Ecological Niche Model of Bacillus cereus Group Isolates Containing a Homologue of the pXO1 Anthrax Toxin Genes Infecting Metalworkers in the United States

Abstract

While Bacillus cereus typically causes opportunistic infections in humans, within the last three decades, severe and fatal infections caused by isolates of the B. cereus group harboring anthrax toxin genes have been reported in the United States. From 1994 to 2020, seven cases of anthrax-like illness resulting from these isolates have been identified. With one exception, the cases have occurred in the Gulf States region of the United States among metalworkers. We aimed to develop an ecological niche model (ENM) to estimate a spatial area conducive to the survival of these organisms based on the presence of known human infections and environmental variables. The estimated ecological niche for B. cereus was modeled with the maximum entropy algorithm (Maxent). Environmental variables contributing most to the model were soil characteristics (cation exchange capacity, carbon content, soil pH), temperature, enhanced vegetation index (EVI), and land surface temperature (LST). Much of the suitable environments were located throughout the Gulf Coast Plain, Texas Backland Prairies, East Central Texas Plains, Edwards Plateau, Cross Timbers, Mississippi Alluvial Plain, and Central Great Plains. These findings may provide additional guidance to narrow potential risk areas to efficiently communicate messages to metalworkers and potentially identify individuals who may benefit from the anthrax vaccine.

Keywords: Bacillus cereus group; ecological niche modeling; pXO1; welder anthrax.

Figure 1

Maxent median suitability model (FC = LQP; RM = 1.5) for B. cereus group bacteria containing anthrax toxins. Black–white dots represent the study occurrence data (n = 7). The model calibration region (M) is outlined in black.

Figure 2

Equal training sensitivity and specificity (sensitivity = specificity) threshold (presence/absence) (value = 0.451) representing potentially suitable environments for B. cereus. Black–white dots represent the study occurrence data (n = 7).

Figure 3

Visualization of the ecological niche of B. cereus based on three environmental dimensions (Set 3 PC2—x, Set 1 PC2—y, and Set 4 PC1—z) representing the principal components with the highest contribution to the Maxent model. The minimum-volume ellipsoid (blue) represents the ecological limits under which the B. cereus human cases were documented.

Figure 4

Study area and corresponding model calibration area (M region) compared to the Köppen-Geiger climate classification [34]. Black–white dots represent the study occurrence data (n = 7).