Mycobacterium avium subsp. paratuberculosis (MAP) is a very slow growing bacterium that is the causative agent of Johne's disease (JD) in ruminants and has long been suggested to be associated with complications of Crohn's disease (CD) in humans. Although there is no direct evidence that MAP is the primary etiological agent for CD, most CD patients are found to have MAP in their intestinal tissues. The current control measures for JD in cattle, sheep, and goats have only been minimally effective, and there are only medications to treat the symptoms of mycobacterial infections associated with CD in humans. Along with not being able to cure MAP infections, there is no established laboratory animal model for testing therapeutics. When mice are infected with MAP they develop systemic infection and do not mimic disease observed in ruminants. J774A.1 murine macrophages typically have a very short lifespan of about 4-6 days, however MAP infected cell cultures can survive up to about 10 days. Using a modified protocol of Estrella et al. (2011), we have been able to establish a 45-60 day long-term MAP infected J774A.1 murine macrophage cell culture model. With the addition of retinoic acid (RA), vitamin D (VD), and phorbol myristate acetate (PMA) in combination in cell culture, we were able to screen novel therapeutics before embarking on in vivo testing in animals. This is a significant step forward in Crohn's and Johne's disease treatment research. We are not only able to test various drugs against specific strains of MAP to determine susceptibility, but we are also able to test a wide variety of drugs at the same time, with relatively minimal cost. We have evaluated the efficacy of clarithromycin, azithromycin, isoniazid, amikacin, ethambutol, ciprofloxacin, levofloxacin, rifampicin, clofazimine, as well as a combination of clarithromycin, rifampicin, and clofazimine using our MAP infected macrophage cell culture model. We were able to determine the drugs' differential ability to kill intracellular MAP in the early stages of infection, versus chronic stages of infection, and against two different strains of MAP, 43015 and 19698 that affect humans and cattle respectively. The minimal inhibitory concentration (MIC) of each drug was determined as per NCCLS protocol in vitro, and the drugs were tested at the MIC value, along with one concentration above and below the MIC in our cell culture model. The antimicrobials were found to be effective at different stages of cell culture infection and in different strains of MAP. Some drugs were more effective at early stages of MAP infection, whereas others were more effective in chronic or latent stages of infections. It is important to note that although a drug may be effective at a certain stage of infection, it may not necessarily be effective against all strains of MAP. The most promising results were seen with a combination of clarithromycin, clofazimine, and rifampicin, which was effective at all stages of infection with both strains of MAP tested. This long term cell culture model will provide researchers with important screening tools for evaluating new therapeutics before embarking on costly in vivo testing, and allow the assessment of therapeutics at different stages of MAP infection but also against an array of intracellular pathogens. / Ph. D.