A gene that helps bugs resist one of our last remaining effective antibiotics has been spreading around the world at an alarming rate, report researchers, and is being spotted in a growing number of animals and humans.

The gene is mcr-1, and it blocks the effectiveness of an antibiotic called colistin – an antibiotic of "last resort" that doctors have been turning to as bacteria develop resistance to most of our existing drugs.

Alarm bells first started ringing last year when scientists discovered that mcr-1 had found a way to hop between species of bacteria by attaching itself to the plasmid part of the cell, a small, circular DNA molecule.

Now it seems mcr-1 is definitely on the march, based on presentations made to a meeting of the American Society for Microbiology (ASM) in New Orleans this week.

One team of researchers found mcr-1 in 497 of 8,000 human stool samples taken over five years from Guangzhou in China, reporting that the number is on the rise.

What's more, 10 percent of the mcr-1 genes were also spotted in particular strains of the gut bacterium Escherichia coli, where were resistant to other antibiotics.

In another study, mcr-1 was found in 25 percent of patients at a Guangzhou hospital. The researchers also came across E. coli strains containing a gene called blaNDM-5, which can block the effectiveness of carbapenems, another of our "last resort" drugs.

None of this is good news – it shows bacteria are getting wiser to our antibiotic drugs, even those drugs that have stayed effective up until now.

Meanwhile, in a study of chickens farmed in Brazil, 60 percent of the 107 samples had E. coli strains carrying mcr-1. Another study in two randomly chosen farms in Portugal found the resistance gene in 98 out of 100 healthy pigs.

Across all these samples, researchers found evidence of mcr-1 in different plasmids and strains of bacteria, suggesting it's getting better at spreading – and the experts still aren't sure how it's happening.

Antibiotics have been saving us from infections for almost 100 years, killing bacteria without causing too much damage to our own cells.

The trouble is, the bugs are getting smarter, evolving through natural selection to find ways to beat the drugs we use, which is where "last resort" antibiotics like colistin come into the picture.

Colistin has remained effective because it's rarely been used on humans in the past – it's associated with a variety of kidney problems, which is why it's only used as a last resort. Farmers have been using it to promote growth in livestock instead, which is where colistin-resistant bacteria might have first appeared.

Now it seems colistin's time as an effective antibiotic is running out as well. Scientists are hoping the new studies act as a wake-up call to the dangers of overusing antibiotics with farm animals.

"It's a crappy drug and I think this is a sign of our desperation that we are so concerned about the loss of a toxic antibiotic," Lance Price, an antibiotic researcher at George Washington University who wasn't involved in the research, told Sara Reardon from Nature.

Meanwhile researchers around the globe are working hard to beat antibiotic resistance before it's too late. Earlier this year a molecule was developed that can reverse antibiotic resistance in several strains of bacteria at once, potentially giving us a new weapon in the fight against superbugs.

Another approach announced last year uses star-shaped polymers to break apart bacteria and kill it off in multiple ways. The next step in this research is to develop a similar system that's effective in humans as well as the lab.

With drug-resistant genes like mcr-1 spreading fast, let's hope we can find a way to fight back sooner rather than later.