At least 52 workers at the Centers for Disease Control and Prevention are taking antibiotics as a precaution because of a lab safety problem that may have accidentally exposed them to anthrax.

The federal agency on Friday raised its estimate of potentially affected workers from 75 to 86, and said the number could rise again as additional workers such as janitors and support staff come forward.

So far, the CDC's occupational health clinic has seen 54 out of 86 potentially exposed employees. Only two have refused antibiotic treatment, which can cut the chances of infection after exposure to the germ. The CDC says 27 of them also began receiving an anthrax vaccine. The others declined or are still considering the vaccine.

The safety lapse was discovered last Friday and CDC revealed it on Thursday. It occurred when a high level biosecurity lab failed to completely inactivate anthrax samples sent to three less secure labs that were researching new ways to detect the germs in environmental samples.

Workers in the less secure labs were not wearing adequate protective gear because they believed the samples had been inactivated. Procedures in two of the labs may have spread anthrax spores in the air. Anthrax is particularly dangerous when inhaled.

Live bacteria were discovered last Friday on materials gathered for disposal, and the CDC began sending emails to potentially affected employees that day, said agency spokesman Tom Skinner.

Top managers at the CDC held a staff meeting Friday morning attended by hundreds of workers, some of whom felt the agency should have communicated with them more fully, Skinner said.

"That's something that management took to heart," and CDC is making every effort to find and notify anyone who may have been exposed, he said.

Anthrax created fear in 2001, when five people died and 17 others were sickened from letters containing anthrax spores sent through the mail. The FBI blames the attacks on a lone government scientist, Bruce Ivins, who committed suicide.

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Online:

CDC anthrax: http://www.cdc.gov/anthrax/

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Marilynn Marchione can be followed at http://twitter.com/MMarchioneAP