Type-specific capsular antigen is associated with virulence in late-onset group B Streptococcal type III disease

Serotypes and clinical manifestations of invasive group B streptococcal infections in western Sweden 1998-2001

This study monitored the serotypes of Streptococcus agalactiae (group B streptococcus; GBS) isolated from invasive infections in western Sweden and investigated possible relationships between serotype, age and clinical manifestations. Invasive GBS isolates were collected prospectively during 1998-2001 at six laboratories, covering two counties with a population of 1.8 million, and were serotyped by coagglutination. Clinical data were obtained from hospital notes. In total, 161 invasive strains (50 from neonates and infants aged < 3 months, and 111 from adults) were serotyped. The commonest serotypes from neonates and infants were serotypes III (60%), V (22%) and Ia (10%), and from adults were serotypes V (42%) and III (25%). Serotype V had doubled in frequency among both children and adults compared to a previous study from the same area in 1988-1997. Most (80%) of the adults had an underlying medical condition. No relationship was found between serotype and clinical manifestations. However, the study demonstrated the importance of active surveillance of GBS serotypes and the difficulties of formulating a multivalent polysaccharide conjugate vaccine against GBS.

Streptococcus suis and group B Streptococcus differ in their interactions with murine macrophages

Streptococcus suis type 2 and group B Streptococcus type III (GBS) are important encapsulated bacterial species causing meningitis. In the present study we compared quantitatively the uptake and intracellular survival of S. suis type 2 and GBS type III with murine macrophages in non-opsonic conditions. The role of the capsule of both pathogens was also studied using previously obtained unencapsulated isogenic mutants. Encapsulated S. suis wild-type strain was practically not phagocytosed, while the unencapsulated mutant was easily ingested by macrophages. On the other hand, the well encapsulated GBS strain and its unencapsulated mutant were both phagocytosed in large numbers. Even if S. suis unencapsulated mutant showed a higher uptake rate than the parental strain, this value was always markedly lower than the numbers of ingested GBS strains. In addition, the intracellular survival of encapsulated and unencapsulated GBS strains was significantly higher than that of S. suis strains. These results suggest that interactions between GBS type III and S. suis type 2 with murine macrophages as well as the role of the capsule as an antiphagocytic factor are different for the two bacterial pathogens.

Identification of cpsD, a gene essential for type III capsule expression in group B streptococci

We showed previously that a mutant strain of group B Streptococcus (GBS) defective in capsule production was avirulent. This study describes the derivation of an unencapsulated mutant from a highly encapsulated wild-type strain of type III GBS, COH1, by transposon mutagenesis with Tn916 delta E. The mutant, COH1-13, was sensitive to phagocytic killing by human leukocytes in vitro and was relatively avirulent in a neonatal rat sepsis model compared with the wild-type strain. No capsular polysaccharide was evident in the cytoplasm or on the cell surface of the mutant strain. The Tn916 delta E insertion site in COH1-13 was mapped to the same chromosomal location as the Tn916 insertion site in the unencapsulated type III mutant COH31-15 reported previously. Nucleotide sequencing of DNA flanking the insertion site in COH1-13 revealed an open reading frame, designated cpsD, with significant homology to the rfbP gene of Salmonella typhimurium. RfbP encodes a galactosyl transferase enzyme that catalyses the transfer of galactose to undecaprenol phosphate, the initial step in O-polysaccharide synthesis. A particulate fraction of a lysate of wild-type strain GBS COH1 mediated the transfer of galactose from UDP-galactose to an endogenous acceptor. The galactose-acceptor complex partitioned into organic solvents, suggesting it is lipid in nature or membrane-associated. Galactosyl transferase activity was significantly reduced in the unencapsulated mutant strain COH1-13. These results, together with the similarity in deduced amino acid sequence between cpsD and rfbP suggest that cpsD encodes a galactosyl transferase essential for assembly of the GBS type III capsular polysaccharide.

