A superbug that doctors dread because it contains a gene resistant to last resort antibiotic colistin has been found in the U.S.

The strain was first discovered in China. Researchers said that the bacteria may have developed resistance to colistin because pigs in China are always fed with the antibiotic. The bacteria was then passed to people who consumed the meat of the animals.

The U.S. Centers for Disease Control and Prevention (CDC) and the U.S Department of Agriculture (USDA) have also earlier found a colistin-resistant strain in one sample from a pig's intestine, but this is the first time that it was found in a person in the U.S.

The strain of E. coli bacteria was detected in the urine of a 49-year-old woman who initially sought treatment for urinary tract infection on April 26.

The discovery, which was reported in the journal Antimicrobial Agents and Chemotherapy on May 26, raises concern among public health officials because they do not know where the patient picked the mutant bacteria albeit the CDC is now working with health officials in Pennsylvania to know how the woman got the pathogen by checking her medical history.

The mutant E. coli strain has a gene known as mcr-1 that is resistant to colistin, the antibiotic of last resort used for dangerous types of superbugs that in some instances can kill up to half of infected patients.

The colistin-resistant gene can be copied and transferred between bacteria through plasmids, fragments of double-stranded DNA that can spread from one species of bacteria to another species of bacteria, making it possible for bacteria to become resistant without evolving.

The strain detected in the woman is still treatable using some other antibiotics, but experts fear the mcr-1 gene found in the bacteria can spread to other types of bacteria that are already resistant to other types of antibiotics.

Scientists fear of the possibility of an E. coli bacteria with mcr-1 gene passing to another drug-resistant bacteria with other mutations, as this can create a super-superbug that can resist currently used antibiotics.

"The emergence of a transferable gene that confers resistance to this vital antibiotic is extremely disturbing," said Patrick McGann, from Walter Reed Army Institute of Research.

"The discovery of this gene in the U.S. is equally concerning, and continued surveillance to identify reservoirs of this gene within the military healthcare community and beyond is critical to prevent its spread."