The secret life of Bacillus anthracis

Bacillus anthracis is the causative agent of anthrax, a serious and globally distributed zoonosis affecting a wide range of wild and domestic animals, invariably also humans. However, although known to humans since biblical times, much remains to be elucidated concerning the ecology and transmission of this bacterium. Of particular interest is the Bacillus anthracis spore, the uptake of which is the predominant way to contract anthrax and which is legendary for its resilience in the environment and thus crucial for persistence and spread of the disease. Hence, the aim of this study is to review the natural transmission of Bacillus anthracis and investigate potential means by which soil persisting Bacillus anthracis spores reach concentrations sufficient to infect susceptible hosts. When reviewing the literature, three different theories can be distinguished. Firstly, “the incubator area” hypothesis suggests that favourable soil factors, possibly in association with amoebas, may constitute an environment supporting repeated spore-bacterium-spore cycling, thus increasing the local amount of spores. Secondly, water runoff from heavy rains or flooding has been proposed to collect spores and dispose them in closely restricted sites, thus creating “concentrator areas” with locally high amounts of spores. Lastly, the outermost layer of the spore, the exosporium, is proposed to tie spores to the environment where they were first shed and hence maintain infectious spore concentrations at a particular site. Considering that these theories all have their agreements and disagreements with the ecology and epidemiology of anthrax, it is reasonable to assume that all three exert an impact on spore concentrations, possibly at different degrees at various sites and regions. Howsoever, the ecology of Bacillus anthracis requires further research to fully understand the mechanism responsible for transmission and spread of anthrax. Only then can efficient methods for eradication of the disease from contaminated fields, and hence reducing the risk of future epidemics, be developed.