Doctors in France say they witnessed a real-life horror tale involving an antibiotic-resistant superbug. In less than a month, their patient’s infection evolved resistance to the last-resort drug they had used to treat it. Thankfully, the doctors were still able to defeat the microscopic threat—and the case may have uncovered a peculiar weakness in the germ.

According to the report, published in the journal Antimicrobial Agents and Chemotherapy, a young child had been dealing with recurrent infections of the bacteria Pseudomonas aeruginosa for over two years. P. aeruginosa is an opportunistic infection that sickens tens of thousands of already weakened people in hospitals and other health-care settings in the U.S. a year. In these people, it can cause serious infections.

Initially, the bug seemed completely vulnerable to the last-resort drug ceftolozane-tazobactam, a combination therapy of two potent antibiotics. But 22 days into treatment, the doctors found a strain of the bacteria in their patient that had evolved resistance to the drug. Other scientists have found that P. aeruginosa can develop resistance to ceftolozane-tazobactam in the lab, but according to the authors, it’s rarely been documented in real-life patients, much less over such a short time.

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