Identification of an avirulent Entamoeba histolytica strain with unique tRNA-linked short tandem repeat markers

Highly polymorphic, non-coding short tandem repeats (STR) are scattered between the tRNA genes in Entamoeba histolytica in a unique tandemly arrayed organization. STR markers that correlate with the virulence of individual E histolytica strains have recently been reported. Here we evaluated the usefulness of tRNA-linked STR loci as genetic markers in identifying virulent and avirulent strains of E histolytica from 37 Japanese E histolytica samples (12 diarrheic/dysenteric, 20 amebic liver abscess (ALA), and 5 asymptomatic cases). Twenty three genotypes, assigned by combining the STR sequence types from all 6 STR loci, were identified. One to 8 new STIR sequence types per locus were also discovered. Genotypes found in asymptomatic isolates were highly polymorphic (4 out of 5 genotypes were unique to this group), while in symptomatic isolates, almost half of the genotypes were shared between diarrhea/dysentery and ALA. One asymptomatic isolate (KU27) showed unique STIR patterns in 4 loci. This strain, though associated with the typical pathogenic zymodeme II, failed to induce amebic liver abscess by animal challenge, which suggests that inherently avirulent E histolytica strains exist, that are associated with unique genotypes. Furthermore, STR genotyping and in vivo challenge of 2 other asymptomatic isolates (KU14 and KU26) verified the covert virulence of these strains. (C) 2009 Published by Elsevier Ireland Ltd.