colony phenotypes. Thousands of colonies from each sample were visually inspected. The initial plating was on sheep blood agar plates to allow the most efficient colony recovery. The plates were incubated at either 26°C or 37°C with room air, or at 37°C with an atmosphere containing 20 percent CO2. These conditions were employed to allow the detection of variants that might be apparent only under particular growth conditions. While the characteristics of colony morphotypes are most obvious following incubation for 48 hours or more after nutrient exhaustion and sporulation, these starvation conditions were avoided in order to diminish the likelihood of the selection of new variants that were not present in the initial evidentiary samples. Plates were inspected after 18 to 22 hours of incubation, which was sufficient to allow the development of sizable colonies. Most classes of colony morphotypes were identified predominantly by the presence of a yellow tint (although the committee notes that this tint is not apparent in Figures 5-1, 5-2, and 5-3, while one class exhibited a smaller, more opaque colony appearance.
FIGURE 5-1 B. anthracis Colony Morphotype “A.” Photograph of colonies formed by growth of B. anthracis cells on blood agar. The colony on the top displays the morphology designated “Type A.” The colony on the bottom displays the typical wild-type morphology.
SOURCE: USAMRIID. This image is a work of the United States Army Medical Research Institute for Infectious Diseases, taken or made during the course of an employee’s official duties. As a work of the U.S. federal government, the image is in the public domain.