Morphology
Balantidium coli has 2 developmental stages: a trophozoite
stage and a cyst stage. (See also Life Cycle.)
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Trophozoites live in the large intestines of the host
animals. They are covered in cilia
and have boring or rotary motility. Balantidium
coli is known for being the largest protozoan parasite of humans, and it
is for the trophozoite stage that it earns this distinction. Trophozoites can measure between 50-130 mm long by 20-70 mm wide. In trophozoites, the two nuclei are clearly visible. The macronucleus is long and
kidney-shaped, and the spherical micronucleus is nestled next to it. Other distinguishing traits of the trophozoite include the opening,
known as the peristome, at the pointed anterior end, which leads to the
cytostome, or cell mouth. Balantidium
coli reproduces during the trophozoite stage either by asexual transverse
binary fission or sexual conjugation. |
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The cyst is the infective stage of the Balantidium coli
life cycle. Encystation is the process of forming the cyst; this event takes
place in the rectum of the host as feces are dehydrated or soon after the
feces have been excreted. Excystation produces a trophozoite from the cyst stage, and it
takes place in the large intestine of the host after the cyst has been
ingested. Cysts are smaller than trophozoites, measuring 40-60 mm across.
Cysts are round and have a tough, heavy cyst wall made of one or two
layers. Usually only the macronucleus
and perhaps cilia and contractile vacuoles are visible in the cyst. |
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See
the table below for a summary of the differences between trophozoites and
cysts.
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Trophozoite |
Cyst |
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Shape |
Oval, pointed at anterior
end |
Spherical |
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Size |
50-130 mm long by 20-70 mm wide |
40-60 mm across |
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Surface |
Covered in cilia |
Covered with thick, hard
cyst wall with cilia sometimes visible underneath |
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Motility |
Rotary or boring motility,
“like a thrown football” |
Non-motile |
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Infectious |
Not infective |
Infective |
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Reproduction |
By binary fission or
conjugation |
Non-reproductive |
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Nuclei |
Macronucleus
(kidney-shaped) and micronucleus (spherical, next to macronucleus) visible |
Only macronucleus (kidney-shaped)
visible; contractile vacuole visible in young cysts; in older cysts,
organelle structures look granular |
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Important cell structures |
Funnel-shaped cytostome
(cell mouth) near anterior end; 2 contractile vacuoles |
Cyst wall made of one or
two layers |
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Diagnosis |
Occasionally found in
feces, often found in tissue biopsies of infected individuals |
Diagnostically found in
feces of infected individuals |
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Drawing |
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