Mycobacterium avium Complex (MAC) are species of the non-tuberculous branch of the Mycobacteria family that is responsible for the development of pulmonary lung disease in immunocompromised individuals. One of the first lines of defence from the host involve a specialised group of tissue macrophages that reside in the lung alveolar space called alveolar macrophages (AM). They have specialised functions that are suitable for recognition of foreign invaders and the ability to tackle these threats though means of a robust immune response. Alternatively, they also feature as key components in a fairly destructive uncontrolled inflammatory response which is the root cause of MAC lung disease. As a facultative pathogen that thus far, almost exclusively infiltrates the macrophage intracellularly, MAC has evolved several tools to establish its foothold in the host. MAC persists intracellularly in the host by evading host immune responses via mechanisms such as preventing phagolysosome fusion and acidification which allows it to proliferate intracellularly thus assisting MAC in the spread of infection. It is vital that these mechanisms are investigated so that the disease process is better understood, and therapeutic strides are made. There is a need for developments in the therapeutic pipeline due to the influx of multi-drug resistant strains of MAC that are ever emerging globally. Current treatments are quickly proving to be inadequate in the face of MAC evasive manoeuvres, and existing literature uses experimental infection models that only cover a short-term infection timeframe. As MAC infection rarely gets resolved in a short duration (i.e., less than a week), the current literature would benefit from the establishment of a long-term infection model as done here in this project. Using MPI cells, this project focused on the modelling of MAC infection in vitro over the span of 20 days. Data from the modified gentamicin protection assays shows a marked difference in number of colony forming units (CFUs) from Day 0 to Day 20 infection period, with numbers declining over this time period in antibiotic treated cultures. Two strains with different phenotypes rough and smooth, were able to induce a pro-inflammatory response in this model which aligns with the previously established knowledge of the over exaggerated immune response associated with severe forms of MAC disease. Methods have been indicated to show the model could successfully sustain a Mycobacterium avium infection for up to 18 days without extracellular contamination and minimal cell death. The persistence of 2 phenotypically different M. avium and M. intracellularae were demonstrated in vitro where the strains continued to replicate until day 18 without elimination. The smooth strain was observed to decline intracellularly at a more rapid rate than the rough. In the analysis of cytokines, both the rough and the smooth strain appeared to induce IL-6 levels that were comparable to non-infected cells which could indicate a mechanism of survival.

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