The mcr-1 'superbug' gene is especially troubling because of its ability to make bacteria resistant to last-resort antibiotics, according to Reuters.
The second 'superbug' gene was found in an E. coli sample from the New York patient whose name and condition haven't been released.
The first case in the U.S. was found last month in Pennsylvania.
In addition to its resistance to last-resort antibiotics, U.S. officials worry that the 'superbug' could have an affect on bacteria that is already difficult to treat with antibiotics.
The mcr-1 gene makes bacteria resistant to colistin, an antibiotic used to treat multi-drug-resistant infections, including carbapenem-resistant Enterobacteriaceae or CRE, which U.S. health officials have dubbed a "nightmare" bacteria.
What is concerning about the mcr-1 gene discoveries in the United States is that bacteria have the capability to share resistance genes. U.S. officials are worried that the mcr-1 gene may find its way into CRE bacteria, potentially creating bacteria resistant to virtually all types of antibiotics.
In the first U.S. case, the bacteria that carried the 'superbug' gene happened to be vulnerable to other antibiotics and was treatable.
Last year, researchers in China discovered the 'superbug' gene in both farm animals and humans. At the time of the initial report, the gene was known to have reached at least 19 countries.
Lance Price, a professor of environmental and occupational health at George Washington University, told Bloomberg last year that he was quite worried about the gene, saying the findings "sort of ruined my Thanksgiving."
Since the gene's discovery in China, researchers have tested over 20,000 E. coli and Klebsiella pneumoniae strains from all over the world and just 19 tested positive for the 'superbug' gene.
According to Reuters, antibiotic resistance causes at least 23,000 deaths annually.
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