Microplastics could be contributing to increased antimicrobial resistance (AMR) and multi-drug resistance in foodborne pathogens like Escherichia coli, suggests a recent study by Boston University (BU) researchers. The study was published in Applied and Environmental Microbiology. 

Aside from the health implications of ingesting plastic-contaminated food and water, a growing body of literature has shown that microplastics could accommodate biofilm communities, chemical contaminants, and AMR genes. The ubiquitous presence of microplastics in the environment is becoming an issue of increasing concern, with studies showing its presence in a diverse range of foods. Scientists have even found microplastics in the human body.

Additionally, the emergence of AMR in foodborne pathogens is a growing public health crisis, with the World Health Organization (WHO) recognizing antibiotic resistance as one of the top ten health challenges facing humanity in the 21^st century. According to WHO, an estimated 4.95 million deaths are caused by antimicrobial-resistant infections each year, globally.

The latest study from BU has augmented the current understanding of the interplay between microplastics and AMR, by showing how these ever-present plastic particles can facilitate biofilm formation, leading to increasingly resilient pathogens. Overall, the researchers found that the presence of microplastics directly correlated with an increase in the rate of development and magnitude of biofilm-associated AMR in E. coli.

“We found that the biofilms on microplastics, compared to other surfaces like glass, are much stronger and thicker,” remarked Neila Gross, a BU Ph.D. student and study author. She called the consistently and significantly higher rate of AMR on microplastics, in comparison to other materials, “staggering.”

“We are demonstrating that the presence of plastics is doing a whole lot more than just providing a surface for the bacteria to stick—they are actually leading to the development of resistant organisms,” added Muhammad Zaman, Ph.D., Professor and Vice Chair in the BU Department of Biomedical Engineering.

For their study, the scientists exposed E. coli to varying concentrations of different types of microplastics, including polyethylene, polystyrene, and polypropylene, in sizes ranging from 3–500 micrometers (µm). Notably, the researchers found that, in the presence of microplastics, E. coli had increased resistance to all of the antibiotics tested—ampicillin, ciprofloxacin, doxycycline, and streptomycin—when compared to media containing no microplastics.

Moreover, when samples were tested for AMR stability over time, 81 percent of the bacteria grown in a media containing both microplastics and antibiotics retained—or even gained—levels of resistance, despite exposure to antibiotics being halted for five days before testing. Of the E. coli grown in the presence of microplastics alone, 44 percent retained or grew in AMR, while only 19 percent of bacteria grown in antibiotics alone retained their levels of AMR.

The researchers also tested the effect of microplastic characteristics on AMR, and found no significant difference in resistance based on the size or concentration of plastic particles. The plastic composition was found to affect AMR, however, with increased resistance seen in the presence of polystyrene. Additionally, when compared to glass particles at the same concentration and size, polystyrene microplastics were also seen to facilitate higher levels of AMR and biofilm development, indicating that plastics may be a unique substrate for bacteria to develop and maintain drug-resistance.

“Plastics are highly adaptable,” said Ms. Gross, explaining that, although the molecular composition of microplastics could help bacteria flourish, the exact mechanism is not yet understood. A theory is that, while the generally hydrophobic nature of microplastics allow bacteria to attach more easily, when plastics do start to absorb moisture over time, it could allow microplastics to absorb antibiotics before reaching the target bacteria.

Aside from Ms. Gross and Dr. Zaman, authors on the study included BU’s Johnathan Muhvich, Ph.D. candidate; Carly Ching, Ph.D.; Bridget Gomez; Evan Horvath; and Yanina Nahum, Ph.D. Their research was supported by the National Science Foundation.