AMOEBAE

M.PRASAD NAIDUMSc, (Medical) Ph.D. (Medical)

• An amoeba (also ameba, amœba or amoeboid) is a type of cell or organism which has the ability to alter its shape, primarily by extending and retracting pseudopods. Amoebae do not form a single taxonomic group, but are found in every major lineage of eukaryotic organisms (domain Eukaryota).

• Amoeoboid cells occur not only among the protozoa, but also fungi, algae and animals.• Among microbiologists, the terms

"amoeboid" and "amoebae" are often used interchangeably for any organism that exhibits amoeboid movement.

• The best known amoeboid protists are the "giant amoebae" Chaos carolinense and Amoeba proteus, both of which are widely cultivated and studied in classrooms and laboratories. Other well known species include the so-called "brain-eating amoeba" Naegleria fowleri, the intestinal parasiteEntamoeba histolytica, which causes amoebic dysentery, and the multicellular "social amoeba" Dictyostelium discoideum.

Naegleria fowleri :

Primary Amoebic Meningo

Encephalitis

(PAM)

Fowler & Carter (1965 ).

Amebiasis Caused by Entamoeba histolytica. (a) Light micrographs of a trophozoite (1,000)

A cyst (1,000).

• TROPHOZOITE :

Vegetative & Feeding stage .

CSF &Tissue .

10 -20 µm , Karyosome with halo .

Actively motile (Lobopodia )

Binary fission .

• Amoebae move and eat by using pseudopods, which are bulges of cytoplasm formed by the coordinated action of actin microfilaments pushing out the plasma membrane that surrounds the cell.

• Groups of amoebae are distinguished by the appearance and internal structure of their pseudopods.

• Amoebozoan species typically have bulbous pseudopods, rounded at the ends and roughly tubular in cross-section (lobose)

• Cercozoan amoeboids, such as Euglypha and Gromia, have slender, thread-like (filose) pseudopods. Foraminiferan emit fine, branching pseudopods that merge with one another to form net-like (reticulose) structures. Some groups, such as the Radiolaria and the amoeboids loosely called Heliozoa, have stiff, needle-like, radiating actinopods supported from within by bundles of microtubules

SHAPE, MOVEMENT

AND NUTRITION

• Free-living amoebae may be "testate" (enclosed within a hard shell), or "naked" (lacking any hard covering). Testate amoebae shells are composed various substances, including calcium, silica, chitin, or agglutinations of found materials like small grains of sand and the frustules of diatoms.• To regulate osmotic pressure, most

freshwater amoebae have a contractile vacuole which expels excess water from the cell.

•  This organelle is necessary because freshwater has a lower concentration of solutes (such as salt) than the amoeba's own internal fluids (cytosol). Because the surrounding water is hypotonic with respect to the contents of the cell, water is transferred across the amoeba's cell membrane by osmosis. Without a contractile vacuole, the cell would fill with excess water and, eventually, burst. Marine amoebae do not usually possess a contractile vacuole, because the concentration of solutes within the cell are in balance with the tonicity of the surrounding water.

• The food sources of amoebae vary. Some amoebae are predatory and live by consuming bacteria and other protists. 

• Some  are detritivores and  eat  dead  organic material. Amoebae  typically  ingest  their  food by phagocytosis,  extending  pseudopods  to encircle  and  engulf  live  prey  or  particles  of scavenged material. • Amoeboid  cells  do  not  have  a  mouth or cytostome,  and  there  is  no  fixed  place  on the  cell  at  which  phagocytosis  normally occurs.  Some  amoebae  also  feed by pinocytosis,  imbibing  dissolved  nutrients through vesicles formed  within  the  cell membrane.

AMOEBAE IN MULTICELLULAR

ORGANISMS: ANIMALS AND SLIME MOLDS

• Amoebae in multicellular organisms: animals and slime molds• Some multicellular organisms have amoeboid cells  only  in  certain  phases  of  life,  or  use amoeboid  movements  for  specialized functions.•   In  the  immune system of humans and other animals,  amoeboid white  blood  cells pursue invading  organisms,  such  as  bacteria  and pathogenic  protists,  and  engulf  them by phagocytosis.

• Amoeboid  stages  also  occur  in the multicellular fungus-like  protists, the so-called slime molds. • Both  the  plasmodial  slime  molds, currently  classified  in  the class Myxogastria,  and  the  cellular slime  molds  of  the groups Acrasida and Dictyosteliida, live as amoebae during  their  feeding stage. 

• The  cells  of  the  former  form  a giant multinucleate amoeboid organism,while  the  cells  of  the latter  live  separately  until  food runs  out,  at  which  time  the amoebae  aggregate  to  form  a multicellular migrating  slug which functions as a single organism

LIFE CYCLE : 

ONE HOST :  Man .

