Abstract
<title>Abstract</title> <p> <bold>Background:</bold> Interleukin-6 (IL-6) is a key pro-inflammatory cytokine involved in the early immune response to bacterial infections. The magnitude of IL-6 production is influenced by multiple factors, including bacterial characteristics, microbial burden, and host immune responsiveness. Although cytokine responses to bacterial pathogens have been widely investigated, limited information is available on how increasing bacterial inoculum affects IL-6 production following stimulation with clinically relevant bacterial isolates obtained from routine patient specimens. Understanding these dose-dependent responses may improve characterization of early host–pathogen interactions and provide baseline information for future investigations of inflammatory biomarker regulation during bacterial infection. <bold>Results:</bold> Clinical bacterial isolates demonstrated heterogeneous IL-6 response profiles across the tested inoculum concentrations. Under the experimental conditions seven of thirteen isolates showed progressive increases in IL-6 concentration across the three bacterial concentrations, while six demonstrated variable trajectories. When the lowest and highest inocula were compared, twelve isolates showed equal or higher IL-6 concentrations at 1 × 10⁸ CFU/mL. The largest fold increases were observed for <italic>Citrobacter freundii</italic> (3.29-fold), <italic>Proteus vulgaris</italic> (2.24-fold), and <italic>Pseudomonas aeruginosa</italic> (2.20-fold), whereas <italic>Proteus mirabilis</italic> showed minimal overall change (0.99-fold). <bold>Conclusion:</bold> Exposure of human whole blood to clinical bacterial isolates resulted in variable but generally dose-dependent IL-6 responses in an ex vivo stimulation model.. Increasing bacterial concentration was associated with enhanced cytokine production, although the magnitude of response differed among bacterial species. These findings provide descriptive evidence that bacterial burden influences early inflammatory responses and support the application of ex vivo whole blood stimulation models for investigating host-pathogen interactions. </p>