Acanthamoeba

Acanthamoeba
Scientific classification
Kingdom: Protista Phylum: Amoebozoa Genus: Acanthamoeba

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Tamar Sifri [2]

Overview

Acanthamoeba is a genus of amoebae, one of the most common protozoa in soil, and also frequently found in fresh water and other habitats. The cells are small, usually 15 to 35 μm in length and oval to triangular in shape when moving. The pseudopods form a clear hemispherical lobe at the anterior, and there are various short filose extensions from the margins of the body. These give it a spiny appearance, which is what the name Acanthamoeba refers to. Cysts are common. Most species are free-living bacterivores, but some are opportunists that can cause infections in humans and other animals.

Acanthamoeba as a human pathogen

Diseases caused by Acanthamoeba include amoebic keratitis and encephalitis. The latter is caused by Acanthamoeba entering cuts and spreading to the central nervous system. The former is a rare disease where amoebae invade the cornea of the eye. In the United States, it is nearly always associated with contact lens use, as Acanthamoeba can survive in the space between the lens and the eye. However, elsewhere in the world, many cases of Acanthamoeba present in non-contact lens wearers. For this reason, contact lenses must be properly disinfected before wearing, and should be removed when swimming or surfing.

To detect Acanthamoeba on a contact lens in a laboratory, a sheep blood agar plate with a layer (a lawn) of E. coli is made. Part of the contact lens is placed on the agar plate. If Acanthamoeba are present, they will ingest the bacteria, leaving a clear patch on the plate around the area of the lens. Polymerase chain reaction can also be used to confirm a diagnosis of Acanthamoeba keratitis, especially when contact lenses are not involved.

Acanthamoeba and MRSA

Methicillin-resistant Staphylococcus aureus (MRSA) is an important pathogen in the hospital setting due to its resistance to many antibiotics. Recent findings demonstrate that MRSA can infect and replicate inside of Acanthamoeba polyphaga; this Acanthamoeba species is widespread throughout the environment. Since A. polyphaga can form cysts, cysts infected with MRSA can act as a mode of airborne dispersal for MRSA. Additionally, it is noted that "evidence with other pathogens suggests that pathogens that emerge from amoeba are more resistant to antibiotics and more virulent." It has been observed that Acanthamoeba can increase MRSA numbers by 1000-fold.

Importance of Acanthamoeba in soil ecology

A. castellanii can be found at high densities in various soil ecosystems. It preys on bacteria, but also fungi and other protozoa.

This species is able to lyse bacteria and produce a wide range of enzymes such as cellulases or chitinases and probably contributes to the break down of organic matter in soil, contributing to the microbial loop.

Acanthamoeba species

Species of Acanthamoeba are distinguished mainly on the form of cysts, and include the following; those marked with an asterisk are known to cause infections.

A. astronyxis*
A. castellanii*
A. comandoni
A. culbertsoni*
A. divionensis
A. griffini
A. hatchetti*
A. healyi
A. jacobsi
A. lenticulata
A. lugdunensis*
A. mauritaniensis
A. palestinensis*
A. pearcei
A. polyphaga*
A. pustulosa
A. quina*
A. rhysodes*
A. royreba
A. terricola
A. triangularis
A. tubiashi

Endosymbiontes of Acanthamoeba

Acanthamoeba sp. contain diverse bacterial endosymbionts which are similar to human pathogens. Because of this they are considered to be potential emerging human pathogens. The exact nature of these symbionts and the benefit they represent for the amoebal host still have to be clarified.

Life Cycle

Acanthamoeba spp. have been found in soil; fresh, brackish, and sea water; sewage; swimming pools; contact lens equipment; medicinal pools; dental treatment units; dialysis machines; heating, ventilating, and air conditioning systems; mammalian cell cultures; vegetables; human nostrils and throats; and human and animal brain, skin, and lung tissues. Unlike N. fowleri, Acanthamoeba has only two stages, cysts and trophozoites, in its life cycle. No flagellated stage exists as part of the life cycle. The trophozoites replicate by mitosis (nuclear membrane does not remain intact). The trophozoites are the infective forms, although both cysts and trophozoites gain entry into the body through various means. Entry can occur through the eye, the nasal passages to the lower respiratory tract, or ulcerated or broken skin. When Acanthamoeba spp. enters the eye it can cause severe keratitis in otherwise healthy individuals, particularly contact lens users. When it enters the respiratory system or through the skin, it can invade the central nervous system by hematogenous dissemination causing granulomatous amebic encephalitis (GAE) or disseminated disease, or skin lesions in individuals with compromised immune systems. Acanthamoeba spp. cysts and trophozoites are found in tissue.

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  Acanthamoeba Life Cycle
  Adapted from CDC

Antimicrobial Regimen

• 1. Granulomatous amoebic encephalitis, meningitis, and disseminated Acanthamoeba disease

• Preferred regimen (1): Pentamidine AND Itraconazole AND Sulfadiazine AND Flucytosine
• Preferred regimen (2): Sulfadiazine AND Fluconazole AND Pyrimethamine
• Preferred regimen (3): Sulfadiazine AND Flucytosine AND TMP-SMX
• Preferred regimen (4): TMP-SMX AND Rifampin AND Ketoconazole
• Preferred regimen (5): Miltefosine AND Amikacin
• Preferred regimen (6): Miltefosine AND Voriconazole
• Preferred regimen (7): Pentamidine AND Itraconazole AND Flucytosine AND Levofloxacin AND Amphotericin B AND Rifampin
• Preferred regimen (8): Pentamidine AND Fluconazole AND Miltefosine
• Note: The mainstay of successful treatment includes early diagnosis and combination therapy with pentamidine, azole, sulfonamide, miltefosine, and possibly flucytosine.

• 2. Cutaneous acanthamoebiasis

• Preferred regimen: Pentamidine AND Sulfadiazine AND Flucytosine AND (Itraconazole OR Fluconazole) AND Chlorhexidine topical AND Ketoconazole topical

• 3. Acanthamoeba keratitis

• Preferred regimen: (Polyhexamethylene biguanide topical OR Chlorhexidine topical) ± (Propamidine topical OR Hexamidine topical)
• Note (1): Azole antifungal drugs (Ketoconazole, Itraconazole, Voriconazole) may be considered as oral or topical adjuncts.
• Note (2): The duration of therapy for Acanthamoeba keratitis may last six months to a year.
• Note (3): Pain control can be helped by topical cyclopegic solutions and oral nonsteroidal medications.
• Note (4): The use of corticosteroids to control inflammation is controversial.
• Note (5): Penetrating keratoplasty may help restore visual acuity.

See also

• Legionella
• Momus (artist)

References

• Khan, N. A. (2006) Acanthamoeba: biology and increasing importance in human health. Fems Microbiology Reviews 30, 564-595.

External links

• Acanthamoeba spp. as Agents of Disease in Humans - Clinical Microbiology Reviews, accessed on 4th February 2006
• Comprehensive resource on Amoeba -
• Eye health and Acanthamoeba