Universal microplastics can become ‘centers’ for antibiotic-resistant bacteria
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- Source: New Jersey Institute of Technology
- Date: 22 Mar,2021
It is estimated that an average-sized wastewater treatment plant serving approximately 400,000 inhabitants will discharge up to 2,000,000 microplastic particles to the environment every day.
Yet, researchers are still learning the ecological and human health impact of these ultra-fine plastic particles, less than 5 millimeters in length, found in everything from makeup, toothpaste and clothes microfibers, to our food, air and drinking water.
Currently, researchers at New Jersey Institute of Technology have demonstrated that ubiquitous microplastics can become’hubs’ for antibiotic-resistant bacteria and pathogens to grow once they wash down household drains and input wastewater treatment plants — forming a slimy layer of buildup, or biofilm, on their surface which allows pathogenic microorganisms and antibiotic waste to attach and comingle.
In findings published in Elsevier’s Journal of Hazardous Materials Letters, researchers found certain strains of bacteria elevated antibiotic resistance up to 30 times while residing on microplastic biofilms that can form inside activated sludge units at municipal wastewater treatment plants.
“All these wastewater treatment plants may be hotspots where various substances, antibiotic-resistant bacteria and pathogens converge and what our research shows is that microplastics can serve as their carriers, posing imminent risks to aquatic biota and human health if they bypass the water treatment procedure.”
“Most wastewater treatment plants are not designed for the elimination of microplastics, so they are constantly being released into the receiving environment,” added Dung Ngoc Pham, NJIT Ph.D. candidate and first author of the study.
“Our aim was to investigate whether microplastics are advancing antibiotic-resistant bacteria from activated sludge at municipal wastewater treatment plants, and if so, find out more about the microbial communities involved.”
In their analysis, 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 industrial microplastics — polyethylene (PE) and polystyrene (PS).
The group used a combination of quantitative PCR and next-generation sequencing techniques to identify the species of bacteria that tend to grow on the microplastics, tracking genetic changes of the bacteria on the way.
The analysis demonstrated that three genes in particular — sul1, sul2 and intI1– known to assist resistance to common antibiotics, sulfonamides, were found to be up to 30 times greater on the microplastic biofilms than in the lab’s control evaluations using sand biofilms after only three days.
After the team analyzed the samples with the antibiotic, sulfamethoxazole (SMX), they found it further afield the antibiotic resistance genes by around 4.5-fold.
“Previously, we believed the presence of antibiotics could be necessary to boost antibiotic-resistance genes in these microplastic-associated bacteria, but it appears microplastics can naturally allow for uptake of these resistance genes on their own.” said Pham. “The presence of antibiotics does have a significant multiplier effect nonetheless.”
Among these species, the team observed two emerging human pathogens typically connected with respiratory infection, Raoultella ornithinolytica and Stenotrophomonas maltophilia, frequently hitchhiking on the microplastic biofilms.
The group say the most common strain found on the microplastics by far, Novosphingobium pokkalii, is probably a key initiator in forming the sticky biofilm that attracts such compounds — because it proliferates it may contribute to the deterioration of the plastic and enlarge the biofilm. At exactly the same time, the team’s study emphasized the role of the gene, intI1, a mobile genetic element chiefly responsible for enabling the exchange of antibiotic resistance genes among the microplastic-bound microbes.
We 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.”
Mengyan Li, Study Corresponding Author and Associate Professor of Chemistry and Environmental Science, New Jersey Institute of Technology
“We have proof that the bacteria developed resistance to other antibiotics such as also, such as aminoglycoside, beta-lactam and trimethoprim,” additional Pham.
Now, Li says the laboratory is further studying the function of Novosphingobium in biofilm formation on microplastics. The team is also trying to better understand the degree to which such pathogen-carrying microplastics could be bypassing water treatment processes, by analyzing resistance of microplastic biofilms during wastewater treatment with disinfectants such as UV light and chlorine.
“Some countries are already considering new regulations on the use of microplastics in consumer products. This study raises calls for additional investigation on microplastic biofilms within our wastewater systems and development of effective means of removing microplastics in aquatic environments,” said Li.
Pham, D. N., et al. (2021) Microplastics as hubs enriching antibiotic-resistant bacteria and pathogens in municipal activated sludge. Journal of Hazardous Materials Letters. doi.org/10.1016/j.hazl.2021.100014.