What is Acanthamoeba?

An Acanthamoeba is a genus of amoebae found worldwide in soil, water, and air. An amoeba is a genus of protozoa, which is a unicellular microorganism belonging to the eukaryote domain. Normally, amoebae in this genus contain typical eukaryotic organelles, cytoplasm, and a single nucleus within an outer layer, called the cell membrane. Under a microscope, they appear as a tiny, spiny, oval or round microorganism that measures between ten and thirty-five micrometers in length.

Members of this genus have temporary cytoskeletal projections, called pseudopodia or acanthopodia, extending from its cell membranes that allow it to move. These threadlike filaments are composed of actin, which binds with myosin near the cell membrane to cause contraction, thus allowing them to move. These small filose projections around the rim of the cell, giving it its characteristic spiky appearance.

The Acanthamoeba genus was accidentally discovered by scientist Aldo Castellani in 1930 when the amoeba contaminated a culture of fungus he was studying. He named the amoeba Hartmannella castellani. Later, as the Hartmannella genus was split into three further categories, or genera: Acanthamoeba, Hartmannella, and Glaeseria. The categorization was changed again when scientists made Acanthamoebidae its own family in the 1970s and then placed Hartmannella in another amoeboid family in the 1980s.

One of the distinct features of amoebae in this genus is their double-walled cysts. Amoebae form cysts when they encounter harsh conditions by contracting into a ball and secreting a membrane, or two membranes in this case, to protect themselves until conditions become more favorable. The other most distinctive feature members of this genus have are their acanthopodia, which help them move, stick to surfaces, and capture prey during vegetative, or eating, stage. During this stage, these amoebae are called trophozoites and may feed on algae, bacteria, yeasts, or other tiny organic matter.

As they eat, members of the Acanthamoeba genus grow exponentially during the trophozoite stage in order to reproduce asexually, via binary fission. In binary fission, the cell doubles in mass and replicates its DNA, or doubles its genetic material. The cell then splits its nucleus, the brain of the cell, into two equal nuclei in a process called mitosis. The large, double-nucleated cell then splits into two equal daughter cells via a process known as cytokinesis. Because the growth and reproduction of the cell places strain on the trophozoite, amoebae often undergo encystations during this period.

Though members of the Acanthamoeba genus are abundantly present in almost every habitat and rarely cause infections, they are most notorious for their pathogenic qualities. The two human infections notably caused by Acanthamoeba organisms are Acanthamoeba keratitis and granulomatous encephalitis. Granulomatous encephalitis is a rare and typically fatal infection that affects the central nervous system. It often presents with skin lesions and neurological symptoms and is usually restricted to people with compromised immune systems.

Acanthamoeba keratitis is a painful infection of the eye usually associated with the wearing of contact lenses. Several factors contribute to the likelihood of infection, including the improper cleaning of contact lenses, wearing a contact lens for excessive lengths of time, and exposure to contaminated water. Trophozoites or cysts adhere to the lenses, particularly worn soft lenses, and then bind to the cornea of the eye. Trophozoites then begin breaking down the cells of the eye with phagocytosis, a sort of "eating" process in which the amoeba envelopes matter with the help of its acanthopodia. This disease is rare and may result in blindness.