Identification of a genetic locus essential for capsule sialylation in type III group B streptococci

The type III capsular polysaccharide of group B streptococci (GBS) consists of a linear backbone with short side chains ending in residues of N-acetylneuraminic acid, or sialic acid. The presence of sialic acid on the surface of the organism inhibits activation of the alternative pathway of complement and is thought to be an important element in the virulence function of the capsule. We showed previously that a mutant strain of GBS that expressed a sialic acid-deficient, or asialo, form of the type III polysaccharide was avirulent, supporting a virulence function for capsular sialic acid. We now report the derivation of an asialo capsule mutant from a highly encapsulated wild-type strain of type III GBS, strain COH1, by insertional mutagenesis with transposon Tn916 delta E. In contrast to the wild-type strain, the asialo mutant strain COH1-11 was sensitive to phagocytic killing by human leukocytes in vitro and was relatively avirulent in a neonatal rat model of GBS infection. The asialo mutant accumulated free intracellular sialic acid, suggesting a defect subsequent to sialic acid synthesis in the biosynthetic pathway leading to capsule sialylation. The specific biosynthetic defect in mutant strain COH1-11 was found to be in the activation of free sialic acid to CMP-sialic acid: CMP-sialic acid synthetase activity was present in the wild-type strain COH1 but was not detected in the asialo mutant strain COH1-11. One of the two transposon insertions in the asialo mutant COH1-11 mapped to the same chromosomal location as one of the two Tn916 insertions in the previously reported asialo mutant COH31-21, identifying this site as a genetic locus necessary for expression of CMP-sialic acid synthetase activity. These studies demonstrate that the enzymatic synthesis of CMP-sialic acid by GBS is an essential step in sialylation of the type III capsular polysaccharide.

Phenotypic diversity in the alpha C protein of group B streptococci

Group B streptococci (GBS) is the leading cause of neonatal sepsis and meningitis. C proteins are an immunologically important group of surface-associated antigens in GBS that remain incompletely characterized. Two C proteins have been designated alpha and beta on the basis of protease susceptibility. We recently used a monoclonal antibody to describe a protective epitope of the GBS alpha (or trypsin-resistant) C protein in the prototype Ia/c GBS strain. In the present study, we examined 51 GBS isolates for expression of C-protein alpha and beta antigens. The alpha antigen, as detected with monoclonal antibody in sodium dodecyl sulfate (SDS) extracts, appears as a heterogeneous series of proteins spaced 8 kDa apart on SDS-polyacrylamide gel electrophoresis, but has a maximum molecular mass that varies among strains from 62.5 to 167 kDa. By immunoblotting with human immunoglobulin A, polyclonal antiserum, or monoclonal antibody, the beta antigen, in contrast, appears as a single protein of molecular mass between 124 and 134 kDa. The amount of alpha antigen expressed by each strain was quantified by enzyme immunoassay inhibition and was found to vary markedly from strain to strain. The susceptibility of strains of GBS to opsonization and killing by human polymorphonuclear leukocytes in the presence of either complement alone or complement with alpha-specific monoclonal antibody was examined. Strains expressing the alpha antigen were less readily killed in the absence of specific antibody than were alpha-negative strains. Killing in the presence of alpha-specific monoclonal antibody was found to correlate directly with the maximum molecular mass of the alpha antigen and with the quantity of antigen on the bacterial cell surface. Isolates of GBS that express the alpha C protein vary widely in the quantity and molecular mass of the alpha antigen produced, and this heterogeneity appears to have biologic importance.

Phenotypic evaluation of acapsular transposon mutants of Vibrio vulnificus

Translucent, avirulent spontaneous phase variants of Vibrio vulnificus MO6-24 reverted back to the original opaque, encapsulated phenotype under both in vivo and in vitro conditions. Two translucent, acapsular mutants, which did not show phase variation, were constructed by using the transposon Tn5 IS50L::phoA (TnphoA). Loss of capsule was accompanied by decreases in virulence, hydrophilicity, and serum resistance. The ability to utilize transferrin-bound iron for growth was lost in only one of the two unencapsulated mutants. Our data emphasize the apparent importance of capsule in the virulence of V. vulnificus and indicate that utilization of transferrin-bound iron is independent of encapsulation.

