Summary

Intracellular bacterial pathogens reprogram host cells to promote their survival, leading to devastating morbidity and mortality worldwide. Our research focuses on revealing how bacterial pathogens usurp host cell function to gain insights into both host cell biology and pathogenesis. In particular, we focus on how intracellular bacterial pathogens (e.g. Rickettsia parkeri and Listeria monocytogenes) hijack host machinery to move throughout tissues via cell-to-cell spread. This critical yet understudied virulence mechanism allows bacteria continued access to cytosolic nutrients and avoidance of humoral immune responses. However, moving through tissues without destroying cell integrity presents challenges, and we are investigating how bacteria spread by identifying the critical host and bacterial factors involved, and their mechanisms of action.


Cell-to-cell spread

Several bacterial pathogens utilize cell-to-cell spread during their infectious life cycle. Our work focuses on two of these human pathogens: Rickettsia parkeri and Listeria monocytogenes. R. parkeri are obligate intracellular Gram-negative bacteria that cause spotted fever and are transmitted to humans by arthropod vectors. In contrast, L. monocytogenes are Gram-positive bacteria that causĀ­e listeriosis and meningitis, and are often ingested from contaminated food sources. Both pathogens can invade non-phagocytic cells, enter the cytosol and hijack host actin to promote actin-based motility. Motile bacteria then initiate cell-to-cell spread, a step-wise process during which bacteria propel themselves to the host cell membrane, form double-membrane protrusions into the recipient cell, and then become engulfed into a vesicle before escape to the recipient cell cytosol.

  R. parkeri  and  L. monocytogenes  rely on secreted proteins (effectors) and actin-based motility to promote spread. Interestingly, even though motility is needed for  R. parkeri  to get to the host cell junction, it loses its tail before a protrusion is formed. In the absence of these actin polymerization forces.  R. parkeri  induces short protrusions that are engulfed into double-membrane vesicles more rapidly than  L. monocytogenes . Given these striking morphological differences, we are excited to dissect the molecular underpinnings that govern each stage of spread. 

R. parkeri and L. monocytogenes rely on secreted proteins (effectors) and actin-based motility to promote spread. Interestingly, even though motility is needed for R. parkeri to get to the host cell junction, it loses its tail before a protrusion is formed. In the absence of these actin polymerization forces. R. parkeri induces short protrusions that are engulfed into double-membrane vesicles more rapidly than L. monocytogenes. Given these striking morphological differences, we are excited to dissect the molecular underpinnings that govern each stage of spread. 

Despite sharing similar life cycles, our work has revealed that the morphological and molecular details of spread differ between these two pathogens. Each pathogen requires a distinct set of host and bacterial proteins for spread, and utilize different force generating machinery to engage host intercellular junctions. This observation evokes several exciting questions with the potential to reveal fundamental biological insights. For example, how do bacteria target membrane junctions to induce protrusions and manipulate forces meant to maintain tissue integrity? How are host membrane-remodeling factors and endocytic pathways usurped during spread to shape and move the plasma membrane? Lastly, what bacterial factors are required and how do they reprogram host pathways during spread?

To tackle these important questions, we use a multidisciplinary approach combining cell biology, microbiology, genetics, biochemistry and biophysics to investigate the mechanisms of cell-to-cell spread. It is our belief that investigating the dynamic interplay between host and pathogen is critical to understanding mechanisms of virulence, and so we focus first on identifying the critical host and bacterial factors involved during spread using a variety of genetic screens. Then we use modern techniques to get a handle on their function. In the end, our goals are to dissect the pathways of spread to reveal important properties of pathogenesis, and use pathogens as tools to better understand host cell biology.