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Scientists were alarmed last year when they found that a woman in Pennsylvania had been infected with bacteria that was resistant to colistin, an antibiotic that is considered the last line of defense against particularly nasty illnesses.

It was a scary reminder that bacteria are increasingly able to survive antibiotics, making some infections extremely difficult or even impossible to treat.

Now California is on a list of six states where patients have been infected with bacteria that contains a gene known as mcr-1, which makes it resistant to colistin. Los Angeles County health officials announced Tuesday that a resident who died last year had been infected with E. coli bacteria carrying the mcr-1 gene.

“This just poses another threat that could make infections more difficult to treat,” said Dr. Benjamin Schwartz, acting director of the acute communicable disease program at the Los Angeles County Department of Public Health.

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