Naegleria fowleri - Classification, History, Habitat, Morphology, Culture

Introduction of Naegleria fowleri

Naegleria fowleri is a free-living amoeba found in the soil, sewage, and water which causes infection of the Central Nervous System (CNS) and is mostly fatal. It causes primary amoebic meningoencephalitis (PAM) which is an acute, fulminant, rapid, and fatal infection of the central nervous system.

Classification of Naegleria fowleri

The classification of Naegleria fowleri is phenotypic as follows:

Domain: Eukaryota

Phylum: Percolozoa

Class: Heterolobosea

Order: Schizopyrenida

Family: Vahlkampfiidae

Genus: Naegleria

Species: N. fowleri

History of Naegleria fowleri

Historically, in 1965, Malcolm Fowler and Rodney F Carter first described Naegleria fowleri from South Australia. Bull coined the term primary amoebic meningoencephalitis in 1966 to distinguish this condition from secondary meningoencephalitis caused by Entamoeba histolytica.

Habitat of Naegleria fowleri

Naegleria fowleri is free-living amoebae that inhabit the soil, fresh as well as brackish water such as lakes, rivers, ponds, etc. They are also found in the throat and nasal cavities in humans.

Morphology of Naegleria fowleri

In its life cycle, Naegleria fowleri exists in three morphologically stages:

• trophozoite

• amoeboflagellate (temporary flagellar stage)

• cyst

Trophozoites

• vegetative stage/feeding stage of the Naegleria fowleri amoeba

• trophozoites measure 10-20 μm in diameter

• the presence of granular cytoplasm, distinct ectoplasm, and multiplies by binary fission

• a characteristic feature is the presence of a large central nuclear karyosome surrounded by a halo

• motility takes place with the help of labopodia

• In addition to motility, the broadly rounded, granule-free Labopodia projection originating from the surface of the amoebae also helps in the ingestion of bacteria, yeast cells, and cellular debris as well as in attachment

• in host tissue, it ingests red and white blood cells which results in the destruction of the host tissue

• found in CSF or in the tissue

Flagellate stage

• this flagellate stage is called the amoeboflagellate stage which is a temporary, non-feeding, and non-dividing form

• formed when the Naegleria fowleri trophozoites are introduced to a change in ionic concentration including the distilled water

• this form usually occurs within 2 hours of exposure and can revert back to trophozoite form within 24 hours

• pear-shaped with a flagellar apparatus at the broader end containing two terminal flagella, two basal bodies, microtubules, and a single striated rootlet called rhizoplast

• movement at this stage is a rapid forward movement or a slow-spinning circular motion

Cyst

• this form is the resistant form which protects Naegleria fowleri from drugs, desiccation as well as shortage of food

• round, 7-10 μm in diameter enveloped in a smooth double-layered 1 μm cyst wall

• contains a single nucleus, contractile vacuoles, and food vacuoles

• in clinical specimens, cysts are not usually observed as the rapid infection results in quick death- before trophozoites are able to encyst

• when environmental conditions are favorable, they excyst to their trophozoite form

Culture of Naegleria fowleri

Naegleria fowleri can be cultured in media as in laboratory animals.

In media

• Non-nutrient agar inoculated with monoxenic culture of bacteria such as Enterobacter species and Escherichia coli

• Axenic culture for Naegleria fowleri – enriched broth without added bacteria

• monolayer cell lines cultures such as Hela cells, MRChuan embryonic lung cells, and monkey kidney cells

In laboratory animals

• laboratory animals such as rabbits, rodents, and sheep can be used for experimental infection

• South American tapir is naturally susceptible to infection by Naegleria fowleri