Study of the chemical stability of the capsular polysaccharide produced by Haemophilus influenzae type b.

Haemophilus influenzae type b (Hib) is a human pathogen that causes severe infections such as pneumonia, sepsis and meningitis. Vaccines for Hib infections are based on its capsular polysaccharide conjugated to a protein. This conjugated Hib antigen is included as one of the components of polyvalent vaccines and accounts for more than 50% of the total cost of the formulations. The instability of the polysaccharide is responsible for the high cost of the vaccine. In this study, the factors affecting the spontaneous degradation of the polysaccharide from Hib were evaluated based on the decrease in its molecular mass, as measured by size-exclusion chromatography. Temperature and pH were found to be the most significant variables, and the results showed that the conditions of bacterial cell growth (37 °C and pH 7.5) are favourable for depolymerization. An increase in the concentration of sodium ions up to 200 mM intensified the effect of pH, allowing higher rates of depolymerization at lower pH values, whereas the presence of magnesium ions showed no effects.