Capsular polysaccharide from Klebsiella pneumoniae KpB-1: a morphological investigation

Klebsiella pneumoniae strain B-1 (KpB-1) is a carbapenem-resistant species (CR-KP) which can cause serious nosocomial infections. Due to its resistance to most of the available antibiotics it was included in the list of pathogens that urgently require new antimicrobial strategies by the WHO. In view of searching for a novel antibacterial target, an interesting one is the capsular polysaccharide (CPS), which is present on the cell surface forming a protective layer. The KpB-1 CPS structure was defined in our laboratory; it is characterised by the presence of 4 Rha, a deoxy-sugar. The presence of a methyl group in place of a hydroxyl function on C6 could confer a weak hydrophobic character to the backbone and favour hydrophobic interactions. Such interactions could ultimately lead to the polysaccharide aggregation, which is relevant for the bacterial capsule and biofilm formation. For this reason, the morphology of KpB-1 CPS was investigated by means of Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM). Solutions of the CPS in different solvents were spray-dried on a mica surface; AFM images revealed that aggregates of different sizes were present in water solution, while single objects of globular morphology were found in methanol solution. The less polar solvent reduces the inter-chain associations thus suggesting a crucial role of the low polar Rha residues. Micrographs of the water solution obtained by TEM show the ability of this polysaccharide to form large aggregates constituted by individual polymer chains with a spherical morphology. Isolated macromolecules were also detected and they seem to have the tendency to form dimers thus confirming the aggregation ability of the polymer. In conclusion, the ability of KpB-1 CPS to exhibit a globular morphology was demonstrated by using AFM and TEM and this behaviour was mainly attributed to Rha-Rha interactions. (This work was funded by grant 2R01GM123283-04 from the US National Institutes of Health)