The new case involved a patient in New York, while the first reported case involved a woman from Pennsylvania. However, the New York patient was actually infected more than a year ago and the resistant bacteria was only spotted recently in lab testing. The Pennsylvania infection occurred last spring, researchers said.
Both patients had E. coli with a gene called mcr-1, which makes bacteria resistant to the antibiotic colistin, the scientists explained.
In the latest study, the researchers tested more than 13,500 strains of E. coli and nearly 7,500 strains of Klebsiella pneumoniae collected from hospitals in North America, Latin America, Europe and the Asia-Pacific region in 2015.
The results showed that almost 2 percent of the E. coli samples were resistant to colistin, and 19 tested positive for mcr-1. Those 19 samples were found in the United States and nine other countries in all of the regions.
The study was published July 11 in the journal Antimicrobial Agents and Chemotherapy, a journal of the American Society for Microbiology.
The findings raise concerns that mcr-1 -- first identified in food, animals and people in China in the 1980s -- may be able to jump to other types of bacteria that are already resistant to other antibiotics, the researchers said. Because mcr-1 is now present worldwide, it needs to be closely monitored, the study authors stressed.
"The fact that the gene has been detected in food livestock and raw meat is also concerning," study corresponding author Mariana Castanheira, director of molecular and microbiology at JMI Laboratories in Iowa, said in a journal news release.
But she noted that the samples that tested positive for mcr-1 were still susceptible to several widely used antibiotics.
That means that E. coli and Klebsiella pneumoniae with mcr-1 are unlikely to cause hard-to-treat infections at this time, Castanheira said.
She said she and her colleagues are continuing their research.
In the first U.S. patient found to have colistin-resistant E. coli, the bacteria was also resistant to first-line antibiotics. That case -- reported May 26 in a journal of the American Society of Microbiology -- triggered alarm bells.
"It is the end of the road for antibiotics unless we act urgently," Dr. Tom Frieden, director of the U.S. Centers for Disease Control and Prevention, warned at the time.
The U.S. Centers for Disease Control and Prevention has more on antibiotic resistance.