Microplastics as Vectors of Antimicrobial Resistance in Aquatic Systems

Microplastic pollution and antimicrobial resistance (AMR) are typically viewed as discrete issues, but there are unexplored dynamics between them. A bridging niche between these planetary threats is the ‘Plastisphere’: microbial communities associated with plastic debris. AMR pathogens may be enriched in Plastisphere communities; however, the unique threat that microplastics pose in supporting AMR remains unknown. To address this gap, successive in vitro, in situ and in vivo experiments are described within this thesis. A literature review (Chapter 1) first highlighted three hypotheses and several outstanding research questions, which dictated the experimental design of four data chapters. In Chapter 2, culture-based biofilm extraction methods were experimentally compared, with an optimised protocol recommended. This method was used in Chapter 3 to explore if different substrates vary in the communities they support; identifying that polystyrene and wood supported AMR faecal bacteria, and sewage-derived polyethylene bio-beads supported pathogenic Escherichia coli. All substrates from Chapter 3 were then incubated along a transect in situ, from highly polluted hospital wastewater to marine surface waters (Chapter 4). Metagenomic analyses identified over 100 unique AMR gene (ARG) sequences in microplastic samples, with an overall increase of ARG prevalence moving downstream across all samples - demonstrating the ability for AMR to persist in environmental settings within the Plastisphere. Polystyrene was again found to support AMR, alongside polyethylene nurdles. From this, it was proposed that microplastics could act as vectors for AMR within the food chain, where microplastic-attached communities may be accidentally ingested by filter-feeders of commercial importance, such as mussels. Chapter 5 addressed this hypothesis by exposing Mytilus edulis to a co-exposure of microfibres and sewage, finding that exposure to microfibres alone selected for faecal indicator organisms and AMR. Finally, Chapter 6 took a broader perspective and explored the ways that plastic raw material extraction, manufacture and use may also influence AMR. Future work should build on these findings so that evidence-based recommendations can be made to improve environmental monitoring and reduce reliance on potentially high-risk substrates for AMR selection, persistence and spread.