Acanthamoeba

contributed by R. Neal Band and Frededrick L. Schuster

Acanthamoeba spp. are found in a variety of soil, freshwater and brackish water environments, and some members of the genus are opportunistic pathogens of humans. In soil, they are part of a collection of organisms referred to as small amebae (compared to Amoeba proteus and other "large" amebae) which constitute a significant component of the soil habitat, reaching up to a million amebae per gram of soil. The Acanthamoeba life-cycle is comprised of a trophic (actively feeding) stage, and a thick-walled cystic (dormant) stage which forms under conditions of adversity, such as desiccation and lack of food. The trophic ameba has distinctive fine-pointed pseudopods (acanthopodia) that project from the surface. Cysts have been reported to remain viable under laboratory conditions for twenty-four years. With return of conditions optimal for growth, cysts germinate to give rise to trophic amebae. Differences in cyst morphology have been used to distinguish species of Acanthamoeba.

Ecology: Acanthamoeba spp. are found in virtually every type of soil and water habitat, and have even been isolated from Antarctic soil samples. The organism can also be isolated from home aquaria, humidifiers, flower pots, domestic water supplies and water taps, and other locations, which puts them in close contact with humans. Amebae isolated from nature are often found to harbor bacteria, implicating them as vectors of potentially pathogenic bacteria. They may, for example, play a role in the survival of bacteria such as Legionella, the causal agent of legionellosis, and may explain the ability of these bacteria to survive in air conditioning cooling towers and hospital water supplies.
In the soil habitat, acanthamebae feed on bacteria and other microorganisms. But soil amebae are also subject to the degradative activities of other soil microbes, such as yeasts and fungi. Cysts of soil amebae, such as Acanthamoeba, are destroyed in the presence of other soil microbes.

Nutrition and cultivation: Acanthamoeba spp can be grown in the laboratory under a variety of conditions. Amebae proliferate when grown with bacteria, but will also grow in bacteria-free (axenic) conditions in an enriched culture medium (peptone, yeast extract, and glucose), reaching numbers of more than a million amebae per milliliter. Under optimal laboratory growth conditions, amebae have a doubling time of several hours. Defined media, in which all components of the growth medium are known, are also available for more exacting studies on nutritional requirements. Their ease of cultivation and the ability of their cysts to be freeze-dried for long-term storage, make Acanthamoeba an organism of choice for cellular and molecular studies, as well as more conventional studies on ecology, pathogenesis, and nutrition.

Evolutionary relationships: In the 1930’s, Andre Lwoff and his collaborators identified thiamine as a vitamin requirement. Subsequent work by others identified vitamin B12 and biotin as additional vitamin requirements, which could be consistent with an algal origin of acanthamebae. This was called into question, however, when the herbicide Trifluralin was found to inhibit the growth of Naegleria but not Acanthamoeba, indicating that microtubules of the former were plant-like while those of Acanthamoeba were animal-like. But the Acanthamoeba cyst wall contains cellulose, a carbohydrate usually associated with plants. Recent studies have made use of the 18S ribosomal RNA gene to determine evolutionary relationships among Acanthamoeba species and strains, as well as relationships to other eukaryotic organisms.

Cytology: Acanthamoeba has a nucleus with a large, central nucleolus. Mitosis, termed mesomitosis, differs from the typical eukaryotic pattern in that centrioles are not present although spindle fibers do terminate in centrosomes. Acanthamoebae have a high degree of resistance to radiation and mutagenic agents, suggesting that they may have a polyploid chromosomal complement or, alternatively, they possess a very efficient DNA repair mechanism.

Pathogenesis: Acanthamoeba spp., although free-living organisms, are also opportunistic pathogens of humans, causing systemic acanthamebiasis (encephalitis, cutaneous, disseminated and nasopharyngeal infections), as well as amebic keratitis. Groups at high risk for systemic infections include immunocompromised and immunosuppressed individuals, while keratitis occurs in otherwise healthy individuals. Several different strains and species of Acanthamoeba have been implicated in these infections, unlike Naegleria where pathogenicity is associated with a single species, N. fowleri.
In Acanthamoeba encephalitis and other systemic infections, the portals of entry are through the respiratory tract and wounds, with passage to the brain and other sites via the circulatory system. Cysts of Acanthamoeba in the soil can be carried by the wind to the nasal passages, and amebae have been isolated from the nasal mucosa of healthy humans. They can also enter the body through breaks in the skin (cutaneous acanthamebiasis) that become contaminated with soil containing cysts. Many of the victims of systemic Acanthamoeba infections have been AIDS patients, whose immune system has been weakened by the human immunodeficiency virus. Immunocompetent individuals have been shown to have antibodies against Acanthamoeba, indicating that many persons may be exposed to these amebae but do not develop disease.
In Acanthamoeba keratitis, in which the amebae attack the corneal surface and stroma of the eye, infection occurs as the result of corneal trauma or, more commonly, because of poor hygiene in care of contact lenses, particularly the extended wear variety. A major source of infection for contact lens users is home-made saline solutions, prepared from non-sterile tap or distilled water containing bacteria and encysted Acanthamoeba. The cysts germinate, giving rise to trophic amebae, which feed on bacteria and proliferate on the contact lens surface or in the lens case. When the lens is placed in the eye, the amebae are then transferred to the corneal surface. Once established in the cornea, they are difficult to eliminate due to the cyst, which is not permeable to many antimicrobial agents.

Antimicrobial therapy for Acanthamoeba infections: Systemic Acanthamoeba infections are often fatal, in part because of difficulty in diagnosing the disease, and in part due to delayed use of antimicrobial therapy. Among the drugs that have been used successfully to treat systemic Acanthamoeba infections are 5-fluorocytosine, fluconazole, and pentamidine isethionate. Optimal drug therapy, however, has not yet been determined.
Acanthamoeba keratitis has been successfully treated with topical applications of chlorhexidine or polyhexamethylene biguanide, with and without propamidine isethionate.
CODENET - website from recent EU project on coccolithophores, includes listing of largest available culture collection.

Pathogenic potential: Pathogenicity and virulence are often associated with production of destructive enzymes or toxins. In N. fowleri, prolonged cultivation in axenic medium causes a significant loss of virulence which can be traced to several differentially expressed genes, one of which codes for a serine protease. Similar studies are lacking for genetic localization of virulence in Acanthamoeba, although secretion of ribonuclease, proteolytic enzymes, and lipases may play a role in determining destructive potential of the amebae.
Most clinical isolates of Acanthamoeba are able to tolerate and grow at 37oC, suggesting that temperature tolerance is an important factor in determining pathogenic potential. There are, however, strains that grow at 37oC and are not pathogenic, and some of the keratitis isolates grow better at 30oC than at 37oC.