Biomedical Engineering Reference
In-Depth Information
Class Discoba is a newly invented class, and its taxonomic origin is
instructive. Recently acquired molecular data suggest that three subclasses
of Class Excavata share a common direct ancestor: Class Percoloza
(Chapter 18), Class Euglenozoa (Chapter 17), and Class Jakobid (which hap-
pens to contain no infectious organisms). The common ancestor of these
three classes is apparently not shared with another subclass of Excavata:
Class Metamonada (Chapter 16). To preserve monophyly within Class
Excavata, a newly named class needed to be inserted under Class Excavata.
This class would contain Class Percolozoa, Class Euglenozoa, and Class
Jakobid and would exclude Class Metamonada. The newly named class is
Discoba [75].
Under Class Discoba is Class Percolozoa, single-celled organisms
containing mitochondria with discoid cristae, and the ability to shift between
three morphologic forms: amoeboid, flagellate, and cyst. The amoeboid form
consists of a non-flagellated feeding cell. Like all amoeboid forms, it moves
slowly, by extending a section of its cytoplasm (the pseudopod). Under adverse
conditions, the amoeboid form can develop flagella, which presumably
enhance its ability to move to a more hospitable location. As you would expect
from organisms in a subclass of Bikonta, two flagella are observed in the flag-
ellate form. Under conditions that are severely unsuitable for growth,
the
organism converts to a cyst form.
Percolozoa
Heterolobosea
Schizopyrenida
Vahlkampfiidae
*Naegleria
There is only one known pathogenic genus in Class Percolozoa: Naegleria.
Naegleria fowleri is the only species of Naegleria that is known to be infec-
tious in humans. In older microbiology textbooks, Naegleria is grouped with
the Acanthamoeba, under Class Amoebozoa (Chapter 22). Naegleria rightly
belongs in Class Percolozoa. The naeglerian life cycle includes three stages:
cyst, trophozoite (amoeboid), and flagellate forms; whereas pathogenic
members of Class Amoebozoa have only two stages: cyst and trophozoite.
This distinction has taxonomic and diagnostic relevance. Though naeglerian
meningoencephalitis can be confused histologically with amoebic meningoen-
cephalitis, flagellate forms in cerebrospinal fluid would indicate a naeglerian
infection (not an amoebic infection).
Naegleria fowleri is found in fresh water and in poorly chlorinated swim-
ming pools, where it is free-living. Infections result from exposure to
free-living organisms in their natural habitat, and not from exposure to infected
individuals. The organism travels from the nose to the brain, where it causes a
meningoencephalitis. Because these organisms are widely found in water, it is
presumed that millions of people are exposed to the organism, but only rare
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