An emerging epizootic of mycobacteriosis currently threatens striped bass Morone saxatilis populations in Chesapeake Bay, USA. Several species of mycobacteria, including Mycobacterium marinum, species resembling M. avium, M. gordonae, M. peregrinum, M. scrofulaceum and M. terrae, and the new species M. shottsii have been isolated from diseased and healthy bass. In this study, we describe the ultrastructure of developing M. marinum granulomas in experimentally infected bass over a period of 45 wk. The primary host response to injected mycobacteria was formation of large macrophage aggregations containing phagocytosed bacilli. M. marinum were always contained within phagosomes. Close association of lysosomes with mycobacterial phagosomes, as well as the presence of electron-opaque material within phagosomes, suggested phagolysosomal fusion. Development of granulomas involved epithelioid transformation of macrophages, followed by appearance of central necrosis. Desmosomes were present between mature epithelioid cells. The necrotic core region of M. marinum granulomas was separated from overlying epithelioid cells by several layers of flattened, electron-opaque spindle-shaped cells. These cells appeared to be formed by compression of epithelioid cells and, aside from a flattened nucleus, did not possess recognizable organelles. Following the development of well-defined, paucibacillary granulomas, secondary disease was observed. Recrudescence was marked by bacterial replication followed by disruption of granuloma architecture, including loss of epithelioid and spindle cell layers. In advanced recrudescent lesions, normal tissue was replaced by macrophages, fibroblasts, and other inflammatory leukocytes. Large numbers of mycobacteria were observed, both intracellular and suspended in cellular debris. KEY WORDS: Mycobacteriosis · Striped bass · Morone saxatilis · Mycobacterium marinum · Granuloma · Ultrastructure Resale or republication not permitted without written consent of the publisherDis Aquat Org 62: [121][122][123][124][125][126][127][128][129][130][131][132] 2004 body granulomas are composed of a solid mass of epithelioid cells and giant cells, with the eliciting agent often visible in the center. The other major granuloma type, the immune granuloma, is formed in response to insoluble particles that stimulate a cell-mediated immune response, such as mycobacteria. Cytokines produced by T cells, such as IL-12 and interferon-γ, play an important role in formation and maintenance of immune granulomas (Cooper et al. 1993, Ehlers et al. 2000. A prominent feature of immune granulomas produced by mycobacteria such as Mycobacterium tuberculosis is caseous necrosis in the lesion core, which produces a highly acidic, anoxic environment that may serve to degrade otherwise refractory bacilli.The structure and development of mycobacterial granulomas in mammals, especially those produced in response to Mycobacterium tuberculosis and BCG (attenuated M. bovis), have been described (Papadimitriou & Spector 1972, Ad...

A large diversity of Mycobacterium spp. has been isolated from striped bass Morone saxatilis in Chesapeake Bay, USA. The new species M. shottsii and M. pseudoshottsii are the dominant isolates, while the classical fish pathogen M. marinum is found much less frequently. M. fortuitum and M. chelonae, other Mycobacterium spp. known to commonly infect fishes, have not yet been aseptically isolated from striped bass within Chesapeake Bay. While M. pseudoshottsii and M. shottsii have been phenotypically and genotypically characterized, other less common mycobacterial isolates have not. In the present study, we describe 17 photochromogenic isolates from Chesapeake Bay striped bass using phenotypic characterization and multilocus sequencing of 16S rRNA, hsp65 and rpoB genes. Genetic characterization reveals that these isolates are related to widely divergent portions of the mycobacterial phylogeny; however, some interesting trends are observed, such as a majority of isolates (10/17) belonging to the M. simiae-related grouping. Five additional isolates were assigned to the slow-growing mycobacteria (including 2 identified as M. marinum), while 2 are clearly shown to belong genetically to the fast-growing mycobacteria.