[Elizabeth_Zeibig]_Clinical_Parasitology__A_Practi(z-lib.org)

CHAPTER 3 The Amebas
69
Cytoplasm
TABLE 3-16
Acanthamoeba species
Cyst: Typical
Characteristics at a Glance
Double cell wall
(outer wall has
ragged edges)
No peripheral
chromatin
Central karyosome
Size range: 8-25 m
FIGURE 3-24 Acanthamoeba species cyst.
Parameter
Description
Size range 8-25 µm
Shape
Roundish with ragged edges
Number of nuclei One
Karyosome
Large and central
Peripheral chromatin Absent
Cytoplasm
Disorganized, granular,
sometimes vacuolated
Other features
Double cell wall—smooth
inner cell wall and outer
jagged cell wall
A disorganized, granular, and sometimes vacuolated
cytoplasm surrounds the nucleus.
FIGURE 3-25 Acanthamoeba species cyst exhibiting a
typical disorganized cytoplasm and vacuoles (iron hematoxylin
stain, ×1000). (Courtesy of WARD’S Natural Science
Establishment, Rochester, NY; http://wardsci.com.)
peripheral chromatin is absent. The cytoplasm
appears granular and vacuolated.
Cysts. The round-shaped Acanthamoeba
species cyst measures 8 to 25 µm (Figs. 3-24,
3-25; Table 3-16). The cyst is equipped with a
double cell wall. The inner smooth cell wall
is surrounded by an outer jagged cell wall.
This appearance is characteristic and aids in
Acanthamoeba cyst identification. The single
nucleus is similar in appearance to that of the
corresponding trophozoite form, a large karyosome
and no obvious peripheral chromatin.
Laboratory Diagnosis
As with N. fowleri, the specimen of choice for
diagnosing Acanthamoeba spp. trophozoites
and cysts is CSF. Brain tissue may also be examined.
Corneal scrapings (see Chapter 2) are the
specimen of choice for recovery of Acanthamoeba
infections of the eye. Suspected corneal
scrapings may be cultured on non-nutrient agar
plates seeded with gram-negative bacteria (specifically,
a viable strain of E. coli). The bacteria
serve as a source of food for the parasites. As
the Acanthamoeba organisms feed, they produce
a set of marks (known as tracks) on the agar.
Histologic examination of corneal scrapings
may also recover Acanthamoeba. Although primarily
used to detect fungi in clinical specimens,
calcofluor white may be used to stain Acanthamoeba
cysts present in corneal scrapings.
Indirect immunofluorescent antibody staining
(see Chapter 2) is the technique of choice for
speciating Acanthamoeba.
Life Cycle Notes
The trophozoites and cysts of Acanthamoeba
convert between these two morphologic forms in
the external environment. Humans may acquire