(36b) Ultrafiltration Behavior of Bacterial Polysaccharides

Bacterial polysaccharides are an important component of newly developed vaccines against pneumococci, and meningococci. These vaccines induce a strong immune response against the capsular polysaccharides that are unique to specific microorganisms. Ultrafiltration can be used for concentration and purification of polysaccharides; and the study aims to further understand the behavior of different polysaccharides during ultrafiltration.

Experiments were performed using purified bacterial polysaccharides provided by Pfizer Inc. Data were obtained in a stirred ultrafiltration cell using Ultracel composite regenerated cellulose and Biomax polyethersulfone membranes with nominal molecular weight cutoffs of 300, 500, and 1000 kDa. Polysaccharide concentrations were determined using high performance liquid chromatography (HPLC) with refractive index detection.  

The filtrate flux and polysaccharide transmission were both strong functions of the solution ionic strength due to inter- and intra-molecular electrostatic interactions.  The filtrate flux attained a pressure-independent value at relatively low pressures (around 1 psi = 6.9 kPa) even in dilute polysaccharide solutions (< 1 g/L).  There was clear evidence of polysaccharide fouling, particularly in the high ionic strength solutions.  Polysaccharide transmission was also a function of the stirring rate, providing clear evidence for the effects of concentration polarization. Much higher transmission could be obtained by using the membranes in the skin-side down orientation due to the strong degree of internal polarization under these conditions.  These results provided important insights into the factors controlling the ultrafiltration behavior of bacterial polysaccharides of interest in bioprocessing applications.