Entamoeba histolytica - Culture, Life Cycle

Culture of Entamoeba histolytica

Entamoeba histolytica can be cultured by the following methods:

Polyxenic culture

• Polyxenic culture was created by Boeck and Drbohlav in 1927 and was the first medium to successfully cultivate Entamoeba histolytica

• contains the egg-serum medium with Lock’s medium and bacterial flora (polyxenic or polybacterial culture) which will provide nourishment for the growing parasites

• other cultures such as Balamuths, Nelsons, or Robertsons’ media can also be used to isolate the amoebae from the stool

Axenic Culture

• Axenic Culture is a bacteria-free culture that was first created by Diamond in 1961

• useful for:

  * studying the pathogenicity of amoebae

  * test anti-amoebic drugs in vitro

  * study immunological property

  * prepare axenic amoebic antigens for immunodiagnosis of amoebiasis

Animal culture

• inoculation of amoebic Entamoeba histolytica trophozoites into the lumen of the caecum by laparotomy in laboratory animals during animal culture such as rats, guinea pigs, dogs, rabbits, etc produces intestinal amoebic lesions.

• inoculation of amoebic trophozoites or of the liver by gelatin sponge soaked with the trophozoites produces amoebic liver abscesses in golden hamsters, gerbils, and newborn mice

• in guinea pigs that have been sensitized with amoebic Entamoeba histolytica extracts followed by repeated low-grade intracecal inoculation of amoebic trophozoites produce amoebic granuloma

Life Cycle of Entamoeba histolytica

The life cycle of Entamoeba histolytica is monoxenous and is completed in a single host- the man

• primary infection occurs by ingestion of water and food infested with mature quadrinucleate cysts.

• infection can also acquire by anogenital or oro-genital sexual contact

• Once reaching the small intestine, the cysts excyst and become active in the small intestine’s neutral or alkaline environment

• the excystation occurs after the cyst wall is lysed by intestinal trypsin liberating a single trophozoite with four nuclei

• the newly released Entamoeba histolytica trophozoite undergoes a series of cytoplasmic and nuclear divisions to release eight small metacystic trophozoites.

• the peristaltic movement of the small intestine carries the trophozoites into the ileocaecal area of the large intestine where they grow and multiply via binary fission

• they eventually colonize the large intestine’s mucosal surfaces as well as the crypts

• other factors such as intestinal motility, transit time, presence/absence of specific intestinal flora, and diet of the host determine the success of colonization by Entamoeba histolytica trophozoite.

Following colonization, the Entamoeba histolytica infection may proceed by following actions:

• the multiplying trophozoites may produce little to no lesions on the intestinal tissues. Hence they only feed on the starches as well as mucous secretions on the surface of the mucosa.

  * as they pass down, the stimulation of defecation triggers the trophozoites to encyst which are then passed along with the feces as cysts

• in other hosts, the trophozoites may invade the large intestinal tissue- although the factor that results in such conditions is poorly understood

  * The Entamoeba histolytica trophozoite form of the parasite produces lesions in the colon by the process of gelatinous necrosis, followed by abscess, and finally results in intestinal ulcer. When the stool is passed in such individuals, a large number of trophozoites are present alongside blood as well as mucus.

• In complicated cases, the large intestine is eroded to a large extent which results in the Entamoeba histolytica trophozoites gaining access to the radicles of the portal veil through which they are carried to the host liver.

  * Following multiplication in the liver, the trophozoites cause non-suppurative liver infection which may progress to suppurative amoebic liver abscess.

In the outside environment, trophozoites degenerate within minutes. The Entamoeba histolytica cysts on the other hand might remain viable for weeks to months if they remain in a moist environment. As cyst is the infective form of this parasite, new infestations may occur through the fecal-oral route as well as the consumption of water and vegetables containing the cysts.