• Microplastics get washed down household drains and enter wastewater treatment plants.
• Bacteria then attach to these microplastics, creating a sludge that attracts more bacteria.
• A new study finds that this bacterial sludge contains genes that promote antibacterial resistance.

Scientists have demonstrated that the bacterial sludge that forms around microplastics in wastewater treatment plants contains genes that promote antibacterial resistance.

The research, which appears in the Journal of Hazardous Materials Letters, provides further evidence of the harmful effects that microplastics can have on human and environmental health.

Microplastics and environmental health

Plastics are one of the defining materials of the modern world. Plastics are so prevalent that researchers have suggested their widespread presence within global archaeological formations indicates a new global epoch: the Anthropocene.

There is currently much focus on whether or not microplastics — generally understood as smaller than 5 millimeters — pose a significant threat to human and environmental health.

Some researchers have suggested there is not yet enough evidence to know what health effects microplastics may have. However, they also recognize that their potential for damaging health is significant enough that further research is urgently needed.

Antibacterial resistance

Although research has typically focused on damage to ecosystems or human health, one area that has had less focus is the possible relationship between microplastics and antibacterial resistance.

According to a recent article by Prof. Zulqarnain Baloch and colleagues in the journal Infection and Drug Resistance, the development and use of antibiotics exploded between the 1930s and 1960s. They have saved countless lives since then, and they are still crucial to contemporary healthcare.

However, pathogens have adapted to some of the key antibiotics, developing a resistance that makes antibiotics less effective or not effective at all.

Although the development of some degree of antibacterial resistance is inevitable, a number of human-influenced factors are exacerbating it.

According to Prof. Baloch and colleagues, these include “overpopulation, enhanced global migration, increased use of antibiotics in clinics and animal production, selection pressure, poor sanitation, wildlife spread, and poor sewerage disposal systems.”

For the authors of the new study, it is at wastewater treatment plants that microplastics may play a key role in promoting antimicrobial resistance.

According to corresponding study author Dr. Mengyan Li, an associate professor of chemistry and environmental science at the New Jersey Institute of Technology (NJIT) in Newark, “[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.”

“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.”

For first study author Dung Ngoc Pham, an NJIT doctoral candidate, “[M]ost wastewater treatment plants are not designed for the removal of microplastics, so they are constantly being released into the receiving environment.”

“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.”

Analyzing two common microplastics

To determine to what extent microplastics might contribute to antimicrobial resistance, the researchers took samples of sludge from three domestic wastewater treatment plants in New Jersey.

In the laboratory, the team introduced polyethylene and polystyrene — two common microplastics — into samples of the sludge to which the bacteria could attach and create biofilms. They also used sand as a control material for a biofilm to form.

They analyzed the samples using quantitative polymerase chain reaction and next generation sequencing. This let them observe the growth of bacteria on the microplastics and how the genetic makeup of the bacteria changed over time.

Increases in 3 resistance genes

The researchers found varying results for the three genes associated with antibacterial resistance — sul1, sul2, and intI1 — depending on which microplastic they used: polyethylene or polystyrene. The results also depended on which wastewater treatment plant the sample came from.

Across the sample locations, polyethylene biofilms resulted in significant increases of almost all three resistance genes. Although polystyrene biofilms had fewer statistically significant results, this varied a lot by the location of the sample.

The researchers then added the antibiotic sulfamethoxazole, which increased the presence of antibacterial resistant genes by up to 4.5-fold.

According to Pham, “[P]reviously, we thought the presence of antibiotics would be necessary to enhance antibiotic resistance genes in these microplastic-associated bacteria, but it seems microplastics can naturally allow for uptake of these resistance genes on their own.”

“The presence of antibiotics does have a significant multiplier effect, however.”

Eight types of bacteria were highly enriched on the biofilm of the microplastics. These included Raoultella ornithinolytica and Stenotrophomonas maltophilia, which are linked to respiratory infections in humans.

The bacterium Novosphingobium pokkalii was the most common strain, and the researchers believe that it plays a key role in helping form the microplastic biofilm. Furthermore, they believe that the gene intI1 is important in enabling other genes that promote antibacterial resistance to exchange between bacteria.

As Dr. Li explains, “[W]e might think of microplastics as tiny beads, but they provide an enormous surface area for microbes to reside. When these microplastics enter the wastewater treatment plant and mix in with sludge, bacteria like Novosphingobium can accidentally attach to the surface and secrete glue-like extracellular substances.”

“As other bacteria attach to the surface and grow, they can even swap DNA with each other. This is how the antibiotic resistance genes are being spread among the community.”

The researchers now hope to investigate if and how bacteria can escape the processing at wastewater treatment plants.

For Dr. Li, “[S]ome states are already considering new regulations on the use of microplastics in consumer products.”

“This study raises calls for further investigation on microplastic biofilms in our wastewater systems and [the] development of effective means for removing microplastics in aquatic environments.”

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