How a squid forms a relationship with a bacterium

July 28, 2014 § Leave a comment

The Hawaiian bobtail squid forms a symbiotic relationship with a bioluminescent bacterium. Photo provided by Spencer Nyholm.

The Hawaiian bobtail squid forms a symbiotic relationship with a bioluminescent bacterium. Photo provided by Spencer Nyholm.

How do you get into a mutually beneficial relationship? That is the question researchers asked in a recent paper in the journal Molecular & Cellular Proteomics, albeit for a squid and its bacterial partner. The researchers showed that in order for the Hawaiian bobtail squid to form a symbiotic relationship with the bioluminescent Vibrio fischeri, proteomic changes have to occur in a set of cells of the squid.

The Hawaiian bobtail squid, formally known as Euprymna scolopes, has a light organ that is exclusively colonized by V. fischeri. The squid feeds the bacterium a solution of sugar and amino acids and, in return for the steady food supply, the bacterium gives off the light that masks the squid’s silhouette while it goes hunting for various species of shrimp for its own meals.

Scientists study the squid and its bacterial partner as a model to understand how beneficial bacteria form associations with multicellular organisms and help animals develop. “Our lab is interested in understanding the role of the host’s innate immune system in establishing specificity,” says Tyler Schleicher at the University of Connecticut, the first author on the MCP paper. “Each generation of squid is colonized by V. fischeri from the environment, and they must distinguish between the symbiont and a huge background of nonsymbiotic bacteria that are found in seawater.” The question is how the squid achieves this feat.

To answer the question, the investigators used two quantitative proteomic techniques to compare a set of special cells taken from squid colonized with bacteria and those that were uncolonized. “This is the first time that two independent, high-throughput proteomic techniques have been applied to the squid-Vibrio association,” says Spencer Nyholm, also at the University of Connecticut, and the senior author on the paper.

The cells the investigators chose to look at are hemocytes, which are blood cells in the squid’s light organ; these cells have properties of the immune system’s macrophages and interact with the symbiotic bacteria present at the light organ.

From the investigators’ analyses of the differences in protein expression in hemocytes taken from colonized and uncolonized squid, they saw that the presence of V. fischeri in the light organ induced changes in the hemocyte proteome to promote the cell’s tolerance of the bacteria and favor symbiosis.  The changes involved the cytoskeleton, lysosome function, proteases and receptors.  Because scientists still don’t understand the precise mechanisms that contribute to host-symbiont specificity, the investigators are now focusing on studying several proteins identified in this study that appear to influence the bacterium’s adhesion to the squid’s light organ.

“A growing body of evidence from a variety of animal model systems suggests that beneficial microbes influence a host’s innate immune system to foster these associations,” says Nyholm. Because macrophagelike cells similar to hemocytes are found in almost all animals, he adds, “our study may provide insight into other host-microbe associations.”

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