Properties of high and low density subpopulations of group B streptococci: enhanced virulence of the low density variant

From the group B streptococcus (GBS) reference strain 090 la Colindale two subpopulations, which differed markedly regarding their capacities for biosynthesis of type-specific polysaccharide, were obtained by separation on a hypotonic Percoll density gradient. In the original strain and the high and low density variants, there was a negative correlation between buoyant density and bio-synthesis of type-specific polysaccharide as determined by ultrastructure and quantitative assays. The invasiveness of these variants was investigated by infecting rabbits via subcutaneously implanted tissue cages. In the animals infected with highly encapsulated bacteria, heavy bacteremia was detected 8 h post-infection, whereas in the animals which received high density bacteria with small amount of capsule, heavy bacteremia was not detected until after five days. All isolates recovered from the blood or organs of these rabbits were of the capsule rich phenotype, indicating a phenotypic shift in the subpopulation of high density bacteria. An apparently similar phenotypic shift was noted in an isolate from a baby with early onset septicemia. There was a dominance of low density bacteria in the isolate obtained from the baby as compared with the colonizing population of bacteria isolated from the cervix of the mother. From these type III isolates, subpopulations with different density maxima were obtained. A reversed shifting towards dominance of less encapsulated, high density bacteria was observed during in vitro passage of these subpopulations.

Effect of differences in antibody and complement requirements on phagocytic uptake and intracellular killing of "c" protein-positive and -negative strains of type II group B streptococci

The influence of antibody and complement on the polymorphonuclear leukocytic uptake and killing of type II group B streptococci (GBS) was examined with 11 adult sera and three type II strains possessing the trypsin-resistant and trypsin-sensitive components (II/TR+TS) of the "c" (formerly Ibc) protein or two type II strains lacking both components (II/no c) of the c protein. All tested sera mediated a greater than 1 log10 reduction in colony-forming units (CFUR) of a type II/no c strain, even in the absence of measurable type-specific antibody (less than 1.08 micrograms/ml), but only 5 of 11 mediated a greater than 1 log10 CFUR of any type II/TR/TS strain, even in the presence of moderate levels of type-specific antibody. The classical pathway of complement activation appeared to be more important than the alternative pathway, and even absorbed or immunoglobulin G (IgG)-depleted serum (IgG, 10 mg/dl) mediated a greater than 1 log10 CFUR without magnesium ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (magnesium EGTA) chelation. Chelation with magnesium EGTA reduced the CFUR in 4 of 11 test sera and greatly reduced the CFUR in absorbed or IgG-depleted sera for type II/no c strains. Despite variation in the phagocytic killing of two representative strains of type II GBS, both strains were well phagocytized, as measured by radiolabeled bacterial uptake or electron microscopy. This study suggested that poorly killed type II/TR+TS GBS were easily phagocytized but apparently resisted intracellular killing.

Identification of a Streptococcus agalactiae protein antigen associated with bovine mastitis isolates

Immunoblotting was used to analyze the immune response of cows to Streptococcus agalactiae. Antibody from the milk of cows immunized (via the superficial inguinal lymph node) with formalinized S. agalactiae cells or from the milk of cows with S. agalactiae mastitis reacted strongly with a group of high-molecular-weight proteinaceous antigens. The two most predominant antigenic polypeptides in this group had apparent molecular weights of 97,000 and 104,000. Because the data indicated that these two antigens, as well as several minor antigens sometimes observed in the 70- to 100-kilodalton size range, seemed to be different forms of the same protein, we refer to the entire group as Sas97/104. A monoclonal antibody that was reactive with Sas97/104 was derived and was used to purify the antigen by affinity chromatography. Whole-cell and colony blot enzyme-linked immunoassays with either the monoclonal antibody or a polyclonal serum sample raised against the affinity-purified antigen indicated that this antigen (or cross-reactive proteins with higher molecular weights) is present on the S. agalactiae strains that were freshly isolated from mastitic cows. However, the antigen was not detected in S. agalactiae of human origin, bovine strains of S. agalactiae maintained for a prolonged period in the laboratory, or other streptococci. The data are consistent with the notion that Sas97/104 is a surface antigen and does not correspond to previously described type-specific antigens of group B streptococci.

Density profile of group B streptococci, type III, and its possible relation to enhanced virulence

