Environmental and genetic factors leading to Mycobacterium Avium biofilm formation
This dissertation investigated the role of environmental and genetic factors that lead to biofilm formation by the environmental and waterborne pathogen Mycobacterium avium. The bacterium causes respiratory and also cutaneous disease predominantly in immunocompromised patients, but infection also occurs in otherwise healthy individuals. M. avium has been frequently isolated from drinking water and it has been shown that the bacterium forms biofilms in drinking water systems. Biofilms show enhanced resistance to antibiotics and disinfectants and therefore are difficult to combat. Biofilm formation is a complex multistep process, and the bacteria change their physiology quite significantly upon entering the attached life style. The process is initiated by a number of triggers that are of both environmental and microbial origin. In this dissertation, biofilm formation was investigated using three different approaches. Firstly, the effect of the universal quorum-sensing signal autoinducer-2 (AI-2) was investigated. The addition of AI-2 to M. avium cultures results in enhanced biofilm formation and transcriptional studies revealed that the bacterium reacts to the compound with upregulation of the oxidative stress response. Oxidative stress was then directly investigated by addition of hydrogen peroxide, also resulting in enhanced biofilm formation. Therefore, it was concluded that oxidative stress leads to M. avium biofilm formation. Furthermore, the importance of the mycobacterial cell wall was studied by comparing biofilm formation of mutants in cell wall-related proteins with the parent strain. These studies revealed a role in biofilm formation that is dependent on the material properties of the surface. Finally, the effect of other bacterial strains and their metabolic by-products on M. avium biofilm formation was studied, as well as the influence of the medium. Although the presence of other bacteria influenced biofilm formation, concentration of the important co-factor iron was shown to be particularly significant in enhancing the process. Collectively, these data suggest that M. avium biofilm formation is enhanced when the bacterium encounters oxidative stress or other bacteria that may represent potential competitors for nutrients or space. It was also shown that the mycobacterial cell wall plays an important part in the attachment process.