Introduction: Proteus mirabilis ( Pm ) has emerged as a significant and widespread opportunistic pathogen affecting both humans and animals, resulting in substantial economic losses within the agricultural sector. While most conventional antibiotics remain effective against Pm infections, the rise of multidrug-resistant strains has considerably complicated disease management. Outer membrane vesicles (OMVs), which are vesicular structures secreted by Gram-negative bacteria, have been identified in numerous studies as potential antigenic components or immune adjuvants for the development of novel vaccines. Methods: In this study, Proteus mirabilis outer membrane vesicles ( Pm _OMVs) were employed to immunize mice, followed by the execution of Pm infection assays. The immune responses elicited by Pm _OMVs and their protective effects in the Pm infection mice were evaluated using quantitative PCR, ELISA, and Western blot analyses. Results: Our findings demonstrated that Pm _OMVs stimulated B cells to produce specific antibodies and induced Th1/Th17-mediated immune responses. Following 24 hours of Pm infection, Th1 and Th17 cells in the Pm _OMVs-infected group were activated, releasing substantial levels of cytokines that facilitated rapid bactericidal activity. After 72hours of Pm infection, Th2 and Treg cells in this group were activated to suppress excessive inflammatory response and achieve tissue repair. Pm _OMVs also specificall increased the survival rate of mice infected with Pm , up to 80%. Discussion: These results suggest that Pm _OMVs can be used as an effective material to prevent Pm infection.

Klebsiella pneumoniae (K. pneumoniae) is an opportunistic bacteria that can result in severe liver abscesses, pulmonary damage, and potentially fatal outcomes. Research has demonstrated that the outer membrane vesicles (OMVs) released by it can provide significant protection to infected animals and may serve as a promising candidate antigen for the development of a novel vaccine. Nevertheless, the specific mechanisms through which OMVs mitigate the detrimental effects of K. pneumoniae infection by promoting the polarization pathways of macrophages and T helper cells (Th cells) remain poorly understood. In this study, we first confirmed that Klebsiella pneumoniae outer membrane vesicles (K. pneumoniae_OMVs) were protective in K. pneumoniae-infected mice, and then we investigated the protective mechanisms by transcriptome data analysis. Then, we constructed a model of in vitro macrophage polarization, an in vivo model for Th differentiation, and a K. pneumoniae infection model in K. pneumoniae_OMVs-immunized mice. qRT-PCR, IHC, Western blotting, and ELISA were used to confirm the polarization indicators. The results showed that K. pneumoniae_OMVs were able to provide specific protection for mice with a maximum protection rate of 80%. In addition, the results of a transcriptome analysis suggested that the protective mechanism might be related to Th cells and macrophage polarization. Mice immunized with K. pneumoniae_OMVs were able to achieve rapid bacterial clearance after K. pneumoniae infection through an M1/Th1 immune response. Subsequently, tissue repair was accomplished through Th2/M2 immune response in the late stage of K. pneumoniae infection to avoid causing inflammatory damage. This study offers a theoretical foundation for the K. pneumoniae_OMVs vaccine’s actual application.