WASHINGTON — Researchers have found bacteria resistant to the antibiotic of last resort in a sample from a second patient in the United States, according to a study published Monday. The patient had surgery at a New York hospital last year, researchers said.

The news comes after researchers reported in late May that a patient in Pennsylvania carried a strain of E. coli bacteria that was resistant to the antibiotic colistin, the antibiotic that doctors use to treat patients who have infections that don’t respond to other drugs.

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 this country after it was first identified 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 around the world, in animals and people.

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

Last week, scientists reported the discovery of another gene that makes bacteria resistant to colistin. Called mcr-2, it 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 swab from a child with leukemia.

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

Researchers at JMI Laboratories in North Liberty, Iowa, tested about 13,000 strains of E. coli bacteria and 7,000 of Klebsiella pneumoniae they 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. The patient recovered.

The biggest fear of public health officials is that the gene will spread to bacteria that are now susceptible only to colistin.