الملخص

Flagellin, the bacterial flagella structural protein, is one of the important pathogen-associated molecular patterns (PAMP) detected by Toll-like receptor 5 (TLR5), which stimulates innate immunity. The aim of this study was to examine the immunological and histopathological impacts of flagellin purified from nine pathogenic bacterial species on rat models. The successful purification of flagellin was confirmed by SDS-PAGE analysis with molecular weights between 35–45 kDa. Enzyme-linked immunosorbent assay (ELISA) revealed high strain-dependent variation in TLR5 levels, with Escherichia coli, Pseudomonas fluorescens , and Pseudocitrobacter faecalis evoking peak responses at day 7 postinjection, suggesting vigorous immune activation. While, Serratia marcescens and Salmonella enterica, produced lower TLR5 levels that can be suggestive of immune tolerance or receptor down regulation. Histopathology of mesenteric lymph nodes demonstrated variable inflammation patterns, including necrotizing lymphadenitis, granulomatous inflammation, and reactive hyperplasia, in accordance with the immunogenicity of each flagellin variant. Worth noting, environmental isolates such as Pseudomonas rhizosphaerae induced milder tissue reactions, reflecting lower pathogenicity. These results indicate the effect of flagellin origin on tissue pathology and host immunity dynamics. The research sets the therapeutic potential of selected flagellins as drugs for immunotherapy, and furthering our understanding of host-pathogen interaction. Further structural and functional investigations are required to identify the mechanisms behind these differential effects.

Conclusion: The integration of TRL5 immune response profiles and histopathological findings provides valuable information regarding the immunogenicity and tissue-specific pathogenicity of the nine bacterial strains that were studied. The ELISA analysis revealed that Escherichia coli, Pseudomonas fluorescens, and Pseudocitrobacter faecalis elicited the most intense TRL5 response, with levels increasing significantly by day 7 (4.98, 4.77, and 4.82 ng/mL, respectively). These pathogens also induced histopathological manifestations of acute immune activation: E. coli and P. fluorescens evoked necrotizing inflammation with lymphoid hyperplasia and vascular damage, while P. faecalis compared lymphoid depletion to focal hyperplasia, suggesting an ineffective but dysregulated immune activation. In contrast, Salmonella enterica and Serratia marcescens were manifested decreaseing TRL5 levels over time (3.56 and 3.74 ng/mL on day 7), as they had histopathological profiles marked by lymphoid depletion and extensive necrosis. This inhibition/inverse relationship with tissue damage indicates potential mechanisms of immune evasion, such as toxin-induced lymphocyte apoptosis or blocking of TLR5 signaling pathways. Strains like Burkholderia metallica and Bacillus cereus elicited modest TRL5 increases (4.32 and 4.26 ng/mL) reflecting their pyogranulomatous inflammation and toxin-mediated stromal remodeling, respectively. The control group's stable TRL5 levels (3.75–3.86 ng/mL) and absence of histopathological changes highlight the specificity of the reaction. Curiously, Bacillus rhizoplanae and Pseudomonas rhizosphaerae caused compartmentalized inflammation and vascular enlargement with minimal noticeable lymphoid depletion, mirroring their intermediate TRL5 levels (4.54 and 4.59 ng/mL).