An antibiotic-resistant strain of E. Coli that has been called the "superbug" has been reported for the second time in the U.S., in news that is causing concern for medical experts.

The findings were reported this week in a new issue of the Antimicrobial Agents and Chemotherapy, ABC reported.

This type of E. Coli has a gene known as mcr-1, which is resistant to most known antibiotics.

The strain was first detected in the U.S. in May on a woman from Pennsylvania with a urinary tract infection, according to the Los Angeles Times.

Researchers write in the paper that the second report of the superbug comes from a sample taken in Iowa more than a year ago.

Experts are concerned because this strain of E. Coli is not responding to colistin, the most common antibiotic to fight the virus.

"It's basically a wake-up call," Dr. Frank Esper of University Hospitals Case Medical Center told ABC News. "It's only going to be a matter of time where the perfect storm happens. ... Next thing you know, you throw your hands up and say we're out of ammunition" to fight certain infections.