Streptococcus agalactiae

GBS grows readily on blood agar plates as colonies surrounded by a narrow zone of β-hemolysis. GBS is characterized by the presence in the cell wall of the antigen group B of Lancefield classification (Lancefield grouping) that can be detected directly in intact bacteria using latex agglutination tests. when cultivated on granada medium that allows its straightforward identification. GBS can also be identified using MALDI-TOF (Matrix Assisted Laser Desorption/Ionization-Time of Flight) instruments. GBS colonies can additionally be identified tentatively after their appearance in chromogenic agar media, nevertheless GBS-like colonies that develop in chromogenic media should be confirmed as GBS using additional reliable tests (e.g.latex agglutination or the CAMP test) to avoid potential mis-identification. A summary of the laboratory techniques for GBS identification is depicted in Ref 7. == GBS colonization==

GBS is a normal component of the intestinal and vaginal microbiota in some people; GBS is an asymptomatic (presenting no symptoms) colonizer of the gastrointestinal tract and vagina in up to 30% of otherwise healthy adults, including pregnant women. GBS colonization may be permanent, intermittent or temporary. In different studies, GBS vaginal colonization rate ranges from 0% to 36%, most studies reporting colonization rates in sexually active women over 20%. It has been estimated that maternal GBS colonization worldwide is 18%, with regional variation from 11% to 35%. These variations in the reported prevalence of asymptomatic GBS colonization could be related to the detection methods used, and differences in populations sampled. == Virulence==

As other virulent bacteria, GBS harbors an important number of virulence factors (virulence factors are molecules produced by bacteria that enhances their capacity to infect and damage human tissues), the most important being the capsular polysaccharide (rich in sialic acid) and a pore-forming toxin, β-hemolysin. Today it is considered that GBS pigment (granadaene) and hemolysin are identical or closely related molecules. Sialic acid is a notable virulence factor in S. agalactiae despite being found normally in humans and many other animals. By expressing an unusually high amount of sialic acid on the bacterial cell surface, S. agalactiae can subvert the innate immune system, convincing leukocytes that the bacteria are human cells. ==GBS infection in newborns==

GBS colonization of the vagina usually does not cause problems in healthy women, nevertheless during pregnancy it can sometimes cause serious illness for the mother and the newborn. GBS is the leading cause of bacterial neonatal infection in the baby during gestation and after delivery with significant mortality rates in premature infants. GBS infections in the mother can cause chorioamnionitis (a severe infection of the placental tissues) infrequently, postpartum infections (after birth) and it had been related with prematurity and fetal death. GBS urinary tract infections (UTI) may also induce labor and cause premature delivery. EOD manifests from 0 to 7 living days in the newborn, most of the cases of EOD being apparent within 24h of birth. The most common clinical syndromes of EOD are sepsis without apparent focus, pneumonia, and less frequently meningitis. EOD is acquired vertically (vertical transmission), through exposure of the fetus or the baby to GBS from the vagina of a colonized woman, either intrautero or during birth after rupture of membranes. Infants can be infected during passage through the birth canal, nevertheless newborns that acquire GBS through this route can become only colonized, and these colonized infants habitually do not develop EOD. Roughly 50% of newborns to GBS colonized mothers are also GBS colonized and (without prevention measures) 1–2% of these newborns will develop EOD. In the past, the incidence of EOD ranged from 0.7 to 3.7 per thousand live births in the US In 2008, after widespread use of antenatal screening and intrapartum antibiotic prophylaxis (IAP), the CDC reported an incidence of 0.28 cases of EOD per thousand live births in the US. Multistate surveillance 2006-2015 shows a decline in EOD from 0.37 to 0.23 per 1000 live births in the US but LOD remains steady at 0.31 per 1000 live births. In 2021 had been estimated a total of 1970 deaths ((0.59/100,000 population) in the US caused by GBS neonatal infections. It was estimated that 226 infants (49 per 100,000) in the United States had a clinically significant GBS infection, and that approximately 11 (2.4%) of those cases resulted in death. It has been indicated that where there was a policy of providing IAP for GBS colonized mothers the overall risk of EOGBS is 0.3%. Though maternal GBS colonization is the key determinant for EOD, other factors also increase the risk. These factors include onset of labor before 37 weeks of gestation (premature birth), prolonged rupture of membranes (≥18h before delivery), intra-partum fever (>38 °C, >100.4 °F), amniotic infections (chorioamnionitis), young maternal age, and low levels of GBS anticapsular polysaccharide antibodies in the mother. GBS LOD affects infants from 7 days to 3 months of age and is more likely to cause bacteremia or meningitis. LOD can be acquired from the mother or from environmental sources. Hearing loss and mental impairment can be a long-term sequela of GBS meningitis. In contrast with EOD, the incidence of LOD has remained unchanged at 0.26 per 1000 live births in the US. S. agalactiae neonatal meningitis does not present with the hallmark sign of adult meningitis, a stiff neck; rather, it presents with nonspecific symptoms, such as fever, vomiting and irritability, and can consequently lead to a late diagnosis. == Prevention of neonatal infection ==