   Asexual Generation cycle .

MOI         :   Swimming in Contaminated water .

Infective form :  Trophozoites .

                      Cysts (Inhalation ) –Rare .

Entry : Nose           Olfactory mucosa.

Olfactory bulbs 

Sub Arachnoid space. 

(Multiplication) 

Choroid plexus 

Ventricular destruction. 

Ependymitis.

• Trophozoites             Cysts (unfavorable conditions ) .

• PATHOGENESIS :

IP : 2 -15 days .(5 days )

Neurotropic .

Brain tissue destruction .

Acute Hemorrhagic  Necrotising meningo 

encephalitis .

Cysts are absent in humans .

CLINICAL FEATURES : 

Children & Young adults .

PAM .

Rapid onset & Fulminant .

  Sudden severe  Persistent Bifrontal /Bitemporal   

     Headache ,Nausea , Projectile vomiting .

   Ageusia (loss of taste function).

   Parosmia (olfactory dysfunction).

   Generalized seizures  

   Photophobia

   Coma

   Death 

   Poor Prognosis .(95 % death )

DIAGNOSIS :

• Specimen : CSF (Antemortem )

Brain Biopsy (Post mortem ).

CSF :

• Wet mount : Plenty of Actively motile

trophozoites .

• Phase Contrast Microscopy .

• Stains : Trichrome , Giemsa , Wright .• Direct Fluorescent Antibody

staining .

• BIOPSY : IF & Immuno Peroxidase method .

• Serodiagnosis : Not Useful .

• Molecular methods : DNA probes & PCR .

• CSF : Features of Pyogenic

Meningitis .

• Treatment : Amphotericin B &

Rifampicin,Miconazole .

• Prevention :

Acanthamoeba• Opportunistic pathogens .

1. A.castellani .

2. A.astronyxis .

3. A.polyphaga .

4. A.culbertsoni .

• HABITAT : Dust , Soil , Sand , Rivers ,

Ponds , Tap water .

• MORPHOLOGY :

2 stages .

1. Trophozoite .

2. Cyst .

• TROPHOZOITE :

• Variable size & shape .

• 10 -40 µm .

• Acanthopodia Characteristic feature .

• CYST :

Polygonal /Spherical /Star shaped .

15 -20 µm.

Double layered cyst wall .

• LIFE CYCLE :

• MOI : Invasion of Broken skin .

• Inhalation of Cysts & Trophozoites .

• ROUTE : Lungs to Blood stream

(Multiplication )

• CNS .

• Sites of Localisation :Basal ganglia ,

• Posterior fossa , Cerebellum .

• PATHOGENESIS :

• Lesions of Brain , Eye , Lungs & Skin .

• Brain: Patchy , Sub Acute lesions of

granulomatous encephalitis .

• Skin: Nodular & Ulcerative lesion. Abscess .

• Cornea : Epithelial inflammation, Hypopyon .

• CLINICAL MANIFESTATIONS :

Granulomatous Amoebic Encephalitis (GAM)

Acanthamoeba Keratitis .

Cutaneous Lesions .

GAE:

• Rare .

Risk factors :

• AIDS ,

• Immunosupression,Organ transplantation

• Malnourished .

• Clinical features : Low grade fever,stiff neck ,

Altered Mental status .

Seizures ,

Cranial palsies ,

Hemiparesis , Ataxia ,

Photophobia

Coma ,

Multi organ Failure ,

Death .

• Acanthamoeba Keratitis :

Contact lens wearers .(trauma )

Chronic , Progressive , Ulceration .

Annular Infiltration & Congested Cornea .

Perforation .

Blindness .

• CUTANEOUS LESIONS :

• Face & Extremities .

• Nodules , papules & ulcers .

• Poor Prognosis .

• GAE Fatal .

• DIAGNOSIS :

1. Microscopy :

2. Trophozoites & Cysts In Brain Biopsy & Corneal

Smears .

3. Wet mount : Trophozoites & Cysts .

1. Stains : Acridine orange , Giemsa , LCB .

2. IF .

3. Culture :

4. Serlogical tests : Not useful .

5. Molecular methods .

6. Imaging Methods .

• TREATMENT :

• GAE – No therapy. Sulphonamides ,

Cotrimoxazole ,

Polymyxin –B .

• Keratitis : Topical Miconazole .

• Kertoplasty .

• PROPHYLAXIS :

• Disinfecting Contact lenses .

Foraminiferan (Ammonia tepida)

Shell of the testate amoeba Difflugia acuminata, made up of mineral particles

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