The thick fog rolling over San Francisco’s Golden Gate bridge is synonymous with the Bay Area. But more than an emblem for the coastal city, the fog may also represent a massive highway for genes that create dangerous, antibiotic-resistant “super bugs,” warn scientists in a study published in Environmental Science and Technology. The rise of super-bacteria through the overprescription and misuse of antibiotics is hard enough to control as it is, but now, it looks like they may be emerging out of the air.

The alarming spread of infections from super gonorrhea and Staphylococcus aureus shows just how bad the antibiotic-resistant bacteria problem has become. In January, the World Health Organization found that 500,000 people across 22 countries had antibiotic-resistant infections that were nearly untreatable. The reason these superbugs are so dangerous is because they carry certain antibiotic resistance genes (ARGs) that protect them from most drugs.

Usually, ARGs spread when one lucky superbug survives after a dosage of antibiotic drugs kills most of its comrades, allowing that superbug to multiply, creating its own colony of minions that share its superior genetic material. But the team behind the new study found that in these ARGs can spread a different way: They can become airborne, traveling from bacteria to bacteria around the world — creating new forms of superbugs.

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The team was surprised to find that cities with thick smog, like Beijing didn't actually have the highest amount of airborne ARGs. 

“ARGs could travel through air to remote regions or other places where antibiotics on the other hand are less used,” study author Maosheng Yao Ph.D. of Peking University’s College of Environmental Sciences and Engineering tells Inverse. “Common bacteria could become resistant to antibiotics when uptaking these ARGs.”

Airborne ARGs make Yao and his team nervous is because they represent bacteria’s second, more difficult-to-manage method of accumulating genes. Unlike animals that have to wait to give birth to pass their genetic material on to the next generation, bacteria can inject each other with genetic material, including ARGs. The effects are permanent because the genes get encoded in the DNA of the recipient bacteria.

This process, called horizontal transfer, is what makes airborne resistance such a threat. Air currents that circulate in urban environments swirl around millions of people every day, greatly increasing the possibility that a typical resistant bacteria might meet an antibiotic-resistant one.

antibiotic resistance cities
The cities in the world with the most diversity of ARGs in the air 

Globetrotting ARGs

The jarring observation raised two questions for Yao. What cities throughout the world are ARG tourism hotspots, and how do they get into the air in the first place?

To investigate this, the team sampled the level of bacteria in the air from 19 cities including San Francisco, Beijing, and Paris using a particularly ingenious method. They randomly selected cars from each city and took samples of particles that had accumulated on the inside of the AC filters, which have been shown to faithfully represent particles in outdoor pollutants. By scraping the filters themselves, the team had plenty of samples to screen for their ARGs.

Their screening revealed 30 different types of ARGs that make bacteria resistant to seven types of antibiotics.

antibiotic resistance cities

Beijing, China and Brisbane, Australia topped the list as the cities with the most diversity of ARGs in the air, but the types of ARGs in the air varied greatly from city to city. For instance, Melbourne’s air contained high levels of airborne genes that conferred resistance to drugs like penicillin and low levels of genes that conferred resistance to ciproflaxin, an antibiotic used to treat typhoid.

This analysis aside, one city topped the list with the highest concentrations of ARGs in their air: San Francisco. Although the city had less overall pollution in the air than particulate-dense cities like Beijing, Yao was surprised to discover that the Bay Area had more airborne ARGs than Paris, Beijing, and Singapore and more than 100 times more ARGs than the relatively small city of Bandung, Indonesia.

“For some cities with good air quality, we have detected higher abundances of ARGs, even including ARGs that are tailored to resist the most powerful antibiotics,” Yao explains.

Where did they come from?

Yao suggests that these genes originate in places like wastewater treatment plants, hospitals or animal feeding operations. Wastewater, specifically, is often treated with antibiotics that, if unsuccessful in killing all bacteria, could lead the survivors to evolve a host of resistance genes. Previous research has also shown wastewater is likely to be aerosolized — meaning that the particles present in the water can become airborne.

wastewater treatment
A wastewater treatment plant near Beijing China 

Whatever is driving the spread of antibiotic resistance, it’s time to double down on containing it, wrote researchers from the University of Exeter Medical School in an industry-funded mBio study on Tuesday. “So far a lot of research effort to tackle this problem has been around hospitals and reducing clinical prescribing, but we now know that the environment is likely to play a part in how resistance to antibiotics can evolve and spread,” said lead author William Gaze, Ph.D. “We all need to think more holistically about environmental management of waste, including how we treat our waste water.”

Even if the genes of those bacteria living in wastewater don’t eventually become airborne globetrotters floating in cities around the world, being better about keeping our sewage under control, Yao suggests, might be a good place to start. In the meantime, do the citizens of San Francisco a favor and stop flushing antibiotics down the toilet.