Since the 1990s, a particular strain of salmonella that’s resistant to several antibiotics has emerged in several countries, including the United States.

The strain, Salmonella Typhimuriam DT104, has caused outbreaks in small numbers, most often linked with contaminated meat, such as beef or pork.

DT104 is resistant to five different antibiotics, one of which has been on the market for more than 80 years, according to U. S. Centers for Disease Control and Prevention (CDC).

While denied by some outside the scientific community, the broad use of antibiotics in food animals — for growth promotion and disease prevention — has helped increase the incidence of drug-resistant bacterial infections in humans.

The World Health Organization has declared the rise of drug-resistant bacteria as a global epidemic. In 2014, the CDC estimated these bacteria cause 2 million illnesses a year, 23,000 of them fatal.

New research helps confirm that the more hosts a strain of bacteria can inhabit, the more chances it has to develop drug resistance, increasing its likelihood of sickening — and even killing — humans.

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Resistance from the 1970s

In a study published today in the journal Applied and Environmental Microbiology, researchers performed genetic testing on samples from salmonella victims from 1969 to 2012, in 21 countries on six continents.

Researchers tracked the rate of mutation in DT104 and estimate it first emerged in 1948 and may have acquired the genetic elements needed to carry and transfer resistance to antibiotics in 1972. It wouldn’t be reported as multi-drug resistant for another 12 years.

Pimlapas Leekitcharoenphon, PhD., a postdoctoral researcher at the Research Group for Genomic Epidemiology, National Food Institute, Technical University of Denmark, Lyngby, says DT104 was able to infect multiple livestock species, including poultry, cattle, pigs, and sheep.

“Having multiple hosts increases the chances of dissemination,” she said in a press release. “If we know and understand the past, we might be able to solve the current resistance problems and prevent future ones.”

But salmonella isn’t the only common bacteria that’s developing impenetrable defenses against modern antibiotics.

Previous research from the CDC has found a “significant upward trend” of E. coli — another common food bug — that is resistant to three or more antibiotics. The rate resistance increased from 2.7 percent in the 1950s to 63 percent in the 2000s.

According to FDA estimates, 70 percent of all antibiotics sold in the U.S. end up in animals that wind up on our plates. Once touted for growth promotion, these antibiotics are now given through feed and water for disease prevention.

While the FDA and other agencies work to protect the usefulness of antibiotics in human medicine, major food producers — such as Chipotle, Subway, and In-N-Out Burgers — are now incorporating antibiotic-free meat on their menus and in marketing messaging.

Read More: E. Coli and Salmonella Infections Are Down, but Other Food-Borne Illnesses Are Up »

This Little Antibiotic Goes to Market

While the first antibiotic was released onto the market in 1911, it took another 40 years for the drug to really take off. During the 1950s, ‘60s, and 70s roughly 50 antibiotics received approval from the U.S. Food and Drug Administration (FDA) within those three decades.

Three of the five drugs DT104 is resistant to — ampicillin, chloramphenicol, streptomycin, sulphonamide, and tetracycline — were the first in their class and were discovered between 1935 and 1961.

Ampicillin is the latest to be discovered and to which DT104 has become immune. It’s most often used to treat pneumonia, bronchitis, and ear, lung, and skin infections. This broad-spectrum antibiotic, however, remains effective against common hospital-acquired infections, such as catheter-associated urinary tract infections.

Expert say the less it’s used the better as the more opportunities bacteria are exposed to antibiotics in non-lethal amounts, like when a patient doesn’t take their full course of antibiotics, the more likely it’s to develop resistance.

Read More: Use of Antibiotics in Agriculture Expected to Skyrocket »

New Antibiotic Development Given a Boost

With current antibiotics losing their effectiveness to these potentially lethal bacteria, the discovery of new antibiotics are needed more than ever.

In the face of an epidemic, many major pharmaceutical companies have backed out of antibiotic development because they’re largely unprofitable drugs.

To spur the discovery and development of new antibiotics, new FDA guidelines give drug makers longer patent protection and fast-track approval options to help recoup expenses.

In January, 85 drug companies and nine industry associations laid out steps for helping to make antibiotic development financially viable and save current ones through an increase in rapid diagnostic testing. This will ensure the right drugs are used against the right bugs.

Among the provisions, it also seeks to remove kick-backs to doctors, veterinarians, and pharmacists who prescribe antibiotics in high volume.

Until new drugs hit the market, medical professionals, as well as those in the agriculture industry, are urged to be judicious with their antibiotic usage. The FDA has issued voluntary guidelines for the meat industry in efforts to slow antibiotic resistance.

Legislation to mandate antibiotics are only used in sick animals has been met with resistance from agriculture and veterinary groups.

One bill, the Preservation of Antibiotics for Medical Treatment Act, has been submitted to Congress more than five times, never making it out of committee.