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By Maggie Fox

Hard-to-kill anthrax spores may explain why the Department of Defense accidentally sent out “live” samples of the bacteria, experts say.

The spores are so tough, and so tiny, that the irradiation procedure used to deactivate the spores might not have killed every single one.

John Peterson, a microbiology professor who works with anthrax at the University of Texas Medical Branch, says the X-rays or gamma rays used to kill anthrax spores might not get every little one.

“They were most likely inactivated but that the inactivation process may not have been complete,” he told NBC News. Anthrax spores are only about 1 micron in diameter and a sample could contain 10 billion individual spores, he said.

“They are very difficult to kill.”

“If a procedure kills 99.999 percent of the bacteria, there would still be 100,000 spores left viable,” he says. “They are very difficult to kill.”

That's why, even though anthrax is not terribly infectious and is not fatal if it gets treated in time, is considered a potential biological weapon. The tough spores can be dispersed into the air and linger for months or even years, putting people at risk of infection.

The DoD is trying to explain how and why live anthrax spores were sent to labs in nine states and to an Air Force base in South Korea in recent months. At least 26 people who handled or were near the samples are taking antibiotics just in case they breathed in any of the spores.

Officials say the investigation is focusing on the irradiation process used to inactivate the spores.

Anthrax bacteria — the scientific name is Bacillus anthracis — live in the soil normally. They make spores when living conditions get tough, for instance, if it gets very dry. “When conditions improve, everything it needs to live again is there,” Peterson said. “It can sense nutrients.”

In nature, that would mean a rainfall, for instance. But if a spore landed in a lung, that could also signal for it to germinate and multiply.

“It only takes a very small number. A single spore will cause the culture to become positive."

A Maryland lab that received one of the shipments first notified DoD about the live spores. Peterson said the lab was almost certainly doing a validation test, which involves putting the spores into a nourishing broth that would encourage them to grow.

“It only takes a very small number. A single spore will cause the culture to become positive,” Peterson said.

But a single spore cannot cause an infection, Peterson said. “It is not the most virulent agent,” he said. Studies of anthrax show it takes 2,500 to 50,000 spores to infect.

Even if lab workers thought the anthrax was completely dead, they would have handled it carefully and it would have been packaged in a way to fully contain the spores, Peterson and other experts agreed. Defense officials and the Centers for Disease control and Prevention say there is no risk to the public.

Officials said the government labs that received the potentially live anthrax samples were at the Army's Edgewood Proving Ground in Maryland and the Naval Surface Warfare Center in Virginia. The Pentagon has declined to identify the other labs.