Streptococcus Group B Vaccines

Clinical Description

Invasive GBS disease in neonates and young infants is characterized by age at onset. Early-onset GBS (EOGBS), defined as isolation of GBS from blood, cerebrospinal fluid (CSF) or other sterile sites, occurs before age 7 days (∼90% have signs of illness within 12 hours of birth), is associated with maternal risk factors for neonatal infection and presents as sepsis without a focus (80–95%), pneumonia (10–15%), or meningitis (5–10%). ^, Clinical findings are non-specific, but respiratory distress is prominent. Illness severity ranges from healthy-appearing newborns only evaluated because of maternal risk factors predisposing to GBS sepsis or inadequate intrapartum antibiotic prophylaxis (IAP) before delivery to profound shock and multiorgan failure. Focal findings are uncommon and are limited to pneumonia or, less often, meningitis. Late-onset GBS (LOGBS) typically occurs between 4 and 5 weeks of age (range 7 through 89 days), and more often manifests as fever, poor feeding, and lethargy, and less commonly with shock and multiorgan failure. Although bacteremia without a focus is the most common presentation of LOGBS, shock occurs in up to 25%, and meningitis occurs in 21% to 35% of infants. Other focal manifestations include septic arthritis, osteomyelitis, soft tissue infections, such as cellulitis-adenitis and necrotizing fasciitis, and rarely several others. ^, LOGBS may also occur in infants older than 89 days (typically very preterm infants whose corrected postmentrual age is younger than 3 months). This group of infants is termed late-LOGBS and accounts for approximately 20% of all LOGBS cases. ^, Recurrent GBS is reported in an estimated 0.5–3% of GBS cases, and is likely the result of persistent colonization at mucosal sites. ^, Invasive GBS infection is rare after age 6 months unless accompanied by an underlying medical condition or immunodeficiency.

In pregnant women genital and/or rectal GBS colonization is a prelude to both invasive disease and EOGBS infant disease. The risk of invasion of amniotic fluid or neonatal bloodstream is related directly to the density of maternal colonization and virulence of the organism. Maternal GBS infection manifests during pregnancy as asymptomatic bacteriuria, urinary tract infection, or pyelonephritis and during the peripartum period as bacteremia, intra-amniotic infection, endometritis, or cesarean section wound infection. Maternal infections can result in spontaneous abortion, preterm delivery, and stillbirth. ^, However, two-thirds of GBS disease cases in adults are not associated with pregnancy. GBS invasive infection rates in nonpregnant adults have increased two- to four fold during the last three decades and cause substantial morbidity and mortality. ^, This GBS-associated disease burden is increasing, especially among older persons, Blacks, and those with coexistent medical conditions (for example, obesity, diabetes, and malignancy). Mortality is higher among patients with shock at diagnosis, alcoholism, cancer, and advanced age, with overall case-fatality rates among all clinical forms ranging from 4% to 25%. ^{, , ,} Skin and soft-tissue infections are the most common clinical manifestation of invasive GBS in adults, with diabetes as the most frequent underlying medical condition. Other clinical presentations including bacteremia without a focus, urinary tract infection, pneumonia, bone or joint infection, endocarditis, meningitis, peritonitis, and catheter-associated infections are described. ^{, , ,}

Complications and Sequelae

GBS bacteremia may result in disseminated disease and involvement of distal sites of infection. EOGBS and LOGBS meningitis may be complicated by the development of seizures, cerebritis, small vessel cerebral thrombosis, ventriculitis or ventricular obstruction, and subdural empyema often requiring prolongation of therapy. Approximately 25% of infants with meningitis die or have obvious neurological deficits at the time of hospital discharge, and long-term 30–40% of cases have permanent neurological sequelae (e.g., vision or hearing loss, cerebral palsy, developmental delay, learning disability). A nationwide matched cohort study in Denmark and Netherlands over a 20-year period (1997–2017) demonstrated that any invasive GBS infection, not only meningitis, was associated with increased use of healthcare resources (outpatient and inpatient) and twice the risk of moderate or severe neurodevelopmental impairment in affected children. However, most infants with bacteremia without a focus or focal infections, such as cellulitis, septic arthritis, or osteomyelitis, typically recover without sequelae following treatment with appropriate antimicrobials. Recurrent infections are uncommon (less than 5% of infants), but the subsequent episode can be more severe than the first. The most likely explanation for recurrence is ongoing immune status susceptibility combined with persistent mucosal GBS colonization even after a course of parenteral antibiotics. For recurrent LOGBS disease, maternal breast milk as a source has been mentioned, but there is no convincing evidence for this conjecture. The most frequent complication among nonpregnant adults is recurrent infection, estimated to occur in 5.7% of patients, and most often among those with skin and soft tissue infections. ^,

