Streptococcus agalactiae (also known as Group B streptococcus or GBS) is a beta-hemolytic Gram-positive streptococcus.
Identification
The CAMP test is an important test for identification. GBS (group B Streptococcus species) are screened through this test. It is characterized by the presence of group B Lancefield antigen and by its ability to hydrolyze sodium hippurate.
It is also sensitive to bile, and will lyse in its presence.
S. agalactiae's polysaccharide antiphagocytic capsule is its main virulence factor.
Clinical significance
S. agalactiae is a member of the gastrointestinal normal flora in some humans and can spread to secondary sites - genitourinary tract of women 20-30%. This is of clinical importance: S. agalactiae can be transferred to a neonate passing through the birth canal and can cause serious group B streptococcal infection. In the western world, S. agalactiae is the major cause of bacterial septicemia of the newborn, which can lead to death or long-term sequelae. S. agalactiae invades via alveolar and pulmonary epithelial cells; newborns are especially susceptible to infection because they lack alveolar macrophages to prevent invasion. Newborn GBS disease is separated into early-onset disease occurring on living days 0â€"7 and late-onset disease which starts on days 7â€"90. Early-onset septicemia is more prone to be accompanied by pneumonia, while late-onset septicemia is more often accompanied by meningitis. 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 late diagnosis. Hearing loss can be a long-term sequela of GBS-meningitis. Infection with GBS is the cause of some instances of stillbirth.
S. agalactiae is present in up to one-third of women of childbearing age, and 1.8 cases per 1000 live births will be affected by group B streptococcal infection. In the elderly or persons with compromised immune systems, septicemia or other serious infections are seen. This can also occur during pregnancy or maternity.
Prevention
There is no reliable way to prevent newborn disease, currently; however, prophylactic treatment with antibiotics (typically intravenous penicillin) during delivery can prevent early-onset disease. Vaccination of adolescent women is considered an ideal solution; though research for the development of such a vaccine is underway, a good candidate vaccine has yet to surface.
Different western countries use different strategies to select which women in labor are most likely to need antibiotics.
- The United States uses the most effective strategy: all pregnant women are screened for S. agalactiae and prophylactic antibiotics are given to all women testing positive . Because of this strategy, the US has seen a marked reduction in babies born with early-onset infection.
- Increasingly, western countries are screening pregnant women routinely for GBS, including USA, Canada, France, Germany, Italy, Spain, Belgium, Canada, Australia, Czech Republic, Slovenia, Kenya and Argentina, with others offering a less effective risk-based strategy at time of delivery, including the UK, Denmark and Bulgaria.
- Part of the neonatal care in Mexico includes undergoing a screening with culture of a cervicovaginal swab during the third trimester of pregnancy, though chemoprophylaxis based on positive results remains below 50%.
Nonhuman infections
S.agalactiae has been found in many animals such as camels, dogs, cats, crocodiles, seals and dolphins.
Cattle
S.agalactiae was recognised as a pathogen in cattle before the Second World War. Its significance as human pathogen was not discovered before the 1950s. In cattle it causes mastitis, an infection of the udder. It can either give acute, febrile disease or sub-acute, more chronic disease. Both lead to diminishing milk production (hence its name: agalactiae meaning "no milk"). Outbreaks in herds are common. This is of major significance for the dairy industry and programs to reduce the impact of S.agalactiae disease have been enforced in many countries over at least the last 30â€"40 years.
Other animals
Fisheries and wildlife officers from Queensland, Australia, have been investigating the deaths of more than 50 grouper fish, between 2008 and 2010, that have washed up dead on beaches in the north of the state. The fish had been infected with S. agalactiae.
