Molecular Epidemiology and Phylogeography of Domestically Established Bacillus anthracis Isolates Collected in the United States from 1982 to 2013

Bacillus anthracis is endemic in the United States causing periodic outbreaks in wildlife and domestic animals. Currently, human anthrax cases in the U.S. are rare but were common in the 1950s–1960s due to industrial work with imported B. anthracis-contaminated animal products. Multiple-locus variable-number tandem repeat analysis (MLVA) initially differentiated B. anthracis into 89 genotypes and two major clades. Recently, whole-genome sequencing (WGS) was implemented to differentiate B. anthracis which allows for higher resolution and can eliminate risk of homoplasy. To assess the molecular diversity of U.S.-established isolates, we performed MLVA and WGS on 81 B. anthracis isolates from domestic animals or soil. By MLVA, most isolates (n = 58, 72%) were in the Western North America (WNA)/A1.a cluster. Isolates were also observed in the Ames (A3.b), Vollum (A4), and Group B clusters. Using WGS, two major clades (A and B) and four clusters (WNA, Ames, Vollum, Group B) were identified. The four WGS clusters correlated with previously established MLVA clusters (A1a, A3b, A4, and B1, respectively). Further differentiation of the WNA cluster showed that isolates collected from the same state generally clustered together and more broadly by region (west, central, Texas). In the current study, we provide an update on the genetic diversity of domestically established B. anthracis strains using MLVA and WGS. WGS was able to provide additional differentiation, particularly within the WNA cluster, which can lend assistance in epidemiological investigations.