Ever since antibiotics were first discovered, a microscopic game of cat-and-mouse has been playing out across the world.

These powerful drugs kill many bacteria, but overuse of antibiotics can allow a few bacteria to become resistant.

Hardy bacteria that survive can pass on their resistance to future generations, and sometimes to other species of bacteria.

So scientists develop new antibiotics.

But some bacteria become resistant to those, too.

And so it goes.

Now bacteria are knocking on the door of the medicine cabinet that holds one of our last defenses against multidrug resistant bacteria.

Read more: Chemicals in dust linked to antibiotic resistance »

Pig farm discovery

On a small family-run pig farm in the United States, researchers discovered several species of bacteria, including E. coli, carrying a gene that makes them resistant to a class of last-resort antibiotics called carbapenems.

The good news is that no meat was contaminated and no people were infected.

Researchers found resistant bacteria mainly in samples taken from the farrowing operation — where pregnant pigs give birth. No resistant bacteria showed up in pigs being fattened for slaughter.

However, in an interview with Popular Science, study co-author Thomas Wittum, Ph.D., a professor in The Ohio State University College of Veterinary Medicine, expressed concern that bacteria carrying this gene could enter the food supply through contaminated fresh pork products.

A person exposed to resistant bacteria — such as by handling the raw meat — might not become sick right away.

But bacteria could colonize in their body, remaining at low numbers until another illness weakened the person’s immune system. People harboring these bacteria in their gut could also infect other people who are already sick.

The gene that provides the bacteria with antibiotic resistance sits on a small piece of DNA called a plasmid.

These are separate from the main DNA. Highly mobile, the plasmids can be easily transferred from one bacterium to another, and even to other species.

The type of plasmid identified in this study is also in a wide range of bacterial hosts.

The authors write that this is the first time that bacteria carrying a plasmid gene for carbapenem resistance have been reported in U.S. livestock populations.

The results were published December 5 in the scientific journal Antimicrobial Agents and Chemotherapy.

Read more: Drug-resistant bacteria common in children »

Surprises and questions

Bacteria can develop resistance to antibiotics by coming into contact with them — and surviving.

Which is why carbapenem resistant bacteria show up in hospitals, where these antibiotics are used to treat some of the toughest bacterial infections.

However, because carbapenems are so important to human health, they are not used in agriculture. So the pigs on the farm would not have been exposed to these antibiotics directly.

Which caught the researchers off guard.

"It's a surprise that they would show up in livestock," said Wittum in a press release.

The farm, which has about 1,500 adult female pigs, has been managed as a closed herd since the 1960s — meaning no pigs have been brought into the farm.

Wittum told NBC News that his group thinks the superbug may have been carried into the farm, although they can’t say if it was on people, equipment, or supplies.

But prophylactic dosing of pigs that are not sick — an FDA use of antibiotics known as “disease control” — may create an environment ripe for bacteria to develop resistance to antibiotics.

On this farm, the farmers do not give pigs carbapenem drugs. They also don’t administer antibiotics to promote growth of the pigs, a practice that has been condemned by many critics.

The pigs, though, do receive another antibiotic, ceftiofur, at birth. And then for males when they are castrated. This is common among U.S. swine production.

Ceftiofur is not a carbapenem, but both antibiotics belong to the same larger group of beta-lactam antibiotics.

Although it has not been shown, the authors think regular use of ceftiofur could push bacteria to develop resistance to carbapenems. Resistant bacteria, though, showed up mainly in the farrowing operation where ceftiofur is frequently used.

To protect the potency of the few remaining last-resort antibiotics in doctors’ medicine cabinets, advocacy groups such as the Natural Resources Defense Council have been pushing for the Food and Drug Administration (FDA) and the states to limit the overuse of antibiotics in the livestock industry.

In light of the results, the researchers write that farms should be monitored for bacteria resistant to carbapenem antibiotics.

And another look at how antibiotics are used in animals.

"We may need to examine some of the practices of farms and evaluate whether they are really appropriate,” said Wittum in the press release, “and whether the benefits outweigh the risks.”

Read more: How one salmonella strain became so powerful »