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Bacillus anthracis-positive encapsulation test demonstrated using two different agar media

This image depicts a view of a Petri dish culture plate, which was partitioned down its center to perform an encapsulation test. On the left is a bicarbonate agar growth medium, and on the right is a blood agar growth medium. Each half of the plate was inoculated with the bacterium, Bacillus anthracis. Note the different morphologic characteristics exhibited by the same microorganism, when grown on different growth media. In this case, this proved to result in a positive encapsulation test, with the colonies produced on the bicarbonate medium exhibiting a smooth appearance, whereas those cultivated on the blood agar medium exhibited a rough appearance. (Courtesy of the Centers for Disease Control and Prevention Public Health Image Library, image 2106, 1980. Photograph by Dr. James Feeley, CDC. Public domain.)

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