NEW YORK—Using a pipette as a makeshift rolling pin, Raymond Schuch spent some of his lab time last summer pressing the guts out of earthworms that he had collected, fresh from Manhattan soil. For his efforts, The Rockefeller University microbiologist extracted what looked like just a small pile of dirt, but was actually a microcosm teeming with phages—viruses that infect bacteria. Schuch was on the hunt for phages that could kill anthrax and become anti-anthrax therapies, but what he discovered were viruses that enable this deadly bacteria to grow and survive when the going gets tough.
Scientists have suspected for decades that some phages have a hand at helping the growth of anthrax, Bacillus anthracis, and its less deadly cousins in the Bacillus genus. Then, four years ago, Schuch, along with Vincent Fischetti, a professor of bacteriology at Rockefeller, found a direct link—a type of phage that made anthrax resistant to an antibiotic commonly produced by other bacteria in soil, such as Streptomyces.
But how anthrax manages to persist in soil for hundreds of years despite environmental extremes, including wind and rain, and even go undetected during outbreaks in humans and livestock has puzzled scientists. In the earthworm intestines Schuch spotted a group of lysogenic phages—viruses that insert their genetic material into anthrax's genome. Instead of killing the pathogen the phages spur its ability both to grow and persist in the soil.
"It's known that there are lysogenic phages—they're very common," Schuch says. "The unusual thing is seeing what dramatic effects they could have on these anthracis strains."
Like other scientists studying anthrax, Schuch and Fischetti assumed that the bacteria's options were limited when it was not inside a host, often herbivores such as cattle or sheep. Although anthracis cells flourish when they are infecting a mammal, they tend to otherwise starve and transform into dormant spores in the soil, where they await their next victim. Typically, spores infect animal hides and human skin, causing infections that are rarely fatal. But inhaling or ingesting them can be deadly in up to 75 percent and 60 percent of cases, respectively.
The phages that Schuch and Fischetti discovered can prompt anthrax cells to remain in a growing, or vegetative, state even when they are in soil. In this condition the cells can form sticky biofilms that resist harsh wind and rain. Moreover, the researchers found that these lysogenic phages bestow anthracis with the ability to colonize earthworm guts, which Schuch calls "a safe haven" for the bacteria. He and Fischetti published their findings of new anthrax phages in the August 2009 issue of PLoS ONE.
"The most surprising [finding] is that anthracis can be grown outside the mammalian host," says Agnès Fouet, a microbiologist at the Institut Pasteur (Pasteur Institute) and the Centre National de la Recherche Scientifique (National Center for Scientific Research), both in Paris. Because of the thinking that anthracis is only in spore form in the soil, scientists typically heat soil samples to try to separate it from the vegetative bacteria that are killed by the heat treatment. "People were just not looking for it," Fouet says, "People were looking for spores." Instead, Schuch says, "If you want to look for anthrax in endemic areas, you look for earthworms in that area." In addition, he says that scientists can turn to heat-free methods to find vegetative anthrax-causing bacteria growing free in the soil.
To date, scientists have only found a handful of phages that can infect anthracis and, other than the case of the phage that bestows the bacteria with antibiotic resistance, few clues exist as to how viral infection affects anthracis. Because previous investigations have uncovered phages even in areas where there was no anthrax—in sewage, soil and water—Schuch looked for new phages in samples that were anthracis-free—fern roots and commercial potting soil.