Sustained neutrophil infiltration and bacterial grain morphology underlie chronic mycetoma pathology in a murine model.

OBJECTIVES: This study aimed to characterize the progression of chronic mycetoma caused by Nocardia brasiliensis in a BALB/c murine model, focusing on the interplay between host cellular immune responses, bacterial burden, and histopathological evolution. MATERIALS AND METHODS: BALB/c mice were inoculated with N. brasiliensis in the left hind footpad to establish the mycetoma model. The mice were divided into four experimental groups: 0, 70, 100, and 365 days post-infection (dpi). Lesion volume was assessed throughout the course of infection. At the defined time points, bacterial load (serial dilution method), percentages of immune cell populations (flow cytometry), serum cytokines (interleukins IL-6, IL-10, and IL-12p70, monocyte chemoattractant protein-1 (MCP-1), interferon-gamma (IFN-γ), and tumor necrosis factor (TNF)) via cytometric bead array (CBA), as well as histopathology and bacterial grain morphology (H&E staining), were evaluated. RESULTS: Chronic mycetoma progression revealed stable bacterial burden and lesion volume stabilization after 70 dpi through 365 dpi. Systemic expansion of CD4+ T cells in the spleen and sustained neutrophil dominance (>90% infiltration) characterized chronic lesions. Progressive tissue necrosis and panniculitis, undetectable by external lesion size, emerged histologically. Serum IL-6 levels surged during chronicity, suggesting a Th17 polarization, contrasting with declining MCP-1. Bacterial grains transitioned from club-shaped to circular over time, suggesting structural grain remodeling. CONCLUSION: In chronic experimental mycetoma, the cell response is mainly characterized by neutrophil infiltration, an altered CD4+ T cell response, and dysregulated cytokine production. The shape of bacterial grains continues to change, and the bacterial load remains constant.