Antibiotics were one of the most important scientific developments of the 20th century, helping to easily control bacterial infections and make previously life-threatening procedures and illnesses safe. But inversely, they might also be one of the biggest medical issues of the 21st century, as bacteria evolve resistances to our best drugs. Now, a bacterial gene that grants resistance to "last resort" antibiotics has been detected in a patient in the US, for the first time.

Salmonella is a bacteria commonly associated with cases of food poisoning, and while infections are often just a case of waiting it out, it can be more severe in young or old people, or those with compromised immune systems. In those cases, antibiotics are often prescribed.

The problem is that like many other bacteria, Salmonella has developed resistance to most of those drugs. All except colistin, an antibiotic that's largely considered the last resort against bacteria like these. And now, it seems like even that won't hold up for much longer.

A gene known as mcr-3.1 is known to give bacteria like Salmonella the ability to resist colistin, making it essentially untreatable by any current antibiotics. This gene has been on a watch list for years, and now it seems to have hit the shores of the United States for the first time.

"Public health officials have known about this gene for some time," says Siddhartha Thakur, corresponding author of the study. "In 2015, they saw that mcr-3.1 had moved from a chromosome to a plasmid in China, which paves the way for the gene to be transmitted between organisms. For example, E. coli and Salmonella are in the same family, so once the gene is on a plasmid, that plasmid could move between the bacteria and they could transmit this gene to each other. Once mcr-3.1 jumped to the plasmid, it spread to 30 different countries, although not – as far as we knew – to the US."

The gene was discovered during routine monitoring designed to keep an eye out for new strains of drug-resistant bacteria. Genome sequencing was conducted on the microbes in 100 clinical human stool samples, taken from people in the southeastern US between 2014 and 2016. The mcr-3.1 gene was found in one sample, from a person who had contracted a Salmonella infection after traveling to China.

Worse still, the sample was from 2014, meaning the gene could be widespread across the country by now. It may even have spread to the more dangerous, related bacteria E. coli.

While this news sounds like another step towards the terrifying future warned by some reports, in which humanity returns to "the dark ages of medicine," there is hope. New antibiotics are always in development, as are other technologies like antibacterial lights and materials. Older drugs could even become useful again, by pairing them up or using other molecules to knock out the bugs' resistance to them.

The new research was published in the Journal of Medical Microbiology.