It’s estimated that an average-sized wastewater treatment plant serving roughly 400,000 residents will discharge up to 2 million microplastic particles each day in its effluent. Yet, researchers are still learning the environmental and human health impact of these ultra-fine plastic particles, less than 5 millimeters in length, found in everything from cosmetics, toothpaste and clothing microfibers, to our food, air and drinking water.  

Now, researchers at New Jersey Institute of Technology have shown that ubiquitous microplastics can become "hubs" for antibiotic-resistant bacteria and pathogens to grow once they wash down household drains and enter wastewater treatment plants — forming a biofilm on their surface that allows pathogenic microorganisms and antibiotic waste to attach and co-mingle. 

In findings published in Elsevier’s Journal of Hazardous Materials Letters, researchers found certain strains of bacteria elevated antibiotic resistance by up to 30 times while living on microplastic biofilms that can form inside activated sludge units at municipal wastewater treatment plants. 

“A number of recent studies have focused on the negative impacts that millions of tons of microplastic waste a year is having on our freshwater and ocean environments, but until now the role of microplastics in our towns’ and cities’ wastewater treatment processes has largely been unknown,” says Mengyan Li, associate professor of chemistry and environmental science at NJIT and the study’s corresponding author. “These wastewater treatment plants can be hotspots where various chemicals, antibiotic-resistant bacteria and pathogens converge, and what our study shows is that microplastics can serve as their carriers, posing imminent risks to aquatic biota and human health if they bypass the water treatment process.”

“Most wastewater treatment plants are not designed for the removal of microplastics, so they are constantly being released into the receiving environment,” says Dung Ngoc Pham, NJIT Ph.D. candidate and first author of the study. “Our goal was to investigate whether or not microplastics are enriching antibiotic-resistant bacteria from activated sludge at municipal wastewater treatment plants, and if so, learn more about the microbial communities involved.”

In the study, the team collected batches of sludge samples from three domestic wastewater treatment plants in northern New Jersey, inoculating the samples in the lab with two widespread commercial microplastics — polyethylene and polystyrene. The team used a combination of quantitative polymerase chain reaction and next-generation sequencing techniques to identify the species of bacteria that tend to grow on the microplastics, tracking genetic changes of the bacteria along the way.

The analysis revealed that three genes in particular — sul1, sul2 and intI1 — known to aid resistance to common antibiotics, sulfonamides, were found to be up to 30 times greater on the microplastic biofilms than in the lab’s control tests using sand biofilms after only three days. 

When the team spiked the samples with the antibiotic, sulfamethoxazole, they found it further amplified the antibiotic resistance genes by up to 4.5-fold.