THE INFLUENCE OF GROWTH-MEDIUM ON SERUM SENSITIVITY OF BACTEROIDES SPECIES

The susceptibility of 12 different Bacteroides strains (representing n ine species) to the bactericidal effect of human serum complement was investigated. When grown in nutrient-rich proteose peptone-yeast extra ct medium, all 12 strains were, to varying degrees, sensitive to serum . However, when grown in Van Tassell and Wilkins's minimal medium, six of the 12 strains became markedly more serum resistant. Five of these six strains became totally resistant to serum when grown in heat-inac tivated (56 degrees C, 30 min) sheep serum. By Percoll discontinuous d ensity centrifugation and light microscopy, the ratio of bacteria with large and small capsules was found to vary with the growth medium use d. Lipopolysaccharide (LPS) was extracted with aqueous phenol after gr owth in the three media. Polyacrylamide gel electrophoresis (PAGE) and silver staining of the LPS showed some differences in LPS profiles in all strains tested. Therefore, variation of growth conditions results in alterations of both the expression of surface structures and, in s ome cases, sensitivity to serum. The biochemical basis for these chang es requires further investigation.