For the first time, the U.S. government is estimating how many people die from drug-resistant bacteria each year — more than 23,000, or about as many as those killed annually by flu.

The figure was released Monday by the Centers for Disease Control and Prevention to spotlight the growing threat of germs that are hard to treat because they've become resistant to drugs.

Finally estimating the problem sends "a very powerful message," said Dr. Helen Boucher, a Tufts University expert and spokeswoman for the Infectious Diseases Society of America. "We're facing a catastrophe."

Antibiotics like penicillin and streptomycin first became widely available in the 1940s, and today dozens are used to kill or suppress the bacteria behind illnesses ranging from strep throat to the plague. The drugs are considered one of the greatest advances in the history of medicine, and have saved countless lives.

But as decades passed, some antibiotics stopped working against the bugs they previously vanquished. Experts say their overuse and misuse have helped make them less effective.

In a new report, the CDC tallied the toll of the 17 most worrisome drug-resistant bacteria. The result: Each year, more than 2 million people develop serious infections and at least 23,000 die.

Of those, the staph infection MRSA, or methicillin-resistant Staphylococcus aureus, kills about 11,000, and a new superbug kills about 600. That bacteria withstand treatment with antibiotics called carbapenems — considered one of the last lines of defence against hard-to-treat bugs.

Germs like those have prompted health officials to warn that if the situation gets much worse, it could make doctors reluctant to do surgery or treat cancer patients if antibiotics won't protect their patients from getting infections.

"If we're not careful, the medicine chest will be empty" when doctors need infection-fighting drugs, said CDC Director Dr. Tom Frieden.

It's not clear that the problem is uniformly growing worse for all bugs. Some research suggests, for example, that MRSA rates may have plateaued.

But CDC officials and others are using the report to press doctors and hospitals to do more to prevent the spread of infections and to more wisely prescribe antibiotics. Experts say as many as half of all antibiotics are not used correctly or are prescribed for the wrong thing — for example, they don't work against flu or colds.

The CDC report warned that misuse creates other problems, too. One is the rise of a nasty strain of intestinal bug called Clostridium difficile, or C-diff. It flourishes in the gut when other bacteria are killed off by antibiotics, and is linked to 14,000 deaths annually.