Variations in the opsonic requirements of group B streptococcus type III.

The opsonic requirements of strains of Type III Group B streptococci (GBS) were studied using Luminol-dependent chemiluminescence and microscopy. Growth and storage conditions, particularly any reduction in the pH of the medium, affected GBS resistance to opsonization. Opsonization was complement-dependent but there was considerable variation in the requirements of individual strains for the classical and alternative pathways. Antibody was also necessary for opsonization. A few type III GBS strains were resistant to opsonization by pooled serum known to contain type-specific antibody, raising the possibility that protective antibody might in certain circumstances be strain- rather than type-specific. Treatment of these strains with neuraminidase, however, rendered them sensitive to opsonization by the same pool of serum. The resistance of GBS to opsonization in the neonate, where complement activity can be reduced and antibody levels are low, may be a major determinant of virulence.