Streptococcus agalactiae (Group B Streptococci)

Classification and Morphologic Characteristics

S. agalactiae is the species designation for streptococci belonging to Lancefield group B. The serologic differentiation of β-hemolytic streptococci by groups is based on capillary precipitin reactions between the group-specific carbohydrate cell wall antigen and hyperimmune rabbit antisera. GBS are facultative, gram-positive diplococci that grow on a variety of culture media. Isolated colonies on sheep blood agar are 3 to 4 mm in diameter and grayish white. The flat, somewhat mucoid colonies are surrounded by a narrow zone of β-hemolysis. One percent to 2% of strains are nonhemolytic. Selective media have been used to enhance the accurate detection of low numbers of GBS from sites such as the genital or gastrointestinal (GI) tract. These usually contain Todd-Hewitt broth with or without sheep red blood cells and antimicrobial agents, such as nalidixic acid and gentamicin or colistin, or chromogenic agar.

Identification

Definitive identification of GBS is based on detection of the group B–specific cell wall antigen common to all strains. A number of serologic methods using hyperimmune group B–specific antisera are used for detection of the group B antigen. Latex agglutination is the most widely used. When the manufacturer's instructions are followed, these products are comparable to the Lancefield capillary precipitin method.

Biochemical methods that permit the presumptive identification of GBS include resistance to bacitracin or trimethoprim-sulfamethoxazole, positive sodium hippurate hydrolysis, and the production of an orange pigment during anaerobic growth on certain media. Production of CAMP (Christie, Atkinson, Munch, Peterson) factor, a thermostable extracellular protein that results in synergistic hemolysis on sheep blood agar with the β-lysin of Staphylococcus aureus, is observed in 98% to 100% of GBS. The combination of the CAMP test with bacitracin susceptibility testing and the bile esculin reaction is adequate for presumptive differentiation of GBS from other serogroups of β-hemolytic streptococci.

Serologic Typing

GBS are differentiated serologically by capsular polysaccharide type and by cell surface–expressed proteins. Lancefield defined two cell wall carbohydrate antigens, the group B–specific or C substance, common to all strains, and the S substance, which allowed classification into types I, II, and III. Later, she reported differences in type I strains and designated these Ia and Ib. These strains possessed capsular polysaccharide Ia or Ib, and some strains also had a surface protein designated C protein, subsequently found to have α and β components. The nomenclature was revised in 1984 to designate the capsular polysaccharides as type antigens, with surface proteins as additional markers. Additional capsular types, IV through IX, have been defined, and additional candidates are being evaluated.

The α C protein is the prototype for a family of proteins (α-like proteins) that includes Rib and Alp1 to Alp4 and is found in most GBS. R proteins (R1 to R4) are found on some strains. Each of the common polysaccharides has a characteristic protein expression pattern, but exceptions occur. For example, Alp1 is found frequently in type Ia strains, Rib is most often associated with type II and III strains, and Alp3 is associated with type V strains. The β C protein binds to human immunoglobulin A (IgA) and is found mainly in type Ib strains.

Antibodies directed against polysaccharide antigens were shown by Lancefield to provide passive protection in mice challenged with homologous, but not heterologous, antigen-containing strains. The α and β C proteins and Rib also elicit protective antibodies in animals, but their role in human infections is not known. Antibodies directed at the group B antigen are not protective.

GBS consist of a large and diverse core population from which recombinant clones with limited diversity have emerged with properties that have allowed them to successfully disseminate and adapt to a special habitat, such as that of the human neonate. The classic method for capsular typing of GBS is by capillary precipitins or immunodiffusion in agarose between acid extracts of the organism and hyperimmune rabbit antisera. Molecular assays for capsule type identification include multilocus sequencing typing, pulsed gel electrophoresis, and polymerase chain reaction (PCR). PCR-based and whole-genome sequencing methods offer powerful tools to document the epidemiologic relatedness of strains and to distinguish among the known types. Such methods can determine whether recurrent infections are caused by separate or identical strains and can identify virulent clone families that may be disproportionate causes of invasive disease. Molecular methods do not permit an assessment of polysaccharide or protein expression. However, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry has allowed rapid detection of proteins specific for highly virulent strains.

Asymptomatic Colonization

GBS colonize the genital area or lower GI tract of women at rates ranging from 10% to 40%; in men, GI tract carriage is estimated to be 10% to 25%. Variations in prevalence of asymptomatic colonization relate to sites sampled, methods of detection, and demographic differences in populations ( Table 201.1 ). When multiple appropriate sites, such as the lower vagina or the periurethral area and the rectum, are sampled and when the selective broth media are used, colonization rates typically exceed 20%. The association of GBS with asymptomatic bacteriuria during pregnancy is a marker for a high density of organisms or heavy genital colonization.