Bacteriology

Streptococcus agalactiae, the species designation for GBS, is a facultative aerobic Gram-positive diplococcus with an ultrastructure similar to other gram-positive cocci. Historically GBS was identified by its narrow zone of β-hemolysis surrounding 3–4 mm, gray-white, mucoid colonies on blood agar plates. Occasional strains (1–4%) are α-hemolytic or nonhemolytic. GBS are readily cultivated in various bacteriologic media, but isolation from respiratory, genital, and gastrointestinal tracts is enhanced by use of antibiotic-containing enrichment broth. A variety of methods can be used for presumptive identification, but definitive identification requires detection of the group B cell wall antigen typically through serologic testing. GBS are divided into 10 serotypes (Ia, Ib, and II through IX) based on their capsular polysaccharides (CPSs). Through whole-genome sequencing, it is clear that virtually all GBS strains contain a capsule gene, and that serologically nontypeable (NT) strains have poor or no CPS expression. These NT strains can be assigned a likely CPS type through multilocus sequence typing or whole-genome sequencing. GBS also has numerous surface proteins (e.g., alp-like proteins and Rib in addition to pilus islands 1, 2a, and 2b) that facilitate further characterization for epidemiologic and virulence studies (described in the subsequent sections). ^{, , ,}

Modeling Pathogenesis Using Animals

Animal models have been developed that provide insights into the pathogenesis, pathophysiology, and disease progression of GBS. These have been used to generate data that helped inform medical care of the process of ascending GBS infection resulting from mucosal colonization, in addition to being deployed to test therapeutic and prophylactic approaches to treat and prevent GBS disease. As such, they have provided an indication as to whether different GBS CPS types, characterized by ST or more recently genetic lineage, are more or less invasive and provided information on the progression of neonatal disease through carriage by the mother followed by ascending infection that reaches the fetus as well as the role of GBS in preterm and stillbirth. ^, A broad range of species have been deployed for this purpose, ranging from higher ones such as nonhuman primates (NHPs), ^, rabbits, rats, mice, ^, and chickens to zebrafish and the wax moth Galleria mellonella . ^, NHPs are considered the closest to humans in pregnancy, including anatomy and placentation, onset of labor, sensitivity to pathogens, and the developmental timeline of the fetal lung and brain. ^, Current NHP models have used catheterization to inoculate bacteria in the choriodecidual space, a site in the uterine mucosa of pregnant NHP between the uterine muscle and placental membranes, where bacteria are thought to first encounter the maternal-fetal interface during ascending infection. ^, The rabbit, rat, mouse, and chicken models that have been developed to study GBS infection typically targeting steps in the pathogenesis, such as organ colonization, septicemia, and meningitis. ^{, , ,} Although these models have generated valuable information about GBS pathogenesis, they utilized atypical routes of infection such as intravenous, intraperitoneal, transurethral, and gavage. ^{, ,} This is being addressed in murine models by the development of ascending mouse models of infection that recapitulates vertical transmission between mother and pup using intravaginal infections. ^, GBS is also a pathogen of fish and impacts the large global aquiculture business, and some isolates have been linked to cases of invasive GBS disease in adults. Zebrafish have been used as a model for exploring molecular pathogenesis due to their susceptibility to GBS, ease of genetic manipulation, and transparency during the first week of life. The pathology of GBS disease in fish and mammals share common features including septicemia, meningoencephalitis, myocarditis, and choroiditis. ^, Likewise, the innate immune systems across zebrafish and mammals are similar. G. mellonella are also susceptible to the pathogen, and as they are relatively easy and economical to keep, have been used to study the virulence of GBS strains from GBS mutation libraries to identify key pathogenicity factors and for testing novel antimicrobial compounds. ^,

