Mycobacterium marinum is an established surrogate pathogen for Mycobacterium tuberculosis because of its strong conservation of thousands of orthologous genes, lower risk to researchers and similar pathology in fish. This pathogen causes TB-like chronic disease in a wide variety of fish species. As in human TB, the microbe grows within the host macrophages, can mount life-long chronic infections and produces granulomatous lesions in target organs One of the fish species known to manifest chronic "fish TB" is the small laboratory fish, Japanese medaka (Oryzias latipes). Recently, our lab characterized the disease progression in medaka with different strains of M. marinum by intraperitoneal injections. However, since this is an unnatural form of transmission, I tested for infection with noninvasive exposure techniques such as by ingestion, cohabitation, and by vertical mechanism. This dissertation demonstrates that transmissions by ingestion and by cohabitation are efficient modes by which medaka can easily become inoculated with M. marinum. While conducting the transmission studies, I observed differences in growth, plasmid stability and virulence depending on which fluorescent reporter construct was present. Here, I describe large negative effects on virulence and organ colonization that occurred with a commonly-used plasmid pG13, that expresses green fluorescent protein (Gfp). The studies presented here, indicate that Gfp overexpression was the basis for the reduced virulence in this reporter construct. I also show that these negative effects could be reversed by significantly reducing Gfp expression levels or by using low-expression constructs of Rfp. Since biofilm formation seems to be an important factor in the virulence of many chronic bacterial infections, I characterized M. marinum groEL1 gene involved in biofilm maturation and its role in virulence of M. marinum in medaka.