Ultrastructure of the Mycobacterium avium subsp. hominissuis Biofilm

Comparison of Ultrastructure, Extracellular Matrix, and Drug Susceptibility in M. avium subs. hominissuis Biofilms

Pulmonary infections with Mycobacterium avium occur in susceptible individuals following exposure to the bacterium in the environment, where it often persists in biofilms. Many methods have been used to generate biofilms of M. avium, and it is unknown whether different approaches generate similar structures and cell phenotypes. To make a parallel comparison of in vitro biofilm ultrastructure, extracellular matrix (ECM) composition, and the drug susceptibility of biofilm resident bacteria, we used two published methods to generate M. avium biofilms: four-week incubation in M63 medium or 24 h exposure to dithiothreitol (DTT). Scanning electron microscopy revealed differences in the biofilm ultrastructure between the two methods, including variation in the appearance of ECM materials and morphology of resident cells, while light microscopy and staining with calcofluor white indicated that both biofilms contained polysaccharides characteristic of cellulose. Measuring the susceptibility of biofilms to degradation by enzymes suggested differences in structurally important ECM molecules, with DTT biofilms having important protein and, to a lesser extent, cellulose components, and M63 biofilms having moderate protein, cellulose, and DNA components. Both biofilms conferred resistance to the bactericidal effects of amikacin and clarithromycin, with resident cells being killed at greater than 10-fold lower rates than planktonic cells at almost all concentrations. These comparisons indicate differences in biofilm responses by M. avium under differing conditions, but also suggest common features of biofilm formation, including cellulose production and antimicrobial resistance.