In this case, although the girl's bacterial strain was resistant to colistin, it was not resistant to all antibiotics, according to the CDC report. The girl recovered fully, and the bacteria did not spread to anyone else in her family or to health-care providers she came in contact with. Officials, however, expect more cases to surface in the United States, and on Friday recommended increased surveillance for bacteria that show resistance to colistin.

Food has been the most common way humans have acquired this superbug in other places, and the Connecticut case suggests that food is also a possible cause, said Maroya Walters, a CDC epidemiologist.

The girl had traveled to the Caribbean for about two weeks to visit friends and relatives. While she was there, she ate chicken and goat meat from a live animal market. She developed fever and bloody diarrhea on June 12, two days before returning to the United States.

When she came back home, she went to the doctor's office, and stool samples were collected, including one from a soiled diaper. An investigation by state and federal health officials found that the strain of E. coli is not what made her sick. But when the Connecticut health department analyzed the genes in that strain, they discovered it was carrying the resistance gene.

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"It was completely an incidental finding," said Alexander Kallen, a CDC medical officer who investigates antibiotic resistance.

The mcr-1 gene was first identified in China almost a year ago, showing up in pork, live pigs and infected humans. Since then, health officials and infectious-disease experts around the world have been looking for it; it has since been found in nearly 30 countries.

"We know this is food-borne, and we know from outbreaks in other countries that's how most people get it," Kallen said.

The CDC is recommending increased testing for the superbug gene among certain types of bacteria that show resistance to colistin. The gene spreads readily among bacteria, and it could make these multi-drug resistant strains almost impossible to treat.

So far, there is no evidence linking any of the four U.S. human cases, Kallen said. Nor have any links been identified between human cases and instances in which the gene has been found in pigs in a slaughterhouse in Illinois and another in South Carolina, he said.

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The second human case was found in a sample of E. coli recovered in May 2015 from a hospital patient in New York after researchers retrospectively examined collections of organisms dating back decades.

The third case, reported last month by Rutgers University scientists, found the resistance gene in an E. coli strain in a 76-year-old New Jersey man dating back to August 2014. The man had multiple medical issues, including prostate cancer and recurring urinary tract infections. Most troubling, researchers said, is that the same strain also had a second gene that made the organism resistant to carbapenems, another critical category of antibiotics. The man was treated successfully with other drugs, and researchers said the bacteria did not cause a major outbreak of drug-resistant infection.

The cases highlight the critical need for more research, heightened surveillance, and increased awareness and preventive measures from hospitals, doctors' offices and other health-care institutions about the growing dangers of antibiotic resistance.

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The biggest fear is that the gene will spread to bacteria that are now susceptible only to colistin. In almost all cases involving mcr-1 worldwide, the gene is carried on a plasmid, a mobile piece of DNA that easily can transfer the gene to other bacteria. That would result in a kind of super-superbug, invincible to every lifesaving antibiotic available.

The CDC on Friday also released new details about the Pennsylvania woman's case, the first reported time the superbug gene had been found in the United States.

Investigators said they don't know how she got the superbug. She had no international travel for one year, no exposure to livestock and a "limited role in meal preparation with store-bought groceries," the CDC found.

CDC and the Pennsylvania health department investigated her household contacts and the two medical facilities where she had frequent, extensive and prolonged interactions with health-care personnel.

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No bacteria with the superbug gene were found among 105 people who were screened. In addition, no colistin-resistant organisms were found among more than 50 samples of a serious drug-resistant bacteria collected from the four medical facilities to which she was admitted during 2016. Although the investigation did not turn up the source of her infection, it also revealed that she may not have spread the bacteria widely to other people.

"These findings suggest that the risk for transmission from a colonized patient to otherwise healthy persons, including persons with substantial exposure to the patient, might be relatively low," the CDC report said.