The only reliable way to prevent EOD currently is intrapartum antibiotic prophylaxis (IAP), that is to say administration of antibiotics during delivery. It has been proved that intravenous penicillin or ampicillin administered for at least 4 hours before delivery to GBS colonized women is very effective at preventing vertical transmission of GBS from mother to baby and EOD. Intravenous penicillin remains the agent of choice for IAP, with intravenous ampicillin as an acceptable alternative. IAP is also recommended for women with intrapartum risk factors if their GBS carrier status is not known at the time of delivery, for women with GBS bacteriuria during their pregnancy, and for women who have had an infant with EOD previously. The risk-based approach for IAP is in general less effective than the culture-based approach because in most of the cases EOD develops among newborns, which are born to mothers without risk factors. to American College of Obstetricians and Gynecologists (ACOG), the American Academy of Pediatrics, and to the American Society for Microbiology. for valid culture results that includes births that occur up to a gestational age of at least 41 weeks. The culture-based screening approach is followed in most developed countries such as the United States, Spain, Belgium, Canada, Argentina, and Australia. The risk-based strategy is followed in the United Kingdom, and the Netherlands. Screening for GBS colonization Though the GBS colonization status of women can change during pregnancy, cultures to detect GBS carried out ≤5 weeks before delivery predict quite accurately the GBS carrier status at delivery. In contrast, if the prenatal culture is performed more than five weeks before delivery it is unreliable for predicting accurately the GBS carrier status at delivery. The clinical specimens recommended for culture of GBS at 36–37 weeks' gestation, this recommendation provides a 5-week window for valid culture results that includes births that occur up to a gestational age of at least 41 weeks Vaccination is considered an ideal solution to prevent not only EOD and LOD but also GBS infections in adults at risk. Nevertheless, though research and clinical trials for the development of an effective vaccine to prevent GBS infections are underway, no vaccine was available in 2023. The capsular polysaccharide of GBS is not only an important GBS virulence factor but it is also an excellent candidate for the development of an effective vaccine. Protein-based vaccines are also in development. == GBS infection in adults ==

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 individuals compromised by underlying diseases such as diabetes, cirrhosis and cancer. GBS infections in adults include urinary tract infection, skin and soft-tissue infection (skin and skin structure infection) bacteremia, osteomyelitis, meningitis and endocarditis. GBS infection in adults can be serious and related with high mortality. In general penicillin is the antibiotic of choice for treatment of GBS infection. Gentamicin (for synergy with penicillin G or ampicillin) can also be used in patients with life-threatening invasive GBS. == Non-human infections ==

Streptococcus agalactiae was historically studied as a disease of cattle that harmed milk production, leading to its name "agalactiae" which means "absence of milk". Strains of bovine and human bacteria are generally interchangeable, with evidence of transmission from animals to humans and vice versa. Cattle GBS is a major cause of mastitis (an infection of the udder) in dairy cattle and an important source of economic loss for the industry. GBS in cows can either produce an acute febrile disease or a subacute more chronic condition. Both lead to diminishing milk production (hence its name: agalactiae meaning "of no milk"). Outbreaks in herds are common, so this is of major importance for the dairy industry, and programs to reduce the impact of S. agalactiae disease have been enforced in many countries over the last 40 years. Vaccination is an effective method to prevent pathogenic diseases in aquaculture and different kinds vaccines to prevent GBS infections have been developed recently. GBS has also been found in many other animals, such as camels, dogs, cats, crocodiles, seals, elephants and dolphins. == References ==