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What makes a Parasite?

Notes from Planet Earth

Symbiotic organisms that live on and within us are rarely discussed, in spite of their benefits for our survival.

Parasites, however, do get our attention and most all organisms have to contend with those other beasts, large and small, that are intent on feeding on them. Leaving out the occasional large cat, crocodile, or snake that is found to have consumed a human, the common parasites of humans generally come from the following well known groups: worms, insects, fungi, bacteria, and viruses. (Perhaps it might be said that humans are also parasites given our dependence on the voracious consumption of animal, vegetable, and mineral resources.) However, the discovery of prion diseases of the brain in the late Twentieth Century added a dramatic new dimension to the field of infectious disease.

When first examined the composition of prions was highly puzzling. They weren’t destroyed by heat and, in spite of containing no genetic material, they multiplied and were infectious when injected into uninfected hosts. The first diseases for which prions were held responsible (and before we knew what they were) included kuru in humans (who ritually ate the brains of other humans), scrapie in sheep and goats, and chronic wasting disease (CWD) in deer and elk. More recently, we learned that “mad-cow-disease” was also caused by a prion.

After much controversy researchers finally concluded prions were composed solely of protein. Most puzzling was the fact that the protein they were composed of was a normal constituent of all brains and some other tissues. Ultimately it was proven that what was different between the normal protein and the infectious prion was the manner in which it was folded. How a protein is folded upon itself is a very important characteristic. Folded correctly, it remains soluble within the cell and carries on its proper function. Folded incorrectly, it will be usually treated as waste by a cell, destined for digestion and recycling. The white of an egg remains soluble (and transparent) in water but heat it (or beat it) and it changes its configuration and becomes insoluble (and opaque).

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