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This 1977 image depicted two Petri dishes, each filled with trypticase soy agar medium, containing 5% defibrinated sheep's blood. The plate on the left had been stabbed and streaked with an inoculum containing
, alpha-hemolytic bacteria, a member of the
group, while the right plate was stabbed with an inoculum containing Group A
(GAS), a typical beta-hemolytic bacteria. The inoculation was performed using a wire loop, which had been dipped into a primary culture medium. The BAPs were incubated in a carbon dioxide enriched atmosphere at 35
C for 24-hours.
All culture organisms were Gram-positive cocci bacteria. The alpha-hemolytic organisms in the left plate grew colonies that were surrounded by a hazy, indistinct zone of partial red blood cell (RBC) hemolysis, and the beta-hemolytic bacteria on the right grew colonies surrounded by a clear, colorless zone of complete RBC destruction. Using the "stabbing" method of inoculation makes the qualitative interpretation of the hemolytic reaction much easier, for the results of colonial interactivity with the blood agar medium is much more pronounced, and therefore, easier to discern.
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CDC/ Richard R. Facklam, Ph.D.
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