Plastic is a cheap and durable material that has been adapted and applied in innumerable ways. Vast amounts of plastic has been produced in recent decades, mainly for stuff that quickly gets thrown away. And all of that plastic breaks down into smaller and smaller pieces. Microplastics, which are under 5 millimeters in length, can be found almost everywhere, even the human brain and the air.
Research has also shown that microplastics can help bacteria move around, and can bring drug-resistant bacteria to new places, expose them to other cells, and help them spread that resistance. But now scientists have also found that microplastics themselves may actually promote antimicrobial resistance. The findings have been reported in Applied and Environmental Microbiology.
In this study, researchers grew Escherichia coli bacteria, which is often used in non-pathogenic forms in the science lab as biofilms or in liquid media. Some were grown along with different types, levels, and sizes of microplastics including polypropylene and polyethylene. As the E. coli grew, the researchers exposed some of them to one of four types of antibiotics that are often found in the environment, like ampicillin, doxycycline, and streptomycin. Other biofilms were left unexposed.
They monitored the bacteria, and found that within ten days, the biofilms that were exposed to microplastics had higher resistance to antibiotics than bacteria that were not grown with microplastics.
The investigators also stopped exposing the bacteria to antibiotics, and then grew bacteria without antibiotics for five days. They determined that E. coli grown along with antibiotics and microplastics still had high levels of antibiotic resistance with some even becoming more resistant to antibiotics. E coli that grew with polystyrene also had higher levels of resistance compared to bacteria grown with polyethylene and polypropylene.
The scientists concluded that microplastics increased the bacteria's ability to resist almost all of the antibiotics.
"Our findings reveal that microplastics actively drive antimicrobial resistance development in E. coli, even in the absence of antibiotics, with resistance persisting beyond antibiotic and microplastic exposure," said first study author Neila Gross, a graduate student at Boston University.
Additional work showed that when E. coli was grown with microplastics, more biofilm formed. Scientists have previously shown that biofilms help bacteria take on new characteristics that make them more tough and tenacious. Biofilms help spread antibiotic resistance through a cellular population and make them harder to eliminate with disinfectants.
This research is particularly important because the antibiotic resistance problem is getting worse, and is expected to cause the deaths of tens of millions of people annually in coming decades. The plastic pollution problem is also getting worse, with no clear solution or end in sight. The study authors noted that these issues are a major challenge for countries where resources are low; there are high rates of bacterial infection; poor access to clean water; and high amounts of plastic waste.
Sources: CIDRAP, Applied and Environmental Microbiology
