Murine infection model for Mycobacterium fortuitum

Mycobacterium fortuitum is an atypical, non-tubercular, pathogenic, rapidly growing mycobacteria. As very little is known about its virulence determinants, the absence of an animal infection model was always sorely felt. A reliable and reproducible murine infection model has been developed in which non-replicating persistence of 105 CFU/g tissue in kidney was observed when a standardised dosage inoculum of 5 × 107 CFU was injected intravenously. The tissue bacillary load was determined at regular intervals (10, 25, 45 and 60 days post-inoculation) in different organs, viz., kidney, spleen, lung and liver. Histopathology of kidney revealed tissue damage and granuloma-like formations which appear to be part of the host's effort to combat the infection. As IFN-γ is known to trigger antimycobacterial effects of murine macrophages, IFN-γ was assayed to determine the correlation between host protective measures and bacillary load in kidney. Fifteen days after infection, the level of IFN-γ secreted by CD4+ and CD8+ T lymphocytes was high, concomitant with high tissue bacillary load, while the level sharply declined as the number of bacilli in kidney decreased 45 days post-inoculation. The invasion and proliferation of M. fortuitum ATCC 6841, when incubated with non-phagocytic (recombination activating genes (RAG) murine kidney) and phagocytic (murine peritoneal macrophages) cell lines, was assessed. M. fortuitum did not invade RAG murine kidney cell line, while the bacilli infected and proliferated freely inside murine macrophages. In conclusion, we present a reproducible murine infection model that sustains a persistent infection, the progression of which correlates meaningfully with host protective response.