Background
Lactobacillus plantarum (L. plantarum) is a probiotic bacterium with diverse health-promoting effects. Recent evidence suggests that these benefits are mediated by extracellular vesicles (EVs) secreted by the bacterium; however, the underlying molecular mechanisms in the context of pathogenic inflammation remain poorly understood.

Results
In this study, we employed super-resolution stochastic optical reconstruction microscopy to elucidate the molecular mechanisms of action of L. plantarum EVs by monitoring alterations in the ultrastructural integrity of cellular organelles in human dermal fibroblasts exposed to Staphylococcus aureus EVs. Pathogenic EV exposure induced characteristic inflammatory changes in organelle morphology. Remarkably, both pre- and post-treatment with L. plantarum EVs restored organelle morphology in a concentration- and time-dependent manner. Cytokine profiling showed selective suppression of interleukin (IL)-6 and IL-8 while preserving IL-10, indicating targeted immunomodulation. We identified nicotinamide adenine dinucleotide (NAD⁺) as a key bioactive cargo, with exogenous NAD⁺ treatment reproducing both structural and cytokine-restorative effects.

Conclusions
These findings establish NAD⁺-mediated organelle protection as a central mechanism through which probiotic EVs mitigate bacterial inflammation. By linking organelle integrity to inflammatory outcomes, our study highlights L. plantarum EVs as nanoscale therapeutic candidates for infection-driven inflammation.