According to the Centers for Disease Control and Prevention (CDC), group B streptococcus or Streptococcus agalactiae is the most common cause of sepsis or blood infection and meningitis in newborns. This is due to the fact that women become colonized with the bacteria in the vagina and rectum, and though it rarely causes disease in the woman, the presence of Streptococcus agalactiae in the female genital tract during birth can lead to serious infection in the newborn. It is important to determine if the pregnant woman is colonized during the 35th to 37th week of gestation so that appropriate antibiotic prophylaxis can be administered to kill the colonized bacteria. This is done by methods to identify group B streptococcus.
Lancefield Grouping
Each of the beta hemolytic streptococci, A, B, C, F and G can be differentiated serologically based on cell wall antigens or in the case of group B streptococci, a capsular antigen. In this test the strep group antigen is extracted from the cell wall. This extraction is then tested against specific corresponding streptococcal group sensitized latex particles to look for agglutination or clumping of the latex. The Lancefield grouping serological procedure is a definitive identification procedure since it detects specific cell wall antigens.
CAMP Test
The CAMP test is a simple, cost-effective test procedure for the presumptive identification of group B streptococcus. Streptococcus agalactiae secretes a protein called the "CAMP" factor and when it comes into contact with a beta hemolysin produced by a specific strain of Staphylococcus aureus causes a synergistic, enhanced hemolysis. The test is performed by streaking the S. aureus strain down an agar plate containing sheep blood. Then placing the unknown beta hemolytic strep perpendicular to the staph streak but not touching. After overnight incubation if the strep is a group B strep, the hemolysins from the staph and strep will synergistically produce a large "arrowhead" of hemolysis.
Hippurate hydrolysis
Group B streptococci can be differentiated from other beta hemolytic streptococci by it's ability to hydrolyze hippurate. This, like the CAMP test is a presumptive test in the identification of Group B strep. The organism hydrolyzes hippurate to its basic components, benzoic acid and the amino acid glycine. Detection of the glycine is accomplished by oxidation with ninhydrin which produces a deep purple color end product. The hippurate test is also used to identify unrelated bacterial species like Campylobacter jejuni, Gardnerella vaginalis and Listeria monocytogenes. There are now rapid tests for hippurate hydrolysis that take just 2.5 hours, as opposed to the classic methods that would take 48 hours.
References
- CDC-Frequently asked questions about group B strep
- Iowa State University- Streptococcosis
- Bailey and Scott's Diagnostic Microbiology; Betty Forbes, Ph.D.; 2002