Colonization with GBS occurs more frequently among black women than in other racial or ethnic groups. Diabetes mellitus also is independently associated with higher rates of group B streptococcal colonization during pregnancy. Multiple partners and frequent or recent sexual intercourse are associated with increased risk for vaginal acquisition over time, possibly because sexual activity alters the vaginal microenvironment to make it more permissive for colonization. Sexual activity and particularly male-to-female oral sex increase the risk for cocolonization with identical group B streptococcal strains in heterosexual college couples. Sexually inexperienced persons have low genital colonization rates. Pregnancy does not influence colonization prevalence.

The principal reservoir for GBS is the lower GI tract. The presence of GBS in the GI tract is a risk factor for vaginal colonization. Changes in capsule expression and recolonization with antigenically distinct group B streptococcal clones can occur over time, and specific sequence types associated with loss or persistence of colonization have been defined. The prevalence of oropharyngeal colonization in adults is about 5% but can approach 20% in men who have sex with men. The colonization rate in healthy older adults is about 20%.

Transmission

Maternal-to-infant transmission occurs in utero by the ascending route or at the time of delivery. Vaginally colonized pregnant women are at an increased risk for premature labor. The rate of vertical transmission among neonates born to women colonized with GBS who do not receive intrapartum antibiotic prophylaxis before delivery is about 50%. A high maternal genital inoculum at delivery significantly increases the likelihood of vertical transmission. Transmission also can occur within the hospital setting, albeit uncommonly, likely through hand-to-hand contact, and in closed populations such as nursing homes.

Incidence and Serotype Distribution of Isolates

The mean incidence of invasive group B streptococcal disease globally in infants younger than 3 months is estimated to be approximately 0.5 cases per 1000 live births. There is marked variation by geographic region, with higher rates in Africa, followed by the Americas and Europe, and the lowest rates in Southeast Asia. The incidence of early-onset (0 through age 6 days) neonatal group B streptococcal infection in the United States, formerly 1 to 3 per 1000 live births, declined by 2016 to 0.21 per 1000 live births in association with widespread use of antenatal culture screening and intrapartum antibiotic prophylaxis for colonized women. Use of this prevention strategy has had no impact on late-onset (age 7 through 89 days) disease incidence, which still occurs at a rate of approximately 0.3 per 1000 live births.

The incidence of invasive disease in pregnancy declined significantly in association with the use of intrapartum antibiotic prophylaxis, but pregnancy-associated group B streptococcal disease is responsible for peripartum febrile morbidity. Intrapartum vaginal colonization with GBS also is an independent risk factor for intraamniotic infection. Women with “heavy” (≥50 colonies on the primary blood agar plate) or high-inoculum GBS colonization are at significantly greater risk for intraamniotic fluid infection than those with “light” (1–10 colonies) vaginal colonization. Among women with invasive group B streptococcal disease for whom pregnancy outcome was known, 61% had a spontaneous abortion or a stillborn infant, 30% had infants without apparent illness, 5% had live-born infants who developed clinical infections, and 4% had induced abortions.

Invasive group B streptococcal disease among nonpregnant adults has been increasing for decades. Adults now account for in excess of 90% of all invasive group B streptococcal disease cases in the United States. Most adults have at least one predisposing medical condition, such as diabetes mellitus, chronic liver or renal disease, human immunodeficiency virus infection, malignancy, or stroke. The incidence of adult disease increased from 3.6 cases per 100,000 population in 1990 to 10.9 cases per 100,000 population in 2016. ^, An estimated 27,700 cases of invasive disease and 1500 deaths occurred in the US population in 2016. A multinational population-based assessment found that bloodstream infection rates increased among older adults across all study regions, especially in association with diabetes. In one population-based surveillance report, adults 65 years of age and older represented one-third of cases of invasive group B streptococcal disease. Nursing home residents have a markedly higher incidence (72 per 100,000 population) than community residents 65 years of age and older.

Group B streptococcal types Ia, III, and V represent more than two-thirds of colonizing isolates in pregnancy and the same proportion of isolates in early-onset neonatal disease. Late-onset infant disease is caused predominantly by type III strains. Among nonpregnant adults, type Ia strains account for 23% of almost 2000 cases of invasive disease in 2016, followed by types II and V (18% each), Ib (15%), III (12%) and IV (11%). Serotypes Ia, Ib, II, III, and V accounted for 86% and 94%, respectively, of invasive strains in nonpregnant adults in contemporary surveillance studies in the United States and France. In Brazil, a surveillance study confirmed the high prevalence of the same serotypes but showed regional variability in the distribution of serotypes. Types IV, VI through VIII, and nontypable isolates are uncommonly associated with invasive infection, but in Japan serotypes VI and VIII are frequently isolated from pregnant women. Type IV strains are increasing as a cause of invasive disease in adults globally