Human Pathogenesis

GBS can cross the epithelial cell barrier by a paracellular route, and translocation of epithelial cell barriers is most likely the mechanism for dissemination of this organism in humans. The first step in the sequence of invasive GBS disease, however, is attachment of the organism at mucosal surfaces with consequent breach of these surfaces to enter the bloodstream followed by possible dissemination to other sterile sites. GBS can adhere to many human cells, including intestinal, genital and respiratory epithelium, placental membranes, and endothelial cells in the blood-brain barrier. Virulence factors promoting adherence are surface protein members of the Alp family, including α-C, Alp1 (Epsilon), Alp2, Alp3 (R28), Alp4 and Rib, pilus-like structures, and the fibrinogen receptor, FbsA. The six α-C and Alp-like proteins are long, tandem repeat proteins that are common to many GBS capsular types causing invasive disease. Pilus-like proteins are associated with cell-wall-anchored proteins, and these three highly conserved structures have been characterized as pilus islands (PI) 1, 2a, and 2b and have a role not only in adherence but also in invasion. Pili can locate in the intercellular space; deletion of pilus backbone protein reduces capacity to transcytose through epithelial cells. ^{, ,} Experimental models of infection have demonstrated that antibodies to these Alps and pili surface structures are protective. The surface protein β-C binds to complement factor H and the complexed factor H then downregulates complement activation. Additionally, BibA a cell-wall–anchored GBS immunogenic bacterial adhesin that is well-conserved and present in all GBS strains, binds human C4 binding protein, and promotes the adhesion of GBS to human epithelial cells. ^, Another adhesin, HvgA, is exclusively associated with a single high-virulence GBS clonal complex, ST-17, which is predominant in infant invasive disease, especially meningitis. Analysis of GBS isolates demonstrated that the serotype III ST-17 clone is associated with >80% of cases of neonatal meningitis. GBS strains that express HvgA adhered more efficiently to intestinal epithelial cells, choroid plexus epithelial cells, and microvascular endothelial cells that constitute the blood–brain barrier. β-hemolysin/cytolysin lyse epithelial and endothelial cells, leading to direct tissue damage to and spread through host tissues. β-hemolysin/cytolysin promotes cell invasion and injury by inducing cytolysis and apoptosis. One example of this is the cardiac myocyte. Another is disruption of placental membranes, allowing for invasion of amniotic fluid. Also, membrane vesicles from hyperhemolytic GBS strains induce greater cell death of neutrophils, T cells and B cells than membrane vesicles from isogenic strains. Immune clearance of GBS is resisted by CPSs and other virulence factor–mediated effects at the cellular and molecular levels, leading to inhibition of neutrophil recruitment, inhibition of opsonophagocytosis and impairment of phagocytosis, and oxidative burst killing. Finally, cell wall lipoteichoic acid, cell wall peptidoglycan, and β-hemolysin/cytolysin trigger cytokine activation and release. ^,

GBS-derived enzymes also play a role in the bacterial pathogenesis. C5a peptidase is a conserved cell-surface–localized serine protease found in most GBS strains, that inactivates human C5a, a neutrophil chemoattractant produced during complement activation, and prevents neutrophil accumulation at the site of infection. An extracellular GBS product, hyaluronate lyase, originally thought to be a neuraminidase, facilitates the bacterial invasion via degrading host hyaluronate, ^, thereby evading host immunity.

CPS allows GBS to evade innate immune mechanisms (complement deposition and phagocytosis), thereby facilitating damage to tissues, with terminal sialic acid components decorating each CPS repeating backbone playing a key role. ^, Tissue damage induces the release of inflammatory modulators with further damage on organ systems. Transposon-induced mutants of serotype III decrease GBS virulence in animal models of lethal infections. Maternally derived, naturally acquired antibodies to the CPS of the colonizing or invading GBS strain, first identified by Baker and Kasper in 1976, are a crucial component of neonatal and infant host defense. Since only immunoglobulin class G (IgG) antibodies pass through the placental circulation, this protection must be derived from the mother, and in many circumstances this does not occur. For adult invasive disease the role of antibodies to CPS or GBS surface proteins is incompletely elucidated. ^,

Diagnosis and Treatment

Invasive GBS infection is diagnosed by isolation of the organism from a normally sterile site. For early-onset infection, the typical source of GBS isolation is blood, but CSF also should be sampled given that 5–10% of cases have concurrent meningitis. Clinically apparent focal sites of dissemination (e.g., CSF, soft tissue, bone, joint, urine, pleural or peritoneal fluid, intravascular device) that are often found in adults and sometimes infants with late-onset disease, also should be sampled. Recovery of the organism from surface sites (e.g., external auditory canal, umbilicus), mucous membranes, urine, in the absence of clinical signs of infection, and placenta is of no diagnostic significance.

Penicillin is the drug of choice for treatment of systemic infections. GBS isolates with point mutations in penicillin-binding proteins and reduced β-lactam susceptibility have been reported from the United States (U.S.) and Japan, but the clinical relevance of these in vitro data, if any, is unclear. GBS is susceptible to other β-lactam antibiotics including all cephalosporins and to vancomycin. Resistance to erythromycin and clindamycin is reported in approximately 55% and 44% of isolates, respectively, although higher rates of clindamycin resistance in serotype IV (up to 80%) and V have been described. ^, This prevalence of resistance of GBS to erythromycin and clindamycin precludes their use for treatment of clinical infections. While typically resistant to aminoglycosides, synergistic killing of GBS occurs in vitro and in vivo when gentamicin is combined with penicillin or ampicillin. Treatment duration depends on the severity of illness and site involved, but a minimum of 10 and 14 days is recommended for bacteremia and uncomplicated meningitis, respectively, and longer for more severe infections (e.g., endocarditis). ^{, ,} Although shorter intravenous courses have been reported, prospective clinical studies are lacking and are not recommended. Also, oral antibiotic therapy has not been demonstrated to be effective for use as step down therapy for neonates and young infant. Despite advances in intensive care and prompt initiation of antimicrobials, GBS infant disease overall mortality (5%) and morbidity remains substantial, ^, and mortality is even higher in adults.