Acanthamoeba sp. is an amoeba best known for its offensive habit of invading the eyes of contact lens wearers and doing a great deal of damage there (a rare but well documented risk of contact lens wear called acanthamoeba keratitis). The amoeba is also sometimes the cause of a chronic type of meningitis known as graulomatous amoebic encephalitis, or GAE. It is one of the few parasites that is easily cultured in the laboratory. Culture is an important tool for acanthamoeba diagnosis because the amoeba themselves are often very rare in eye specimens, and are therefore easily missed The key to getting acanthamoeba to flourish in the laboratory is to give it the right thing to eat. Just as it likes to eat bacteria in contaminated contact lens cases and lens solutions, it likes to eat bacteria in the wild and in the laboratory. The best approach is to use a non-nutrient agar base (a medium that does not provide nutrients to bacteria or amoebae) and coat the surface with bacteria. Commonly isolated bacterial strains, in particular Pseudomonas aeruginosa and Enterobacter aerogenes, are perfect for acanthamoeba culture. The specimen from a case of acanthamoeba keratitis or GAE is placed in the middle of the medium. Acanthamoeba cultures are generally incubated at body temperature, but the amoebae do quite well at room temperature as well. After just a few days, they can be seen through the microscope, moving slowly across the agar surface, engulfing bacteria as they go, dividing, and encysting (surrounding themselves with a tough protective cell wall) when the limited food supply runs out. Acanthamoeba have a very typical look: they have spiny, spiky pseudopodia (extensions of their cell wall that project and then retract, used for locomotion and engulfing food). The appearance of a typical spiky acanthamoeba from a suspected case of acanthamoeba keratitis is diagnostic. In GAE, there is another amoeba that we have to watch for, but the symptoms of disease are quite different for the two diseases, so a mistake in identification is relatively unlikely. One must keep in mind too, that these are environmental amoebae that are present in soil and water supplies - there is always the possibility of contamination. A culture containing thousands of Acanthamoeba organisms is a potentially dangerous thing, especially if you are a contact lens wearer: laboratory culture of acanthamoeba should only be attempted in an accredited laboratory with the proper safety equipment. |
