Acanthamoeba

Acanthamoeba

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.

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 granulomatous encephalitis is an opportunistic protozoan pathogen that rarely causes disease in humans. Approximately 400 cases have been reported worldwide with a survival rate of only two to three percent. Infection usually occurs in patients with an immunodeficiency, diabetes, malignancies, malnutrition, systemic lupus erythematosus, or alcoholism. The parasite's portal of entry is via lesions in the skin or the upper respiratory tract or via inhalation of airborne cysts. The parasite then spreads hematogenously into the central nervous system. Acanthamoeba crosses the blood brain barrier by means that are not yet understood. Subsequent invasion of the connective tissue and induction of pro-inflammatory responses leads to neuronal damage which can be fatal within days. A post-mortem biopsy reveals severe oedema and hemorrhagic necrosis. A patient that has contracted this illness usually displays subacute symptoms including altered mental status, headaches, fever, neck stiffness, seizures, focal neurological signs such as cranial nerve palsies and coma all leading to death within one week to several months. Due to the rarity of this parasite and our lack of knowledge there are currently no good diagnoses or treatments for Acanthamoeba.

Infection usually mimics that of bacterial leptomeningitis, tuberculous meningitis, or viral encephalitis. The misdiagnosis often leads to erroneous treatment that is ineffective. In the case that Acanthamoeba is diagnosed correctly, the current treatments such as amphotericin-B, rifampicin, trimethroprim-sulfamethoxazole, ketokonazole, fluconazole, sulfadiazine, albendazole are only tentatively successful. Correct and timely diagnosis as well as improved treatment methods as well as understanding of the parasite are important factors in improving the outcome of infection by Acanthamoeba.

MRSA

Methicillin-resistant Staphylococcus aureus (MRSA) is an important pathogen in the hospital setting due to its resistance to many antibiotics. Recent findings from the University of Bath 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 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.

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 (renamed A.castellanii Poussard)
  • 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.

See also

References

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

External links

Search another word or see Acanthamoebaon Dictionary | Thesaurus |Spanish
Copyright © 2014 Dictionary.com, LLC. All rights reserved.
  • Please Login or Sign Up to use the Recent Searches feature
FAVORITES
RECENT

;