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A patient with a throat infection has a blood agar culture of his throat swab performed. The type of hemolytic reaction displayed on the blood agar is described as being beta-hemolytic. This means :

a) there is complete lysis of red cells surrounding the colony
b) there is partial lysis of red cells surrounding the colony
c) the patient is infected with Group B streptococci
d) there is hemolyis of red blood cells in a helical pattern
e) there is evidence of hemolysis of B-lymphocytes


beta hemolysis - complete clear hemolysis
can be gr B (strep agalactae) or gr A (pyogenes)
gamma - no hemolysis, alfa - green hemolysis, partial, might be like D.






The correct answer is A


Throat culture has been used as the gold standard for determining the presence of streptococci in a throat infection.

The type of hemolytic reaction displayed on blood agar has long been used to classify the streptococci. Beta -hemolysis is associated with complete lysis of red cells surrounding the colony, whereas alpha-hemolysis is a partial or "green" hemolysis associated with reduction of red cell hemoglobin. Nonhemolytic colonies have been termed gamma-hemolytic. Hemolysis is affected by the species and age of red cells, as well as by other properties of the base medium.

Group A streptococci are nearly always beta-hemolytic; related Group B can manifest alpha, beta or gamma hemolysis. Most strains of S. pneumoniae are alpha-hemolytic but can cause -hemolysis during anaerobic incubation.
The Lancefield procedure is the gold standard for identifying Streptococcal Group Antigens. It requires a pure culture of beta-hemolytic streptococci and standardized grouping antisera, and it is the most complex GAS identification procedure, with a minimum of 48 hours between swabbing the throat and identification of the bacteria. Beta-hemolytic streptococci are subdivided into groups A-D, F, and G using antibodies against the heat- and acid-stable carbohydrate antigens in their cell walls.
Beta -hemolysis is associated with complete lysis of red cells surrounding the colony.Colonies of Streptococcus pyogenes on blood agar exhibiting beta (clear) hemolysis.


I think you are wrong about Group B strep. I think you mean Group _D_ strep can be alpha, gamma, or beta. They are mostly gamma hemolytic, but vancomycin resistant enterococci (Group D) can be a dark alpha/gamma - usually E. faecium. I see these on a daily basis in the lab.

Group B strep (S. agalactiae) can be non-hemolytic. They are a larger colony than Group A and sometimes are not hemolytic at all or only weakly hemolytic. They still type with Group B latex, however.

Edited by bactitech on Apr 26, 2007 - 9:36 PM

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