In both cases, the bacteria carried a gene, known as mcr-1, that allows the organism to withstand colistin. The Pennsylvania case was the first time this colistin-resistance gene had shown up in the United States after its identification in China last fall. Health officials and infectious-disease experts around the world have sounded the alarm because the gene has since been found in more than two dozen countries, in animals as well as people.

The latest research, published in Antimicrobial Agents and Chemotherapy, a journal of the American Society for Microbiology, comes as scientists intensify efforts to find threats to their antibiotic arsenal.

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David Hyun, senior officer at Pew’s Antibiotic Resistance Project, said the findings are significant because they underscore the need for more research and better surveillance to document “how widespread this resistance is and how patients are impacted.”

Last week, scientists reported the discovery of another gene that makes bacteria resistant to colistin. The gene, called mcr-2, was found in E. coli bacteria from pigs in Belgium and is similar to mcr-1. On Monday, Italian investigators said they discovered a variant of the mcr-1 gene in a rectal swab from a child hospitalized with leukemia.

Experts tracking infectious disease say they’re not surprised that more genes and variants are showing up as researchers increase their search for them by retrospectively examining collections of organisms that date back decades. That’s how the most recent discovery in the New York patient occurred.

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Researchers at JMI Laboratories in North Liberty, Iowa, tested more than 13,000 strains of E. coli bacteria and more than 7,000 strains of Klebsiella pneumoniae that they had collected from hospitals in Asia, Latin America, Europe and North America in 2015. They found 390, or nearly 2 percent, were resistant to colistin, and 19 tested positive for mcr-1.

The 19 included a strain of E. coli originally recovered from the New York patient in 2015, said Mariana Castanheira, director of molecular and microbiology at JMI. “I know the patient went through surgery, and the isolate was deemed the cause of the infection,” she said in a recent interview. She declined to identify the hospital, which she said was investigating the case.

The biggest fear of public health officials and experts 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.

JMI researchers said the strains that have tested positive for mcr-1 were still susceptible to several commonly used antibiotics, including carbapenems. The researchers suspect the gene is on a plasmid and are working with other labs to confirm that.

Testing for mcr-1 is not something that most U.S. hospitals do because resistance to colistin remains relatively rare. In addition, the test requires more molecular biology analysis than hospitals are equipped to do, said Mike Bell, deputy director of healthcare quality promotion at the Centers for Disease Control and Prevention.

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The CDC is planning to establish seven regional laboratories this fall that will have the capacity to do better and faster testing for a broad range of antimicrobial resistance.

Lance Price, director of the Antibiotic Resistance Action Center and a George Washington University professor, said the United States and other countries need to push for a global effort to control antibiotic use in human medicine and in livestock production. Colistin is not used in livestock in the United States but is widely used to promote livestock growth elsewhere.