After the death of its vertebrate host, viable cells of the causative agent of anthrax, Bacillus anthracis are released along with blood into soil. Nutrients released from carcasses may promote the growth of grasses, attracting new potential hosts to infectious sites. Although B. anthracis produces hardy spores, spore concentrations at carcass sites typically decline and appear to fluctuate. An environmental life cycle may help to explain how spores persist at some sites despite weathering, competition with other microbes, and predation by lytic bacteriophages. Although once held to be contentious, the idea that replication can also occur outside of a host is strongly supported by studies investigating activity in the rhizosphere and effects of lysogenic bacteriophages.
In this seminar, I will discuss current evidence for an environmental life cycle in B. anthracis and I will present results from a manipulative field experiment. Using a B. anthracis isolate from a zebra carcass, I tested whether B. anthracis persists as vegetative cells as well as spores within soil and whether a native grass Enneapogon desvauxii promotes its survival. In addition to monitoring spore and vegetative cell abundance over time, I tested whether the presence or absence of B. anthracis and the grass, E. desvauxii, affected soil bacterial community structure. This research was conducted in Etosha National Park, Namibia. Within an enclosure placed in a grassland savanna, we established eight experimental treatments: 2 B. anthracis treatments (plus/minus spores) x 2 grass treatments (plus/ minus seeds) x 2 zebra blood treatments (plus/minus blood). We detected both spores and vegetative cells in the soil throughout the duration of the experiment. During this time, B. anthracis maintained pathogenicity. All isolates tested contained both pX01 and pX02 plasmids. The addition of B. anthracis spores significantly increased the likelihood of establishment of a native grass (P< 0.01). In soil, B. anthracis concentrations (spores and vegetative cells) decreased over time. Grass and zebra blood had no significant effect on spore or vegetative cell numbers in soil. This study provides the first evidence from the field for activity of B. anthracis outside of a host and demonstrates a significant interaction between B. anthracis and a native grass.
Link to video.