Fun With Microbiology (What's Buggin' You?): Entamoeba polecki

Entamoeba polecki

This amoeba most likely has worldwide distribution and can be found as an intestinal parasite in pigs and monkeys. It is generally considered to be non-pathogenic for humans but has occasionally been recovered from patients with loose stools and intestinal discomfort. Several sources have stated that in Papua New Guinea, E.polecki has been recovered as the most common intestinal parasite from the inhabitants. Living closely with these animals my increase the chances of carrying the parasite. E.polecki makes its home in the large intestine of man.

Trophozoites:

E.polecki trophs have an average size of 12 µm to 18 µm but can range from 10 µm to as large as 25 µm. Its cytoplasm can be vacuolated and may contain ingested bacteria and yeast. The size and cytoplasmic appearance may cause it to be confused with Entamoeba coli. The karyosome may not be visible which has the nucleus appear empty. When present, the karyosome is usually minute and centrally located, or nearly so. It however may occasionally appear diffuse. Peripheral chromatin can vary from being fine and evenly distributed as in E.histolytica, or coarse and irregularly distributed as in E.coli. This variability may make diagnosis a challenge.

Cysts:

E.polecki cysts are about 9 µm to 15 µm but can be as large as 24 µm and usually contain only one nucleus (usually E.histolytica has 4, E.coli has 8). It has rarely been reported as being binucleate. The karyosome again can be minute can compact or it may be larger and diffuse – it may be centrally located within the nucleus or somewhat eccentric. Peripheral chromatin may be delicate to coarse but is evenly distributed on the nuclear membrane. Chromatoidal material is usually abundant but highly variable in shape and size. It may be present as larger rods with rounded or splintered ends and they may be arranged parallel to each other in the cyst. Unique to these cysts may be an inclusion body of variable size. It stains a monochromatic grayish-purple with Iron hematoxylin stain or greenish with the trichrome stain. In an iodine stained preparation it usually appears light brown in contrast to the intensely dark brown staining of glycogen vacuoles. The nature of these inclusions is not known but does not appear to be glycogen.

Differentiation:

Differentiating the E.polecki trophozoites from E.histolytica and E.coli may be difficult as the size ranges overlap and both the cytoplasmic and nuclear appearance can mimic both. If examining a specimen that has amoeba showing characteristic of both E.histolytica & E.coli, the presence of E.polecki should be considered. Chromatoid bodies are usually are more numerous and show greater pleomorphism in E.polecki than E.histolytica. E.polecki cytoplasm often stains very darkly making it difficult to see the nucleus, inclusion mass and chromatoid bodies in the same plane of focus

Note: The photographs which follow were of E.polecki found in a fecal specimen obtained from a patient with gastrointestinal discomfort. Only trophozoites were present in the sample. I’m rather unhappy with the quality of the photos I took and present here. The features lack resolution and the amoeba appears darker in the photos than they did when viewed through the light microscope. The features appear to be over saturated in colour and “bleed” together, obscuring details. I was unable to correct for this satisfactorily with photo software post exposure. This specimen also contained Entamobea hartmanni trophs. Identification was confirmed by our provincial public health laboratory. Here they are for what they’re worth.

All photos below were taken using the Nikon 'Coolpix' camera at 1000X magnification and are from an Iron-Hematoxylin stained preparation.

E.polecki trophozoite exhibiting a coarse cytoplasm. Peripheral chromatin in the nucleus is rather evenly distributed. Karyosome is central but not distinct.

E.polecki trophozoite showing rather evenly distributed peripheral chromatin in the nucleus and as above the karyosome is rather diffuse and slightly eccentric.

E.polecki trophozoite showing a rather coarse cytoplasm with ingested material (bacteria). The karyosome shows even, dense peripheral chromatin but the karyosome appears to be absent in all planes of focus. Above the E.hartmanni troph may be a somewhat distorted Blastocystis hominis.