Strain Differences in Virulence of Group B Streptococci

Summary and for antibody by indirect immunofluorescence (1, 6). Over 20 Strain differences in virulence for types la, I1 and 111 group B streptococci were assessed using an animal model of group B streptococcal disease. Human isolates of group B streptococci, which have been shown to have strain specific differences in opsonic requirements, were also found to exhibit differences in virulence within the same serotype. Serum, possessing opsonic and protective antibody against one strain, often did not have activity against other strains of the same serotype. Our results point to a direct correlation between strain resistance to opsonization and increased virulence within a given serotype. different sera were employed in these studies. ~e~iesentative experiments are shown. For opsonic assays, sera was heated at 56°C for 1 h and then reconstituted with whole human complement as previously described (6). Rabbit antibody to the group specific carbohydrate of group B organisms was purchased from Difco Laboratories (Detroit, MI). Typing serum was prepared by slight modifications of Lancefield's techniques as previously described (6). All sera were absorbed with equal volumes of heterologous type organisms to remove cross reactions (6). Chemiluminescence procedure. This procedure has been described in detail elsewhere (6). Briefly, s&a to be tested for opsonic Speculation antibody activity were incubated at 37OC for 30 min with a Some strains of group B appear to be more virulent reference 0' wild type (recent clinical isolate) strain of group B than others in experimental animals. -,-hese strains are not uni- streptococci types Ia, 11, or 111 at a ratio of 0.5 ml of bacteria (At fody susceptible to the actionof antibody andcomplement. Such 108-1.0 log colony-foming units per ml) m1 of strain differences in virulence and opsonic requirements must be Se"m. The O~sonized bacteria were washed considered in the development of active or passive immunotherapy ~nbound antibody. In dark adapted scintillation vials, polymerof this most fulminant of neonatal diseases. phonuclear leukocytes (0.5 ml at lo7 cells/ml) were mixed with 0.5 ml of the bacterial suspension. The volume was adjusted to 3.5 ml with sterile PBS. The vials were capped, and placed in a Group B streptococci continue to be a major cause of morbidity Beckman LS-100~ liquid scintillation counter (Beckman Instruand mortality in neonates and in other patients with compromised ment% Inc.7 Fullerton, CA) and counted at approximately 10 min host defense mechanisms (2, 13). In previous studies we have intervals for a tot?l of 100 min. Chemiluminescence (CL) peaks of shown strain specific differences in opsonic requirements for type 7500 cpm or greater were always associated with phagocytic I1 and I11 group B streptococci (8, 18). Using a chemiluminescence uptake as detmnined by visual mkroscopic examination (61, opsonic assay (6), a radiolabeled bacterial uptake technique (l), which was performed as described below. Phagocytic uptake was and an indirect immunofluorescence method (6), we have system- also assessed in selected sera by the radiolabeled bacterial uptake atically examined a number of sera for opsonic activity to multiple technique described below (1). The correlation coefficient between strains of group B streptococci isolated from infected infants. We CL peaks and ixx~~lated CPm approximates 0-79. It should be found some strains to be highly resistant to opsonization by noted that the level of chemiluminescence, which indicates the antibody containing serum (18). The strains which have this presence of functional opsonic antibodies in these studies, is higher pattern of resistance to opsonization are considered to be resistant than in previously published repofis (6) and reflects a higher strains. In contrast to these isolates, other strains are opsonized by overall background from an older scintillation apparatus. Thus, the majority of antibody containing sera examined and are con- bacterial uptake using the visual and radiolabeled technique was sidered sensitive strains (8, 18). associated with CL peaks of 7500 rather than 6500 cpm. The purpose of this study was to assess the biologic significance ViSUal ~~C~OSCO~~C e~aminati~n of reaction mixtures. After Schof these differences in opsonic requirements using an animal tillation counting, each reaction mixture was centrifuged at 5000 model of group B streptococcal disease. Resistance to opsonization Tm for 5 mh the supernate was removed, and the cell button was found to correlate closely with virulence.