Tuberculous granuloma formation is enhanced by a mycobacterium virulence determinant.

Granulomas are organized host immune structures composed of tightly interposed macrophages and other cells that form in response to a variety of persistent stimuli, both infectious and noninfectious. The tuberculous granuloma is essential for host containment of mycobacterial infection, although it does not always eradicate it. Therefore, it is considered a host-beneficial, if incompletely efficacious, immune response. The Mycobacterium RD1 locus encodes a specialized secretion system that promotes mycobacterial virulence by an unknown mechanism. Using transparent zebrafish embryos to monitor the infection process in real time, we found that RD1-deficient bacteria fail to elicit efficient granuloma formation despite their ability to grow inside of infected macrophages. We showed that macrophages infected with virulent mycobacteria produce an RD1-dependent signal that directs macrophages to aggregate into granulomas. This Mycobacterium-induced macrophage aggregation in turn is tightly linked to intercellular bacterial dissemination and increased bacterial numbers. Thus, mycobacteria co-opt host granulomas for their virulence.

Group B streptococcal beta-hemolysin/cytolysin promotes invasion of human lung epithelial cells and the release of interleukin-8.

Pneumonia and lung injury are hallmarks of early-onset neonatal group B streptococcal (GBS) infections. Production of a beta-hemolysin/cytolysin (beta-h/c) encoded by the cylE gene is associated with GBS virulence in vivo. To elucidate the contribution of the beta-h/c toxin to lung injury, the interactions of GBS wild-type strains and isogenic cylE mutants with A549 lung epithelial cells were examined. Compared with wild-type GBS strains, cylE mutants did not produce cytolytic injury, even at high inocula, and exhibited decreased cellular invasion. Additionally, cylE mutants induced less A549 cell release of the neutrophil chemoattractant interleukin (IL)-8. GBS invasion and IL-8 induction were significantly reduced in the presence of dipalmotyl phosphatidylcholine, a major constituent of lung surfactant and a known inhibitor of beta-h/c activity. These data indicate that the GBS beta-h/c contributes to invasion and immune activation of lung epithelial cells and may represent a multifunctional virulence factor in the early pulmonary stages of GBS infection.