TWENTY-FIFTH ANNUAL EASTERN FISH HEALTH WORKSHOP MARCH 10-13, 2000   Ultrastructural Interaction Between Mycobacterium Spp. And Peritoneal Mononuclear Phagocytes Of Striped Bass (Morone saxatilis)       David T. Gauthier, P. Mason, M. Rhodes, H. Kator, S.L. Kaattari, and W.K. Vogelbein   Dept. of Environmental Sciences, Virginia Institute of Marine Science, College of William and Mary, Gloucester Point, VA 23062, U.S.A.     Mycobacteriosis is a potentially important emerging disease of striped bass (Morone saxatilis) in the Chesapeake Bay.   It is characterized by formation of granulomatous lesions in the skin and viscera, and may lead to significant morbidity in affected fish.  Most details of piscine mycobacterial pathogenesis remain unknown, but survival of mycobacteria within professional phagocytes is thought to be an important factor, as it is in mammalian mycobacterial infections.   In this study, the in vitro interaction between mycobacteria and M. saxatilis peritoneal phagocytes was studied ultrastructurally.  Peritoneal cells were recovered from wild-caught fish and exposed to either Mycobacterium marinum, a known fish pathogen, or Mycobacterium gordonae, generally considered to be a nonpathogenic saprophyte. Cells were harvested at various times post-exposure and processed for transmission electron microscopy (TEM).  M. marinum was phagocytosed readily by peritoneal cells as early as 0.5 hr.  Bacteria were found within large and small phagosomes, the former being highly expanded.  Very little lysosome fusion with large phagosomes was observed, while fusion with smaller phagosomes was fairly common.  Bacteria appeared to be protected from lysosomal degradation by an electron transparent zone (ETZ), which may be associated with an outer cell wall layer. Disruption of infected phagocytes was observed after 2 hours, and was nearly total at 72 hours. Phagosomal escape by mycobacteria in viable, intact cells was not observed.  Interestingly, the interaction of the supposed non-pathogen M. gordonae with peritoneal phagocytes was similar to that of M. marinum.  Although ETZs were observed around intracellular M. gordonae, the presence of an additional layer was uncertain.  Bactericidal assays run in parallel to this ultrastructural work also support the conclusion that, under the conditions of this experiment, peritoneal phagocytes of striped bass were unable to kill M. marinum and M. gordonae.     Return to 25th Annual Eastern Fish Health Workshop Return to Leetown Science Center Home Page