The nation faces “potentially catastrophic consequences” if it doesn’t act quickly to combat the growing threat of antibiotic-resistant infections, which kill an estimated 23,000 Americans each year, the Centers for Disease Control and Prevention warned Monday.

In a 114-page report, the agency detailed for the first time the toll that nearly two dozen antibiotic-resistant microbes are taking on humans — ranking the threat of each as “urgent,” “serious” or “concerning.” Should the trend continue, it said, some infections could become essentially untreatable.

“If we’re not careful, the medicine chest will be empty when we go there to look for a life-saving antibiotic,” CDC Director Thomas R. Frieden told reporters Monday in a telephone news conference. “Without urgent action now, more patients will be thrust back to a time before we had effective drugs.”

One family of bacteria atop the agency’s “urgent” list of infections is carbapenem-resistant Enterobacteriaceae (CRE), which typically strike patients in medical facilities and have become resistant to nearly all existing antibiotics. Known as the “nightmare bacteria,” CRE cause life-threatening diarrhea. They have continued to proliferate, showing up in medical facilities in 44 states, including the National of Institutes of Health’s Clinical Center in Maryland, where seven people died from one type of CRE in 2011 and 2012.

Likewise, Neisseria gonorrhoeae, which causes the sexually transmitted disease gonorrhea, has begun showing resistance to the antibiotics typically used to treat it, the CDC said. The condition, which can cause severe reproductive complications, shows up in an estimated 800,000 cases annually in the United States.

An overuse of antibiotics has led to the emergence of superbugs, disease-causing microbes that are becoming increasingly unaffected by even the most powerful drugs.

Clostridium difficile, or C. difficile, infections, which cause about 14,000 deaths per year, also made the agency’s urgent list. While resistance to the antibiotics used to treat C. difficile infections hasn’t yet become a problem, the agency said the bacteria spreads rapidly because it is naturally resistant to many drugs used to treat other infections.

The CDC estimated that more than 2 million people in the United States are sickened each year by antibiotic-resistant infections, with 23,000 dying as a result. Frieden said those numbers are “a bare minimum, a very conservative estimate.”

The report said such infections lengthen hospital stays and require more extensive treatment, adding “considerable and avoidable” costs to the nation’s already overburdened health-care system.

The overuse of the drugs is the strongest factor contributing to antibiotic resistance around the globe, the agency said. The more a particular germ is exposed to antibiotics, the more rapidly it can develop resistance.

The CDC said as many as half of all antibiotics prescribed are either unnecessary or used inappropriately. Agency officials urged doctors and patients to think twice before using them, though that can be difficult in practice.

“It’s very complicated. Often when people are sick with viral infections, they want an antibiotic. . . . There is a misperception that antibiotics help everything, even viral infections,” said Vicky Fraser, chairman of the department of medicine at the Washington University School of Medicine in St. Louis and a member of the Infectious Diseases Society of America’s antimicrobial resistance committee.

Add to that increasing pressures for people to return quickly to school and work, along with the lack of fast, accurate diagnostic tests for determining when antibiotics aren’t necessary, and the result is a recipe for overuse, she said. “Often people use antibiotics because they’re worried,” Fraser said. “There’s pressure from the patients and the family, as well as from the doctors, to feel like they are doing something.”

The vast majority of antibiotics in the United States — by some estimates 80 percent — are used in animal agriculture to promote growth and prevent and treat disease in livestock. Consumer advocates have continued to push Congress and the Food and Drug Administration to more tightly regulate the amount of antibiotics given to animals, though Frieden suggested Monday that the most urgent threat still involves medical treatment.

“Right now, the most acute problem is in hospitals,” Frieden said. “The most resistant organisms in hospitals are emerging in those settings because of poor anti-microbial stewardship among humans.”

While Monday’s report detailed growing threats in the United States, the CDC reiterated that the problem is not defined by borders. “New forms of antibiotic resistance can cross international boundaries and spread between continents with ease,” Monday’s report stated. “Many forms of resistance spread with remarkable speed.”

Steve Solomon, the CDC’s director of antimicrobial resistance, said in an interview that, while the agency is concerned about the 17 bacteria and one fungus detailed in Monday’s report, the most troubling are those on the brink of resisting all current treatments.

“The ones at the edge of pan-resistance, we’re sounding an urgent alarm,” Solomon said. “We need to act now. We do not have antibiotics in the pipeline that are going to be available soon enough to address those problems.”

For those microbes and others that are increasingly resistent, the search for new antibiotics is essential. But according to CDC officials and Monday’s report, that pipeline has all but run dry in recent years, with few new antibiotics on the horizon. If new drugs aren’t developed to replace the ones that have lost effectiveness, patients will face a dire predicament, said Michael Bell, deputy director of the CDC’s division of health-care quality promotion.

“We’re getting closer and closer to the cliff,” Bell said. “When we no longer have that second-line drug to rely on, that’s when it’s a life or death matter.”

Beyond developing new treatments for antibiotic-resistant bacteria, the CDC on Monday detailed an array of actions that local communities, government officials and public health professionals should take to reduce the sickness and deaths caused by the problem each year.

Those include more aggressive work to prevent infections, closer monitoring of resistant bacteria and additional steps to ensure that antibiotics are used more judiciously and wisely in humans and in animals.

“Only through concerted commitment and action,” the CDC wrote, “will the nation ever be able to succeed in reducing this threat.”