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Updated 12:00 noon February 16, 2004



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Anthrax spores can germinate, grow and reproduce in soil

For Bacillus anthracis, the bacterium that causes anthrax, nothing beats the inside of a warm human or animal host for triggering an intense spurt of rapid growth and reproductive activity. But when a warm-blooded animal isn't available, new research by scientists in the Medical School shows that ordinary dirt can do the job as well.

Microbiologist Philip Hanna announced his findings in a Feb. 14 presentation at the American Association for the Advancement of Science (AAAS) meeting. His results are surprising because most scientists believe B. anthracis spores will not germinate and grow unless they are inside a living host.

Hanna described what happened when he seeded soil samples with anthrax spores, added water and let the mixture incubate in the laboratory.

"All stages of the anthrax life cycle were found to occur in soil, including germination of spores, bacterial reproduction and formation of new spores," said Hanna, assistant professor of microbiology and immunology. "Our research demonstrates that anthrax can complete its full life cycle without a mammalian host."

Researchers in Hanna's laboratory collected ordinary soil from the bank of Miller's Creek—part of the Huron River watershed—in Ann Arbor. The soil was taken to the laboratory where researchers added water and dormant spores of an attenuated, or non-infectious, strain of B. anthracis, which was modified to make it safe to handle in university laboratory facilities. Then, scientists cultured the soil samples to see whether the spores would germinate and grow.

"The spores germinated and continued to replicate until they ran out of nutrients in the soil," Hanna reported. "At that point, the bacteria formed new spores and became dormant. In every case, we ended up with more spores than we added to the original soil samples."

Hanna said the Ann Arbor soil samples did not contain naturally occurring anthrax spores, and the attenuated strain of anthrax used in the study is unable to infect or produce disease in people.

Scientists have detected dormant anthrax spores in soil before, but vegetative or "growing" anthrax has never been found in nature outside a mammalian host. According to Hanna, this led most scientists to assume that B. anthracis had to get inside a person or an animal in order to reproduce. Hanna's research suggests that this deadly pathogen may be even more versatile and resilient than scientists originally believed.

Collaborators on the soil study included Nicholas Bergman, a University research investigator, and Brendan Thomason, a graduate student.

Hanna is part of a collaboration of scientists from U-M, The Institute for Genomic Research and The Scripps Research Institute who are working together to identify all the genes and proteins involved in the deadly transformation of B. anthracis from a dormant spore to an active, growing organism.

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