Scientists have found a secret 'switch' that lets bacteria resist antibiotics — and it's been evading lab tests for decades

In 2021, a man in his 50s was transferred to the intensive care unit at Emory University Hospital in Atlanta. He was in septic shock due to a bacterial infection that had spread to his bloodstream. The culprit? Klebsiella pneumoniae, which was resistant to most antibiotics.

Finally, hope emerged: A two-week course of an antibiotic called cefiderocol seemed to clear the infection. But just 10 days later, the man was rushed back to the ICU, where doctors discovered a pus-filled mass covering his liver. The same bacteria had returned with a vengeance.

The doctors sent the patient's blood sample to David Weiss, a microbiologist at Emory Antibiotic Resistance Center. Weiss discovered that the bacteria were now highly resistant to cefiderocol.

He quickly notified the doctors. But even after switching antibiotics, the man died.

The man's case highlights an elusive strategy that bacteria use in their arms race against antibiotics — a hidden type of resistance that can be switched on almost instantly but leaves no genetic trace. And it can be almost impossible to detect using standard lab tests. Researchers are increasingly recognizing that this bacterial strategy, called "heteroresistance," may play a significant role in antibiotic failures. Recognizing this bacterial strategy, experts say, is the first step in fighting it.

"While this is a worrisome phenomenon that we're studying now, in the big picture and in the future, I think we'll be able to use this knowledge to greatly help patients," Weiss told Live Science.

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