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Microbes and the Environment: No Microbes, No Life |
Chapter: 15
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The Horseshoe Crab in Microbiology The horseshoe crab, Limulus polyphenus (lim´u-lus pol-i-phe´nus), has existed essentially unchanged for about 200 million years. Many biologists refer to it as a living fossil because it is apparently unrelated to anything in the sea. Despite its name, zoologists point out that it is not even a crab but more closely related to a waterborne spider.
Since 1968, the horseshoe crab has assumed a position of importance in the microbiology laboratory. That year scientists discovered that a solution of broken Limulus blood cells quickly forms a gel in the presence of endotoxins. Soon thereafter, they developed a test for detecting microorganisms.
The test, called the Limulus endotoxin assay, detects minute amounts of endotoxins in fluids. Polluted water, for example, is mixed with a solution of horseshoe crab blood cells, and the preparation is incubated at 37°C for 60 minutes. If the mixture shows an increase in viscosity or if a gel forms, the test is considered positive for endotoxins. Inasmuch as endotoxins often signal the presence of Gram-negative bacteria, microbiologists have encouraged using the test as a presumptive sign for these organisms.
Because the test is highly sensitive, endotoxins may also be detected in such specimens as urine, cerebrospinal fluid, and serum, and many diagnostic laboratories are now incorporating the Limulus assay into their standard procedures. Though it has no known relative and is misnamed, the humble horseshoe crab has apparently found a future in microbiology. |
