications^{40 (link)}. Exponential phase cultures were centrifuged at 3000 g for 10 min, and the pellet re-suspended in PBS. 10 µl of bacterial suspension was mixed with 1 µl of 2000 kDa FITC-dextran (Sigma-Aldrich) and pipetted onto a microscope slide. The Nikon Eclipse 80i fluorescence microscope (100 × magnification) was used to view the slides and photographs were taken using a Hamamatsu C4742-95 camera. ImageJ was used to determine the zone of exclusion (area in pixels), a value proportional to capsular thickness.