Submitted to: Biomed Central (BMC) Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/1/2001
Publication Date: N/A
Interpretive Summary: A hallmark of mycobacterial disease is the ability of these bacteria to survive within host immune cells, called macrophages, designed to kill bacteria. In this study we evaluated growth and survival of Mycobacterium avium subspecies paratuberculosis (MAP) inside cultured macrophages. Data show that M. paratuberculosis is not growing within macrophages, and in fact, there is a slight decline in viability over time. We also show degraded intracellular forms of the bacteria within macrophages by electron microscopy. These results have strong implications in the pathogenesis of M. paratuberculosis, the etiologic agent of Johne's disease.
Technical Abstract: Mycobacterium avium subspecies paratuberculosis (M. paratuberculosis) is a facultative intracellular pathogen that resides within host macrophages during infection of ruminant animals. The objective of this study was to define the kinetics of M. paratuberculosis growth and survival within J774.16 macrophages. Serial plating of M. paratuberculosis infected macrophage lysates on Herold's egg yolk medium showed that mycobacterial replication takes place between 0 and 24 hours post-infection. This initial growth phase was followed by a slow decline in viability over the next six days. Electron microscopy demonstrated that initial intracellular bacterial growth was the consequence of simultaneous intracellular killing and replication of bacteria within macrophages as degraded mycobacteria were observed even at early times postinfection. Immunogold labeling of infected macrophages with antibody against M. paratuberculosis showed degraded intracellular mycobacteria that were unrecognizable by morphology alone. When macrophages were heavily infected with M. paratuberculosis, no degraded forms were observed and macrophages were killed, presumably by apoptosis. We present a general description of M. paratuberculosis survival within cultured macrophages using transmission electron microscopy and other techniques.