Saving the horseshoe crabs
Longtime BIO230 fans know of my fondness for Limulus polyphemus, or the common Atlantic horseshoe crab. My fondness for them goes back over 20 years, to the work done by these researchers at my graduate alma mater. I was quite frustrated back then that the lab my spouse worked in chose to study oxygen-carrying proteins from something as inedible as these guys. Just as well, I’d have felt guilty about eating the science experiment anyway.
Limulus has a big role in infectious disease, as its circulatory fluid, specifically its amoebocytes, are very sensitive at detecting endotoxin. Horseshoe crabs are harvested, and the amoebocyte-containing hemolymph is extracted using a syringe into the back of the carapace. The horseshoe crab can actually tolerate donation of a sizable volume pretty well, and it is generally returned back to the ocean at this point.
The hemolymph is briefly processed in the laboratory, and then is used by many aspects of the biomedical industry (drug companies, medical device manufacturers, basic and clinical laboratories) to detect the presence of minute levels of bacterial endotoxin. Recall that endotoxin comes from fragments of the outer membrane of Gram negative bacteria, and small amounts of these active compounds can have profound adverse effects on health. Additionally, it doesn’t require any living bacteria to cause these symptoms; fragments of outer cell membrane released as the body combats an infection can be sufficient to initiate the signs and symptoms of endotoxin shock. Conceivably any contamination by a Gram negative bacterium can lead to toxemia due to endotoxin release, so there is a strong medical need to ensure that medical products are free of endotoxin.
The Limulus Amoebocyte Lysate (LAL) assay is currently the FDA-approved standard for detection of endotoxin, however as you might imagine an alternative approach might have its benefits. A recent report from the journal Science and summarized in Science Daily highlights a breakthrough that might further standardize the detection of endotoxin. Researchers at the University of Wisconsin-Madison have been examining liquid crystal droplets, and how the interfaces of these compound change when they come into contact with impurities. They noticed that the appearance of the liquid crystals changed visibly under the light microscope when mixed with picogram levels of bacterial lipids, particularly the lipids that comprise endotoxin. Strikingly, the visible changes of the liquid crystal droplets in the presence of endotoxin did not seem to be due to disruption of the surface as they had previously surmised, but instead is likely due to a wholesale mixing of the two components leading to a change in the structure of the liquid crystal matrix itself.
The assay using liquid crystals works very fast, in less than a minute, and also appears to be several orders of magnitude more sensitive than the LAL method for endotoxin detection. The researchers caution though:
We have found a fundamental phenomenon, but it’s a long path to have a validated technology that can replace the horseshoe crab.
Let’s hope that we’ll continue to see the horseshoe crabs continuing to come up on the Delaware and New Jersey beaches for many summers still.