Comment on “Different microplastics distinctively enriched the antibiotic resistance genes in anaerobic sludge digestion through shifting specific hosts and promoting horizontal gene flow by Luo et al. [Water Research 228 (2023), 119356]”

Microplastics may enrich for resistance genes by shifting community composition but what can gene enrichments prove?

The recent publication by Luo et al. (2022) in Water Research reported the surprising and important finding that the addition of microplastics to an anaerobic digester resulted in an increase of the relative abundance of antibiotic resistance genes.

The authors identified a shift in the microbial community composition towards Proteobacteria and Bacteroidetes, primarily an increase in a particular Acinetobacter sp., as the cause of the increase in the relative abundance of antibiotic resistance genes. This study is thus a good demonstration that shifts in community composition can look like selection for resistance when the taxa that increased carry more resistance genes than the taxa that decreased, a point Luo et al. (2022) have clearly and admirably recognized.

Such ‘fake’ selection, observed in this study, can happen when groups in a community that differ in ecology also happen to differ in the carriage of resistance genes (Dai et al., 2022; Huijbers et al., 2019; Marano et al., 2021). In other words, if there is an association between ecological niche and resistance, any change in the environment that shifts the composition of the community can change the relative abundance of antimicrobial resistance genes without any selection for or against resistance. The study by Luo et al. (2022) nicely demonstrates that the addition of microplastics leads to an increase of Acinetobacter sp., presumably forming biofilms on the plastic surfaces, which is coupled with an increase in the relative abundance of resistance genes without any addition of antibiotics or other selective agents.