The ability of anthracis to grow in earthworms has been "one of those puzzles that people have talked about for years, but no one's really taken the trouble to answer," says Nicholas Bergman, a bacteriologist at the National Biodefense Analysis and Countermeasures Center (NBACC) at Fort Detrick in Frederick, Md.
In lab-grown microcosms of earthworms and soil, the researchers also found that infected bacteria shed viruses both in the earthworm guts and soil, suggesting that anthracis can encounter and become infected with new viruses in both environments.
In future studies, Schuch says that he and Fischetti plan to import earthworms from areas where anthrax is endemic, which include sub-Saharan Africa, parts of Canada and western regions of the U.S., and examine whether they are colonized with anthracis. Scientists already know that related Bacillus bacteria live in wild earthworms. Schuch thinks that the invertebrates could provide a reservoir in which anthracis can grow out of season, when the soil is too cold and dry.
Ghosts of anthracis past
In the late 1800s cowboys on horseback drove cattle from Texas ranches to railroad stations in Kansas, Oklahoma, New Mexico and Colorado so that the cattle could be transported farther north and out west. Today, 120 years after the last cattle drives, trails of soil contaminated with anthrax spores still remain, left from where infected cows fed and rested.
"Normal weather processes and things of that nature really ought to diminish the spore population [in the soil]," says Bergman of the NBACC. Anthracis spores have no means to keep from being washed or blown away. But, "if phages are directing them to vegetative growth," Bergman adds, "they might be able to form a biofilm and produce something that would help them stick."
Another example of anthracis's puzzling persistence are the outbreaks that occurred in the early 1990s after the Soviet Union dissolved and unfurrowed land was farmed for the first time in about 80 years. Anthrax started infecting and killing the livestock, probably because the soil was still contaminated from agricultural activity four scores earlier.
Bergman points out that scientists are now using historical land surveys and town records to help them predict areas that might be harboring anthracis. "Mostly they tend to be areas that have had outbreaks before, or had high volume of either containment or trafficking of livestock," he says.