Mucin stimulates the growth of Legionella pneumophila

Previously, it was demonstrated that a chitinase (ChiA) secreted by the Legionella pneumophila type II secretion system (T2SS) degrades mucin, thereby facilitating bacterial movement through a mucin layer. Here, we discovered that the addition of mucin to a chemically defined medium (CDM) greatly stimulates the growth of L. pneumophila, indicating that the bacterium can very effectively utilize mucin as a food source. This growth-stimulatory effect was evident in broth and agar media and for all WT strains tested. Remarkably, the growth of L. pneumophila on mucin-containing CDM agar rivalled the growth of the bacterium on buffered charcoal yeast extract (BCYE) agar, the standard medium used for cultivating legionellae. A L. pneumophila mutant lacking the T2SS was majorly impaired for growth in mucin-containing CDM, suggesting that exoenzymes are needed for mucin assimilation. In support of this hypothesis, mutants lacking either ChiA or the secreted protease ProA were also impaired for growth in the presence of added mucin. Finally, we observed that L. pneumophila, but not a mutant lacking secreted surfactant, exhibits a marked spreading phenotype (i.e. sliding motility) when grown on CDM agar vs. BCYE agar. However, WT bacteria grown on CDM agar containing mucin did not show this spreading, suggesting that sliding motility is induced under low-nutrient conditions but repressed by mucin byproducts. Taken together, these results provide new insight into the versatility of L. pneumophila physiology and suggest that L. pneumophila may grow well in extracellular spaces in the lungs that contain mucin.