The effects of small plastic particles on antibiotic resistance gene transfer revealed by single cell and community level analysis

Small plastic particles with sizes comparable to bacterial cells, widely exist in environment. However, their effects on antibiotic resistance gene (ARG) dissemination remain unclear. Using polystyrene (PS) particles (0.2 µm, 2 µm, 5 µm, 10 µm, 15 µm, 20 µm) as models, conjugative transfer of ARGs between the donor E. coli and different recipients (E. coli or sludge bacterial community) was investigated. Compared to the pure strain, the sludge bacterial community exposed to PS particles showed higher transfer frequencies (1.67 to 14.31 times the blank control). The transfer frequencies first decreased and then increased with particle size, and plastics similar in size to bacteria (e.g., 2 µm) appear to be a transitional zone with minimal impact on ARG transmission. Furthermore, using microfluidics, in-situ observation at single cell level found that 2 µm plastics can act as barriers between donor and recipient bacteria inhibiting growth, but conjugation events mostly occurred around them. Conversely, nanoplastics (e.g., 0.2 µm) and larger microplastics (e.g., 20 µm) significantly promote conjugation, mainly due to increased reactive oxygen species production and cell membrane permeability, or facilitating bacterial adhesion and biofilm formation, respectively. This study aids in assessing environmental risks of small plastic particles on ARG dissemination.