Variation in growth of Group B streptococci (GBS) and capsular polysaccharide production according to the manufacturing source of Todd-Hewitt broth

For quality assurance, we studied Todd-Hewitt broth (THB) from three manufacturing sources, since lot-to-lot variation of a medium can influence growth characteristics. We compared our current lot of Bacto THB with a lot from the new company, and a lot from a third manufacturer. All three had essentially the same chemical composition, except for the exact designation of quantity and source of heart infusion and peptone. Growth of GBS serotypes in the three sources of THB, with and without yeast supplementation, was assessed at several points in the growth curve by measuring absorbance and quantitation of colony-forming units. Capsular polysaccharide (CPS) production was examined by double immunodiffusion in agarose slides (Ouchterlony) using HCl cell extracts. While growth in the three lots was similar during log phase, the immunoprecipitin lines were more intense when strains were grown in our current THB. Addition of 1% yeast extract to any of the three lots enhanced absorbance and CFU counts, but reduced the quality of the precipitin lines. The inverse correlation between growth kinetics and intensity of the immunoprecipitin reactions may be due to modifications in the formulation of the three sources of THB, resulting in differences in the amount and/or composition of CPS.