Mycobacteria have a cell envelope that can act as a shield against host defense. This study shows that mycobacteria survive in host macrophages by targeting the innate host receptor dectin-1 through a noncanonical ligand. Compared with wild-type (WT) mice, dectin-1–deficient mice were more resistant to infection to mycobacteria. Dectin-1–deficient mice presented with substantially reduced bacterial burdens, inflammatory cytokines, and infiltrating myeloid cells, such as neutrophils and macrophages. Intracellular survival of these bacteria was reduced in macrophages derived from dectin-1–deficient mice compared with those from WT mice. Cellular characterization of mycobacteria-infected macrophages indicated that the presence of dectin-1 altered phagosomal maturation and association with markers of autophagy. Activity-based purification and nuclear magnetic resonance spectrometry identified branched α-glucan as the dectin-1 mycobacterial ligand. This branched glucan was essential for activating dectin-1. These results show that mycobacterial α-glucan targets dectin-1 to facilitate intracellular bacterial survival.

Mycobacteria form cellulose-containing biofilms and exhibit drug resistance at infection sites. However, the function of mycobacterial biofilms in host defense remains unclear. Herein, we demonstrate that reductive stress-induced mycobacterial biofilms evade macrophage capture and protect bacilli from nitric oxide stress. We first determined the optimal conditions for biofilm formation by mycobacteria. Using green fluorescent protein (GFP)-labeled mycobacteria, we then examined the protective effect of biofilms on the capture of bacilli by macrophages and found that macrophage capture was inhibited in the presence of biofilms. Furthermore, we constructed GFP-expressing mycobacteria that respond to acidic pH and nitric oxide stress, both of which are bactericidal factors. The results showed that mycobacterial biofilms protect bacilli from nitric oxide-mediated stress, but not from acidification. Finally, the removal of biofilms by cellulase enhanced the capture of mycobacteria by macrophages and the exposure of mycobacteria to nitric oxide. These findings highlight the protective roles of mycobacterial biofilms in innate immunity, particularly against macrophage capture and nitric oxide-induced stress.