Silver nanoparticles resistance in Listeria monocytogenes: morphologic, virulence and cellular response

Bacterial antibiotic resistance poses a growing global health threat. To address this, silver nanoparticles (AgNPs) have been extensively applied for their broad-spectrum antibacterial activity. However, the recent emergence of AgNPs resistance in bacteria presents a critical challenge, as the underlying resistance mechanisms remain poorly understood. In this study, spherical citrate-capped AgNPs were synthesized with an average particle size of approximately 12 nm. Listeria monocytogenes developed resistance to AgNPs (L.M_AgNPs) after repeated exposure, and the underlying mechanisms were investigated through phenotypic and genetic changes. The results showed that the hemispherical poles of rod-shaped cells became sharply tapered after long-term exposure to AgNPs, and the cell wall thickness in L.M_AgNPs increased by approximately 17.4 % compared to wild type strain, with p ˂ 0.0001. Differential gene expression analysis showed that the relative expression of virulence genes was significantly down-regulated in L.M_AgNPs (padj ≤ 0.05). Some genes were barely expressed upon exposure to AgNPs, and this was further validated by qRT-PCR and hemolysis assay. The gliding motility of L.M_AgNPs was significantly increased. Based on these studies, we emphasize that the mechanism of AgNPs resistance in L. monocytogenes may be mediated by the sharp-tapered poles of resistance strain, which make them more rigid, and by altered virulence responses and enhanced gliding motility, which are distinct from the reported AgNPs resistance. This work not only advances the understanding of the relationship between antibacterial stress responses and virulence in pathogen, but also facilitates safer utilization of AgNPs by clarifying bacterial resistance mechanisms.