A photograph shown to the committee of a dense lawn of B. anthracis grown on such a plate at Dugway reveals the presence of many papillae, small outgrowths of bacteria indicative of mutants that are overgrowing their neighbors in the lawn or have some other distinctive feature (Martin, 2010). The scientist who repeatedly prepared these materials for the multiple production runs told the committee that the presence of numerous papillae was the typical outcome. The committee believes that the presence of these papillae can be taken as evidence that the agar slants already contained mutants, that growth of the bacterial population on the blood agar plates selected for mutants, or both. Because B. anthracis cells sporulate on blood agar once they reach high density, the papillae could have been outgrowths of sporulation-defective mutants that continued to grow for several generations after other nonmutant cells stopped growing and formed spores.

Second, according to one DPG scientist, the biological material (lawns, including papillae) scraped from these plates was then used directly to inoculate the fermentors at Dugway (Martin, 2010). This material was collected from the plates after the lawn population had largely converted to spores. Because spores must germinate before growth can resume, unsporulated cells (including from sporulation-defective mutants in the inoculum) would likely have had a growth advantage in the fermentor. Because material prepared at Dugway comprised the bulk of the material that was pooled in the RMR-1029 flask, and because the inocula used to prepare the spores had visible evidence of mutants that may have been defective in spore formation, the committee suggests that at least some of the morphotypes identified in RMR-1029 originated from Dugway. Various biological factors would have affected the resulting presence and abundance of the genetic variants, including their growth rates, germination rates, and sporulation efficiencies under the specific cultivation conditions used as well as the rate at which each variant arises by mutation.

6.5 ANALYSES OF THE REPOSITORY SAMPLES AND STATISTICAL INTERPRETATION OF THE EVIDENCE

The assays developed by the various contractors (Commonwealth Biotechnologies, Inc. [CBI], Midwest Research Institute [MRI], IIT Research Institute [IITRI], and the Institute for Genomic Research [TIGR]) and described in Chapter 5 were used to analyze the samples submitted to the FBIR under the subpoena protocol or gathered through the searches performed by the FBI. The purpose of these assays was to search for the presence of the genetic mutations—A1, A3, D, and E—among the FBIR samples to determine whether any of the samples had a genetic profile that matched that of the evidentiary material and could be a possible source of the material used in the letters.

It is important to reiterate here that the existence in laboratories of the Ames strain of B. anthracis dates back only to 1981 and that all samples of the Ames strain at laboratories outside USAMRIID had been derived either



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement