A newstudy by researchers at Boston University indicates the presence of microplastics in the environment might facilitate antimicrobial resistance (AMR).
The study, published today inApplied and Environmental Microbiology, adds a new layer to concerns about the role that the extremely small (less than 5 millimeters in length) pieces of plastic, which are ubiquitous in the environment, play in promoting the emergence and spread of drug-resistant bacteria. While previous studies have shown how microplastics can serve as vehicles on which bacterial communities can form and share resistance genes, this study suggests there's an interaction occurring between the microplastics and bacteria that influences the development of AMR.
"This challenges the notion that microplastics are merely passive carriers of resistant bacteria and highlights their role as active hotspots for antimicrobial resistance evolution," lead study author Neila Gross, a Boston University PhD candidate, said in apress releasefrom the American Society for Microbiology.
Microplastics 'actively drive' development of resistance
To investigate the interaction between bacteria and microplastics, Gross and her colleagues exposedEscherichia coli bacteria grown in liquid media to varying concentrations of different sizes and types of microplastics, including polyethylene, polystyrene, and polypropylene, until biofilm growth was detected. They then added subinhibitory levels of four different antibiotics commonly found in the environment—ampicillin, ciprofloxacin, doxycycline, and streptomycin—and tested for antibiotic susceptibility every 2 days.
For comparison, the researchers tested antibiotic susceptibility inE coli grown without exposure to microplastics. They also measured the minimum inhibitory concentration (MIC, the amount of antibiotic needed to kill bacteria) in cells grown with a single subinhibitory antibiotic with or without microplastics.
Within 10 days, they found that exposure to microplastics led to increased resistance to all four antibiotics compared withE coli grown in liquid media without microplastics.
"Our results suggest that the addition of MPs [microplastics] led to an increase in AMR for nearly all antibiotics," the researchers wrote. "In each case where bacteria were grown and tested in the same antibiotic, the addition of MPs to antibiotics in the media led to an MIC increase of at least five times more compared to cells grown in the antibiotics alone."
Furthermore, when the antibiotic exposure was halted and the bacteria were grown in antibiotic-free media for 5 days, theE coli that had been grown with antibiotics and microplastics retained the same resistance level, and some even gained resistance. Additional analysis showed thatE coli grown with concentrations of polystyrene developed higher levels of resistance compared with polyethylene and polypropylene.
This challenges the notion that microplastics are merely passive carriers of resistant bacteria and highlights their role as active hotspots for antimicrobial resistance evolution.
"Our findings reveal that microplastics actively drive antimicrobial resistance development inE. coli, even in the absence of antibiotics, with resistance persisting beyond antibiotic and microplastic exposure," Gross said.
To better understand why resistance was higher in the samples containing microplastics, particularly polystyrene, the researchers used a specialized form of standard fluorescence microscopy to visualize the surface of the particles. What they found was that theE coli samples grown with microplastics had significantly more biofilm growth than those grown without microplastics.
"Overall, these data suggest that the presence of MPs select for better biofilm formers, and these cells display increased resistance," they wrote.
An environmental and public health threat
Gross and her colleagues say the findings are important because global plastic use has risen 20-fold since 1964 and microplastics have infiltrated ecosystems throughout the planet. At the same time, there is increased recognition of the role that environmental contamination from antibiotic residues, particularly in wastewater in low- and middle-income countries, plays in the spread of AMR.
As a result, they argue, the role of microplastics in AMR development makes them both an environmental and public health threat. In particular, they suggest that understanding how microplastics, bacteria, and antibiotics interact in low-resource countries—where infection rates are high, wastewater treatment is poor, and there is significant plastic waste—could be critical for efforts to address the growth and spread of drug-resistant pathogens that pose a threat to human health.
"The difficulty in treating infectious diseases in these areas, combined with inadequate wastewater treatment—which may result in higher concentrations of MPs—may contribute to the observed increase in AMR cases among vulnerable populations," they wrote. "Therefore, understanding the fundamental interactions between MPs and AMR development is imperative."