Acanthamoeba sp. is a genus of amoeba, a microscopic organism composed of a single cell. The first part of the name means "spiny," so acanthamoeba is the spiny amoeba, and if you saw one through the eyepiece of a microscope, you would understand why: as it moves about, it extends long pointy projections like the eye stalks of a snail, so that it looks like it is spiny all over.

Acanthamoebae generally live in the environment, in fresh and salt water, in soil, decaying organic matter, and in sewage. These amoebae avidly consume bacteria, engulfing them, drawing them in, and digesting them. In the environment, they are simply members of the vast array of organisms responsible for recycling dead plants and animals and their waste products. Acanthamoeba becomes important, however, when it invades a human eye and causes acanthamoeba keratitis (inflammation of the cornea).

Acanthamoeba keratitis, and quite rarely, other tissue infections, can be caused by any of a variety of species: A. culbertsoni, A. polyphaga, A. hatchetti, A. castellanii, and A. rhysodes. The problem usually occurs in contact lens wearers when a lens solution, lens case, or the lens itself becomes contaminated with bacteria. If an acanthamoeba is then introduced from the environment, a food source is readily available, and the amoebae are able to reproduce.

When amoebae busy engulfing bacteria on a contact lens are popped into an eye, and the eye is irritated, or has even the slightest little scratch, the amoebae can get inside the cornea, and there they live quite comfortably, now ingesting and getting nutrients from the eye itself. Acanthamoeba keratitis is initiated.

Acanthamoeba species thrive in water supply pipes, faucets and sink drains. When conditions are dry, or the food supply dwindles, they encyst - wrap themselves up in a tough protective cell wall - and wait it out. As cysts, they can hang around houses and institutions in dust and on dry surfaces. They are out there, and it's difficult to avoid having them come in contact with contact lenses. The key to avoiding acanthamoeba keratitis is to prevent the organisms from making themselves at home: lenses and cases should never come in contact with tap water, lenses should be regularly cleaned with a commercial disinfectant lens solution to prevent bacterial contamination. Above all, a contact lens should never be put in an irritated eye.