Super-resolution fluorescence microscopy study of the production of K1 capsules by Escherichia coli: evidence for the differential distribution of the capsule at the poles and the equator of the cell.

The production of the Escherichia coli K1 serotype capsule was investigated using direct stochastic optical reconstruction microscopy (dSTORM) with live bacteria and graphene oxide coated coverslips; overcoming many morphological artefacts found in other high resolution imaging techniques. Super-resolution fluorescence images showed the K1 capsular polysaccharide is not uniformly distributed on the cell surface as previously thought. These studies demonstrated that on the cell surfaces the K1 capsule had a bimodal thickness at the poles of 238 ± 41 nm and 323 ± 62 nm, whereas at the equator there was a monomodal thickness of 217 ± 29 nm. This bimodal variation was also observed in HPLC measurements of purified K1 capsular polysaccharide. Particle tracking demonstrated the formation of the capsule was dominated by the expansion of lyso-phosphatidylglycerol (lyso-PG) rafts that anchor the capsular polysaccharide in the outer membrane and the expansion of these rafts across the cell surface was driven by new material transported through the capsular biosynthesis channels. The discovery of thicker capsules at the poles of the cell will have implications in mediating interactions between the bacterium and its immediate environment.