Characteristics of Mycobacterium avium Biofilms and Implications of Biofilm Growth for Pathogenesis

Pulmonary infections with Mycobacterium avium subs. Hominissuis occur in susceptible individuals following exposure to the bacterium in the environment, where it often persists in biofilms. M. avium is able to survive and grow in many conditions, placing the opportunistic pathogen in frequent proximity to humans. Understanding of M. avium biofilms and implications of biofilm growth on subsequent pathogenesis is limited. One specific limitation is the use of different in vitro models to characterize biofilm development in different studies, leading to uncertainty about whether findings are characteristic of M. avium biofilms in general or whether they are specific to a given model. To elucidate similarities and differences between biofilms formed using different methods, we made a parallel comparison of in vitro biofilm ultrastructure, extracellular matrix (ECM) composition, and drug susceptibility of biofilm resident bacteria, using two published methods to generate biofilms. To understand transcriptional adaptations in M. avium during residence in a biofilm and following engulfment by macrophages, we undertook the first study of global differential gene expression in M. avium using RNAseq and used aerosol infections of mice with M. avium grown in planktonic culture or in a biofilm to assess the effect of biofilm growth on infectivity. Taken together, the findings presented in this dissertation indicate that the mechanism of biofilm formation in M. avium, including patterns of gene expression, varies under different environmental conditions. However, some characteristics may be common to M. avium biofilms, including the production and export of cellulose, increased antibiotic resistance, and increased expression of certain genes. While growth in a biofilm may cause patterns of expression in some genes that are similar to those observed during infection of macrophages, we did not observe a clear benefit for biofilm derived cells in establishing and persisting in lungs or tracheae in our mouse infection model.