As many as 84 scientists and staff may have been exposed to live anthrax bacteria after researchers failed to follow safety procedures, the U.S. government said Friday.

The Centers for Disease Control and Prevention (CDC) announced Thursday that at least 75 people had been affected, prompting an investigation by federal authorities.

CDC spokesman Benjamin Haynes said in an email on Friday that the number has increased to 84 potential exposures.

As of early Friday, 32 staff members were taking the powerful antibiotic ciprofloxacin, or Cipro, and 20 were taking another antibiotic called doxycycline, Haynes said in a statement.

CDC Anthrax

This electronmicrograph from the official U.S. Department of Defence anthrax information website shows Bacillus anthracis vegetative cells in a monkey spleen. (Associated Press)

In addition, as many as 27 people were getting the anthrax vaccine to prevent infection. No illnesses have been reported, but the agency expects the number of possible exposures to rise as more people step forward now that news of the anthrax scare is public.

The safety breach, which originated in the CDC's bioterror lab, raised new concerns about the way laboratories around the world conduct research into the deadliest known pathogens, from anthrax to Ebola and avian flu. The CDC has already faced repeated scrutiny over security lapses and mechanical malfunctions at some of its labs dating back to at least 2007.

CDC cedes control of investigation

The CDC said it will cede control of the investigation to the U.S. Department of Agriculture "to avoid potential conflicts of interest."

U.S. President Barack Obama was briefed on the matter on Friday by his homeland security and counterterrorism adviser, Lisa Monaco, the White House said. The incident also is drawing scrutiny from Congress about whether the CDC has the appropriate safety procedures in place to protect federal employees from contamination.

A week ago, on Friday the 13th, bioterrorism researchers at the CDC discovered they had mistakenly sent live anthrax bacteria out to fellow scientists in two lower-security clearance labs at the agency, instead of what they thought were harmless samples of the deadly pathogen.

The CDC's bioterror rapid response and advanced technology laboratory is a high security lab that was trying out a new protocol for inactivating anthrax, using chemicals instead of radiation.

In an interview, the CDC's Dr. Paul Meechan described some of the events that led to the discovery that as many as 75 agency staff had been exposed to live anthrax.

The scientists in the bioterror rapid response units had been preparing an especially dangerous strain of the bacteria for use in two lower-security CDC labs, the biotechnology core facility and the special bacteriology reference laboratory, Meechan said.

Those teams were experimenting with methods to more quickly identify anthrax in substances and powders sent to the United States.

"If there was a bioterrorism incident, we could more quickly identify yes or no, this sample has anthrax," said Meechan, director of the CDC's environmental health and safety compliance office.

Assumed it was safe

Meechan said the team in the bioterror lab used a new process to purify anthrax samples that they had not had a lot of experience with. To check their work, they took a sample of what they thought was dead bacteria and put it on a nutrient-rich lab dish called an agar plate to see if the bacteria would grow.

"They waited 24 hours. They took a look at the plate and they didn't see any new growth," Meechan said. "At that point they assumed the material was safe."

Researchers took the slides to the two lower-security CDC labs which were trying to develop the new tests. Their experiments did not work and a week later, one of the labs asked for additional inactivated samples.

At the time, researchers in the bioterror lab discovered that they had left the agar plates in an incubator for an additional week, Meechan said.

As they were about to dispose of them, they noticed growth on one of the agar plates. "The growth turned out to be anthrax," he said.

That is when the scientists realized the samples they sent to the two lower-security labs may have contained live anthrax bacteria. People working in those labs take fewer safety precautions and were unlikely to be wearing a respirator, putting them at higher risk for infection.

Meechan said the team immediately pulled back the samples and contacted the staff members who had handled them.

That was on the evening of Friday, June 13th.

Meechan said they reached some of the lab workers that same night. Since then, they have been interviewing managers and using electronic surveillance and key card data to identify anyone who might have been inside one of the two labs testing the samples.

The CDC has reached out to all identified individuals, who have been offered antibiotics and a vaccine.

No instances of illness have been reported.

Corrections

  • An earlier version of this story said incorrectly that 86 researchers may have been exposed to anthrax. The number was lowered to 84 after the Centers for Disease Control and Prevention corrected its estimate.
    Jun 20, 2014 1:41 PM ET