Our research focuses on host responses to interactions with beneficial microbes. Within this context, the studies of my laboratory address five major questions:
- How are environmentally rare bacteria harvested from the host’s habitat during the onset of a horizontally transmitted symbiosis?
- By what mechanisms does the host recognize its specific symbiotic partner(s)?
- What are the influences of symbiotic bacteria on the developmental of the host tissues with which they associate?
- How is the symbiont population maintained in balance over the host’s lifetime, such that neither does the symbiont overgrow the host nor does the host eliminate the symbiont?
- What are the similarities and differences between pathogenic and beneficial animal-bacterial interactions?
We use the squid-vibrio (Euprymna scolopes-Vibrio fischeri) light organ system as a model to approach these broad questions. Over the past ten years of our development of this system, we have focused on defining the anatomical and biochemical influences of the microbial symbiont on the host, using standard techniques, including confocal, electron and light microscopy and routine protein biochemistry. In recent years, we have added to our approaches the role of symbionts in the induction of host gene expression. To this end, in collaboration with the laboratories of Drs. Bento Soares and Thomas Casavant of the University of Iowa, we have developed a unigene set from the cDNA libraries of the host squid light organ, which contains nearly 14,000 unique genes. We are now launched into RNAseq technology to describe the changes in host gene expression with symbiosis.
Using these techniques, our principal findings with respect to the above-mentioned questions have been as follows:
- We have described the mechanism by which the newly hatched host enriches for V. fischeri, which accounts for less than 0.1% of the bacteria in the surrounding seawater. Non-specific, environmental peptidoglycan (a cell surface component of all bacteria) induces the host to shed mucus from an epithelial field near the site of colonization. V. fischeri specifically aggregates in this mucus for several hours and then invades host tissues.
- We have found that specificity begins outside the sites of eventual colonization, i.e., in the host-shed mucus. Further specificity determinants include: the provision of high levels of nitric oxide, a toxic oxygen species, in the ducts through which symbionts must migrate to colonize host crypt spaces; mannose-glycan interactions between the symbiont surfaces and host crypt epithelial cells.
- We have documented that V. fischeri induces profound developmental effects on the host light organ tissues, including apoptosis, macrophage-infiltration, cell swelling, increase in microvillar density, and a total remodeling of light organ morphology. Much of this development is mediated through bacterial factors, most commonly known as ‘microbe associated molecular patterns’ or ‘MAMPs’. Specifically, components of symbiont surface molecules, lipopolysaccharide and peptidoglycan, interact with host crypt epithelial to induce developmental pathways. The specific peptidoglycan fragment was identified as ‘tracheal cytotoxin’ or TCT, a molecule that has thus far only been reported to be exported by two pathogens, Neisseria gonorrheae and Bordetella pertussis.
- We have found that the symbiosis is controlled by a dramatic diel rhythm. Each day at dawn, the host vents 90-95% of its symbiont culture into the surrounding environment. Coupled with this rhythm is a marked remodeling of the crypt tissues. After venting, the tissues organize into highly polarized epithelia with dense lobate microvillar surfaces. With progression over the day, the tissues begin to take on a stressed appearance, with effacement of the epithelia and blebbing of the apical surfaces of the epithelial cells. Just before venting, the tissue is highly disorganized.
- The finding that ‘MAMPs’ mediate harvesting and much of development in the squid-vibrio system suggests that, at least in this system, beneficial and pathogenic bacteria are likely to use a similar molecular language to foster interaction.
Our immediate goals are to use the information that is becoming available through bioinformatics to deepen our understanding of the interaction between the host and symbiont in the squid-vibrio association. These data will provide heuristic value for the continued study of the above-described aims. However, although bioinformatics will be brought into the study of this system, the conceptual focus will always remain at the more organismic level, i.e., understanding how two species, from very different phylogenetic origins, manage to form a persistent relationship.