The LAL Test
Beth Osia reports on a sensitive test to detect bacterial toxins using an extract from the blue blood of the horseshoe crab.
Originally shared by Beth Osia
Ah, the majestic Horseshoe crab. Scientifically known as Limulus polyphemus, this creature not only looks like a tank, but is a real blue blood. That is, the horseshoe crab literally bleeds blue. This is due to their blood containing copper associated hemocyanin for oxygen transport rather than hemoglobin, the transport protein which also gives our blood its red color.
Now, you’re probably wondering, why the hell would anyone want to capture and bleed a horseshoe crab? Well, I’m sure there are a number of reasons, and I’m sure all of them are very good. However, I am only personally concerned with their amebocytes as I’ve heard they taste rather rubbery. Besides, after the bleeding, I hear they’re returned to the ocean (probably because nothing else wants to eat them either).
Moving on, there is something very interesting that happens when you combine Limulus amebocyte lysate (LAL) with a nasty little molecule known as Lipopolysaccharide (LPS). I suppose it is unfair to call LPS nasty since it’s not really toxic or anything, but because it resides and is expelled from the cell walls of Gram negative bacteria, our silly human bodies think it’s the end of the world. I suppose you could liken the process to propaganda. LPS normally alerts the immune system to a deadly bacterial infection in the blood or deep tissue, so even if it’s found freely floating about and not attached to any bacteria at all, the immune system blows the whole thing out of proportion and generates a massive immune response. This includes inflammation, but what’s most dangerous is the fever. It’s kind of like cellular fear mongering.
LPS is known as a Pyrogen or fever generating substance, and this can be a problem because it’s pretty much everywhere in large-enough-to-kill-you quantities (1 nanogram per kilogram is baseline for pyrogenic activity in mammals), and it’s not easy to destroy. Now, before you freak out and take a hydrogen peroxide bath (this would effectively oxidize the molecule, but is not at all recommended), know that it still has to end up in your blood before your body decides it’s world war III and tries to save you by killing you. This makes it a pretty big obstacle for Hospitals, since much of the healing they do involves jabbing you with sharp things that pump stuff directly into your blood. So how do you avoid giving your patients a deadly fever every time you stick them with an IV?
It’s all in the blood.
Well, in limulus blood, that is. Getting back to our horseshoe crab friends, the amebocytes in their blood contain coagulogen, a protein which forms a clot when it comes into contact with the LPS molecule, even if it is not attached to a bacterium’s cell wall. We’ve managed to take advantage of this quality by processing the blood into various types of test enzymes capable of detecting extremely small quantities of LPS. This allows for spot testing of medical equipment and fluids (before they enter your body), and has saved countless patients from complications due to pyrogenic endotoxin. If not for this, we’d still be trying to detect LPS contamination by giving lots of fuzzy bunnies fevers, and nobody likes a hot bunny, let alone thousands of them.
Well, there it is. Horseshoe crabs save lives. Try to remember that next time you’re at the beach and you almost step on one.
Photo via Fresh Photons