Group B streptococcal infection

Group B streptococcal infection, also known as Group B streptococcal disease or just Group B strep, is the infection caused by the bacterium Streptococcus agalactiae (S. agalactiae).

It known as group B streptococcus or GBS. 

GBS infection can cause serious illness and sometimes death, especially in newborns, the elderly, and people with compromised immune systems.

GBS harbours an important number of virulence factors: capsular polysaccharide, rich in sialic acid, and a pore-forming toxin, β-haemolysin.

The GBS capsule is the key virulence factor because it helps GBS escape from the host defense mechanisms interfering with phagocytic killing of GBS by human phagocytes.

GBS is usually a harmless commensal bacterium being part of the human microbiota colonizing the gastrointestinal and genitourinary tracts of up to 30% of healthy human adults.

S. agalactiae is a Gram-positive coccus with a tendency to form chains, beta-hemolytic, catalase-negative, and facultative anaerobe. 

GBS grows readily on blood agar plates, surrounded by a narrow zone of β-hemolysis. 

GBS’s cell wall of the group B antigen of the Lancefield classification that can be detected directly in intact bacteria using latex agglutination tests.

The CAMP test and its ability to hydrolyse hippurate, are test that  can be used to identify GBS.

Hemolytic GBS strains produce an orange-brick-red pigment when cultivated on medium.

Identification of GBS could also be carried out by laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry and Nucleic Acid Amplification Techniques.

GBS is found in the gastrointestinal and genitourinary tract of humans and is normal component of the intestinal and vaginal microbiota in some women.

GBS vaginal colonization rate ranges from 4 to 36%, with most studies reporting rates over 20%.

Vaginal or rectal colonization may be intermittent, transitory, or persistent.

GBS is an asymptomatic and harmless colonizer of the gastrointestinal human tract in up to 30% of otherwise healthy adults, including pregnant women.

This opportunistic harmless bacterium can, in some circumstances, cause severe invasive infections.

Though GBS colonization is asymptomatic and, in general, does not cause problems, it can sometimes cause serious illness for the mother and the baby during gestation and after delivery. 

GBS infections in the mother can cause intra-amniotic infection or severe infection of the placental tissues infrequently, postpartum infections and it had been related with prematurity and fetal death.

GBS urinary tract infections may induce labour in pregnant women and cause premature delivery and miscarriage and requires antibiotic treatment. 

Neonates with meningitis often have an initial clinical presentation identical to presentation in those without meningeal invasion, and study of the cerebrospinal fluid is often necessary to rule out meningitis.

Colonization with GBS during labor is the primary risk factor for the development of GBS-early onset of disease, as it is acquired vertically (vertical transmission), through exposure of the fetus or the baby to GBS from the vagina of a colonized woman, either in utero  from ascending infection or during birth, after rupture of membranes. 

Infants can also be infected during passage through the birth canal; but these colonized infants usually do not develop GBS-early onset of disease.

National guidelines in most developed countries advocate the use of universal screening of pregnant women late in pregnancy to detect GBS carriage and use of IAP in all colonized mothers. 

GBS is also an important infectious agent able to cause invasive infections in adults. 

Serious life-threatening invasive GBS infections are increasingly recognized in the elderly and in individuals compromised by underlying diseases.

GBS infections in adults include urinary tract infection, skin and soft-tissue infection bacteremia without focus, osteomyelitis, meningitis and endocarditis.

GBS infection in adults can be serious, and mortality is higher among adults than among neonates.

In general, penicillin is the antibiotic of choice for treatment of GBS infections. 

Gentamicin plus penicillin (for antibiotic synergy) in patients with life-threatening GBS infections may be used.

Toxic shock syndrome (TSS) is an acute multisystem life-threatening disease resulting in multiple organ failure. 

TSS is caused primarily by some strains of Staphylococcus aureus and Streptococcus pyogenes that produce exotoxins.

GBS infection can be infrequently complicated, by streptococcal toxic shock-like syndrome.

Low levels of maternal antibodies against the GBS capsular polysaccharide are correlated with susceptibility to GBS-EOD and GBS-LOD. 

Maternal-specific antibodies, transferred from the mother to the newborn, are able to confer protection to babies against GBS infection.

GBS6 vaccine elicited, anti-capsular poly saccharide antibodies against Group B streptococcus in pregnant women that were transferred to infants at levels associated with reduce risk of invasive Group B streptococcal disease.

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