The susceptibility of 12 different strains (representing nine species) to the bactericidal effect of human serum complement was investigated. When grown in nutrient-rich proteose peptone-yeast extract 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-inactivated (56°C, 30 min) sheep serum. By Percoll discontinuous density centrifugation and light microscopy, the ratio of bacteria with large and small capsules was found to vary with the growth medium used. Lipopolysaccharide (LPS) was extracted with aqueous phenol after growth 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 some cases, sensitivity to serum. The biochemical basis for these changes requires further investigation.


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