Scientists tested lab-grown Escherichia coli biofilms in various environments, including on microplastics, on tiny glass fragments and in cell culture media. They found that pathogens in the plastisphere not only grew faster—reconfirming earlier research—but, importantly, were harder to kill when treated with several different antibiotics. The effects “were significantly larger than what we were expecting,” says Zaman, who was senior author of the study.

So how do antibiotic resistance genes arise on microplastics in the first place? A July study in Scientific Reports investigates one theory: antibiotics can cling to microplastics, too. The study’s authors showed that common antibiotics, such as amoxicillin and tetracycline, stuck to microplastics—and that, the older the microplastics were, the more readily the antibiotics attached to them. As they age, microplastics become rougher and more electrostatic, which makes them even better at trapping antibiotics. The combination of antibiotics and pathogenic bacteria biofilms on microplastics could theoretically drive the evolution of antimicrobial resistance.