By comparison, the drug-resistant Gram-negative germs for the most part threaten only hospitalized patients whose immune systems are weak. The germs can survive for a long time on surfaces in the hospital and enter the body through wounds, catheters and ventilators.

What is most worrisome about the Gram-negatives is not their frequency but their drug resistance.

“For Gram-positives we need better drugs; for Gram-negatives we need any drugs,” said Dr. Brad Spellberg, an infectious-disease specialist at Harbor-U.C.L.A. Medical Center in Torrance, Calif., and the author of “Rising Plague,” a book about drug-resistant pathogens. Dr. Spellberg is a consultant to some antibiotics companies and has co-founded two companies working on other anti-infective approaches. Dr. Rice of Cleveland has also been a consultant to some pharmaceutical companies.

Doctors treating resistant strains of Gram-negative bacteria are often forced to rely on two similar antibiotics developed in the 1940s  colistin and polymyxin B. These drugs were largely abandoned decades ago because they can cause kidney and nerve damage, but because they have not been used much, bacteria have not had much chance to evolve resistance to them yet.

“You don’t really have much choice,” said Dr. Azza Elemam, an infectious-disease specialist in Louisville, Ky. “If a person has a life-threatening infection, you have to take a risk of causing damage to the kidney.”

Such a tradeoff confronted Kimberly Dozier, a CBS News correspondent who developed an Acinetobacter infection after being injured by a car bomb in 2006 while on assignment in Iraq. After two weeks on colistin, Ms. Dozier’s kidneys began to fail, she recounted in her book, “Breathing the Fire.”

Rejecting one doctor’s advice to go on dialysis and seek a kidney transplant, Ms. Dozier stopped taking the antibiotic to save her kidneys. She eventually recovered from the infection.

Image Dr. Louis Rice of Case Western Reserve University says hardy Gram-negative bacteria “are becoming more and more common.” Credit... David Maxwell for The New York Times

Even that dire tradeoff might not be available to some patients. Last year doctors at St. Vincent’s Hospital in Manhattan published a paper describing two cases of “pan-resistant” Klebsiella, untreatable by even the kidney-damaging older antibiotics. One of the patients died and the other eventually recovered on her own, after the antibiotics were stopped.