Adaptive responses of Bacillus subtilis underlie differential nanoplastic toxicity with implications for root colonization

Positively charged nanoplastics are more toxic to microorganisms than their negatively charged counterparts, prompting further investigation into their antimicrobial properties. While many studies have shown that positively charged nanoplastics bind to bacteria, the fate of these nanoplastic coatings during bacterial growth remains unclear. Here, we report how amine-modified polystyrene nanoplastics (PS-NH₂) reduce the viability of the plant growth-promoting rhizobacterium Bacillus subtilis and impair its ability to colonize plant roots. We found that upon exposure to PS-NH₂, the nanoplastics form stable, multilayer coatings on the surface of the bacteria. In response, B. subtilis initiates processes to remove these nanoplastics—a behavior heavily influenced by their growth environment, whether at air or liquid interfaces. Consequently, we observed differential toxicity under varying growth conditions. Using tomato plant as a model system, we found that these nanoplastics severely inhibit bacterial attachment to plant roots. Our results demonstrate that nanoplastics can disrupt beneficial interactions between soil bacteria and plants, potentially compromising the effectiveness of microbial biofertilizers. Given that current practices introduce large amounts of plastics into agricultural areas, the adverse effects of nanoplastic pollution need to be mitigated