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Description:Magnified 100x, this 1977 photograph depicted a Petri dish filled with heart infusion agar medium containing 5% defibrinated rabbit blood, i.e., blood agar plate (BAP). A loop of diluted culture of Streptococcus anginosus was put into the melted agar (50oC) just before the blood was added to the melted agar. The agar was allowed to solidify, and then incubated at 35oC for 24 hours in a normal atmosphere. The culture grew subsurface bacterial colonies, one of which is seen here, surrounded by what is known as "wide zone alpha hemolytic" (WZα) color changes. Characteristics of WZα reactivity are described as, "the area immediately adjacent to the colony has some red blood cells (RBCs), but an area outside of that may be completely, or nearly completely, cleared of RBCs. Therefore, there are no reactive zones where "complete" RBC hemolysis has occurred, as in the case of beta-hemolytic reactions, hence the Wide Zone "alpha" terminology.
The technician who must interpret the color changes associated with hemolytic reactions found in inoculated culture media such as the blood agar plate (BAP), must be highly skilled at recognizing the subtle differences, particularly between those associated with WZα and beta-hemolytic reactions. WZα can be misinterpreted as being beta-hemolytic in nature. The important fact is being that the WZα streptococci are members of the Viridans group of streptococci (VGS), these are usually non-pathogenic in humans, and are actually normally found as commensal flora in the nose and throat.
High Resolution: Click here for hi-resolution image (6.23 MB)
Content Providers(s):CDC/ Richard R. Facklam, Ph.D.
Creation Date:1977
Photo Credit:
Links:CDC - Nat. Cntr. for Infectious Diseases; Div. of Bacterial and Mycotic Diseases; Group A Streptococcal (GAS) Disease
CDC Organization
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Copyright Restrictions:None - This image is in the public domain and thus free of any copyright restrictions. As a matter of courtesy we request that the content provider be credited and notified in any public or private usage of this image.