For 50 years, hospitals have used a single test to decide how to treat the most stubborn infections. But according to a growing body of research, that test is now wrong more often than we'd thought. All because of the rise of antibiotic-resistant bacteria that behave one way in lab tests and another way in the human body.

The findings have huge implications for how doctors fight the growing problem of so-called superbugs, which can't be easily treated with antibiotics. The bacteria infect 2 million people each year in the U.S. alone, and kill 23,000, according to the Centers for Disease Control.

"We're saying the standard way the world does this is wrong," said Michael J. Mahan, a professor of microbiology at the University of California, Santa Barbara. That standard protocol, established in the 1960s, is called antibiotic susceptibility testing: Bacteria are grown in a solution called Mueller-Hinton broth, and then attacked with various antibiotics to see which one works best. Doctors around the world use the test to decide which antibiotic to use for which bacterial infection.