Group B streptococcal infection causes
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Overview
Streptococcus agalactiae also known as Group B Streptococcus is a beta hemolytic gram-positive streptococcus characterized by the presence of group B Lancefield antigen. The species name "agalactiae" meaning "no milk", alludes to this. These bacteria cause Group B streptococcal infection. Streptococcus agalactiae is a species of the normal flora of the gut and female urogenital tract. Early perinatal infection causes pneumonia or septicemia, which are associated with a high mortality. During delivery, a baby can acquire S. agalactiae and develop neonatal sepsis, neonatal meningitis, and/or neonatal pneumonia. Neonates with meningitis do not present with the hallmark sign of adult meningitis, a stiff neck. They display nonspecific symptoms such as fever, vomiting, poor feeding, and irritability. The polysaccharide antiphagocytic capsule is this bacterium's main virulence factor.
S. agalactiae displays beta-hemolysis when cultured on a blood agar plate and produce zones of hemolysis that are only slightly larger than the colonies themselves. Group B streptococci hydrolyze sodium hippurate and give a positive response in the CAMP test. S. agalactiae is also sensitive to bile and will lyse in its presence.
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Taxonomy
cellular organisms; Bacteria; Firmicutes; Bacilli; Lactobacillales; Streptococcaceae; 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.
Shown below is an image depicting an example of a positive CAMP test proving that GBS is present. The arrowhead formation on the Blood agarose gel, BAP (5% sheep blood) indicates that this bacteria is GBS.
Shown below is an image depicting colonies of GBS under microscope.
Shown below is an image depicting the growth of colonies of GBS on ChromID CPS chromogenic agar.
Shown below is an image depicting the growth of colonies of GBS on Columbia horse blood agar.
References
- Brooks, Geo F., Janet Butel, and Stephen Morse. Jawetz, Melnick, and Adelberg's Medical Microbiology, 22nd edition. 2001.
- Gillespie, Stephen and Kathleen Bamford. Medical Microbiology at a Glance. 2000.
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