Bacterial intercellular signaling
Although bacterial cells can function autonomously, most bacteria in the environment live in close proximity with others of their kin. Evidence is accumulating to suggest that, much like multicellular organisms, bacterial populations can exhibit coordinated behaviors. In multicellular organisms, this coordination is mediated by a vast diversity of intercellular signaling mechanisms. In contrast, relatively few intercellular signaling pathways have been defined in bacteria. Quorum sensing, or the ability of bacteria to sense and respond to diffusible small molecules that accumulate under conditions of high cell density, is one of the few prominent examples.
A) Discovery – We are employing a number of genetic, biochemical and bioinformatics approaches to uncover intercellular signaling pathways previously unidentified in bacteria.
Type VI secretion
The type VI secretion system (T6SS) is a complex protein export pathway present in hundreds of known bacterial species. Our laboratory discovered that this pathway is utilized by bacteria to deliver toxic effector proteins to neighboring bacterial cells. An interesting facet of the T6SS is that it targets cells of its own kind in addition to those of other species. Specialized cognate immunity proteins protect organisms against their own toxins.
A) Regulation – As a cell contact-dependent process that is energy intensive, the T6SS is tightly regulated. This is multifactorial, occurring at the transcriptional, translational and posttranslational levels. Our work has predominantly focused on posttranslational regulation governed by a eukaryotic-like signaling cascade. Recently, we discovered that regulation of T6SS in Pseudomonas aeruginosa is integrated into a global, multi-faceted response to intercellular antagonism.
B) Effectors – The activity of a secretion system is defined by its substrates. Using both proteomic and bioinformatic approaches, we have identified new families of T6S effectors. A major objective of our work in this area is to define the activities of these effectors at a molecular level under physiological conditions.
C) Mechanism – Currently, little is known about the mechanism by which the T6SS translocates effectors from one bacterial cell to another. We have discovered that the T6SS structural protein Hcp serves as a molecular chaperone to stabilize and target a subset of effectors for secretion. We also found that the secretion of a second subset of effectors requires an association with the structural T6SS protein VgrG. Using structural and biochemical approaches, we are working to further elucidate the process of effector secretion.
D) Pathogenesis – T6SSs are widespread among Gram-negative bacteria, including those that commonly exist in human polymicrobial communities. We are exploring the hypothesis that antagonistic interactions mediated by the T6SS could influence disease outcome. We are also analyzing the role of eukaryotic cell-targeting T6SSs in infection.
Ticks are important vectors for a number of widespread and emerging diseases. The success of ticks as disease vectors can be linked to their life history. At each of their life stages, ticks take blood meals from a vertebrate host, and pathogens acquired at one feeding can be transferred to new host at a subsequent life stage. In addition to the pathogens they transmit, ticks associate with a diverse assemblage of other microbial species, which can vary based on tick species, host animals, and other factors.
A) Innate immunity – Despite its crucial importance for regulating populations of transmitted pathogens, the tick immune system remains poorly characterized. We recently discovered that genes encoding toxic T6SS effectors employed in competition between bacteria have been horizontally acquired by a range of eukaryotic species, including the common ancestor of ticks and mites. In the deer tick, Ixodes scapularis, we have demonstrated that one of these proteins controls proliferation of Borrelia burgdorferi, the bacterium responsible for Lyme disease. We are working to identify the mechanism by which the transferred toxin regulates this bacterium, and to define the role these proteins play in innate immunity of ticks more broadly.
B) Bacterial interactions – The Lyme disease bacterium, B. burgdorferi is acquired by ticks during feeding on one host animal, and must persist in the tick gut through molting, until it is transmitted to a new host at the next feeding. We are working to characterize the mechanisms employed by B. burgdorferi to survive in the tick gut in the face of competition from other tick-associated bacteria