The buoyant densities of virulent and colonizing group B streptococci, type III, were determined by centrifugation of bacteria on a linear, hypotonic density gradient. A total of 28 strains were investigated. Eleven strains were obtained from blood cultures of babies with early-onset disease, and eight strains were isolated from the cerebrospinal fluid of babies with late-onset septicemia and meningitis. Nine colonizing strains were genital isolates from pregnant women subsequently giving birth to healthy children. In each strain the buoyant density was determined before and after neuraminidase treatment. All strains showed an increase in the buoyant density after enzymatic removal of sialic acid, and the density differences before and after desialylation were calculated. The mean values of these differences for blood, cerebrospinal fluid, and colonizing isolates were 23.4, 25.3, and 10.6 mg/ml, respectively. The mean value for the colonizing strains differed significantly from the mean value for each group of virulent strains. All colonizing strains banded singly in the gradient, whereas five of the virulent strains divided into two density populations. Extracts of the low-density cells produced markedly more dense immunoprecipitates with type antiserum than did extracts of the high-density bacteria. One double-banding strain was positive for R protein. After separation of the two density populations, this antigen was detected only in the low-density population. The results indicate that bacterial buoyant density is inversely related to the amount of capsular polysaccharide enveloping the cell and that a determination of the density profile of the bacteria may be used for discriminating strains with an increased pathogenic potential.

Influence of polysaccaride capsule and ionic strength on buoyant density of group B streptococci

Density fluctuation during the cell cycle was investigated with group B streptococcus (GBS), type Ia, in isotonic medium. Maximum density occurred at late lag-phase, with a minimum at the middle of logarithmic growth. During the stationary phase the density remained stable. The buoyant density of serotypes Ia, Ib, II, III and a rough variant was determined by means of gradient centrifugation. In an isotonic milieu at the early stationary phase, the density of the strains varied in the range between 1.06 and 1.11 g/ml. Low ionic strength reduced the density of all strains. Neuraminidase-treatment increased the buoyant density of types Ia, Ib and III. The same pattern was seen in isotonic and hypotonic medium, although low ionic strength markedly augmented these differences. The influence of tonicity and neuraminidase-treatment was most prominent with types Ia and III, which may be due to relatively thicker polysaccaride capsules in these types.

The type-specific polysaccharide and the R protein antigens of the L-phase from a group B, type III Streptococcus

The type-specific polysaccharide and the R protein antigens from filtered culture supernatants of the bacterial phase and L-phase of the group B, type III streptococcal strain 76-043 were studied by several immunological methods. In the L-phase of growth, the two antigens were separate and distinct molecules which were found principally in the culture supernatant even on the 254th serial subculture in the cell-wall-defective state. Only trace amounts of these antigens were detected in extracts of L-phase cells. The type III polysaccharide antigens in the supernatant of cultures of the parent bacterium and the L-phase gave reactions of identity in immunodiffusion. Precipitin bands obtained by immunoelectrophoresis (IEP) revealed that the type-specific antigen of the bacterial phase of growth migrated toward the anode, whereas that of the L-phase remained near the antigen well. The R protein antigen in the L-phase supernatant was immunologically identical to the R protein of the supernatant and 1% trypsin-extracted antigens from whole cells of the parent bacterial strain, and other groups A, B and C streptococcal strains sharing a common R antigen. Immunologically, the R antigen appeared to be the species R4. The R protein of the L-phase and bacterial phase cultures was resistant to 5% trypsin but sensitive to 0.5% pepsin at 37 degrees C/2hr. Antiserum prepared in rabbits against L-phase cells contained an antibody reactive with the R protein antigens of the bacterial and L-phase cultures. The soluble, naturally released type III and R protein streptococcal antigens of the L-phase of growth permitted immunological confirmation of its bacterial origin.

Influence of nutrient limitation and low pH on serogroup B Neisseria meningitidis capsular polysaccharide levels: correlation with virulence for mice

Neisseria meningitidis strain M986, which possesses a polyanionic sialic acid capsular polysaccharide, was resistant to the bactericidal effects of normal rabbit serum, but sensitive when immune serum and complement were present. An isogenic strain PRM102, deficient in the ability to produce capsular polysaccharide, was sensitive to normal serum. Strain M986, when grown under conditions of low pH or nutrient limitation, synthesized increased levels of capsular polysaccharide. This was accompanied by an increase in cell surface hydrophilicity and virulence for mice. Cells grown in low-pH, iron-limited medium synthesized the highest concentration of polysaccharide and exhibited the highest cell surface hydrophilicity and virulence among the cases examined. The increase in capsular polysaccharide was partly explained by a decrease in the specific activity of a membrane-bound cytidine monophosphate-N acetylneuraminic acid hydrolase. The results suggest that conditions of nutrient limitation and low pH exert profound effects on the physicochemical nature of the meningococcal cell surface which, in turn, cumulate in enhanced virulence of this organism for mice.