A λgt11 cDNA library was constructed from the poly(A)+ RNA of trophozoites of Entamoeba histolytica HM-1:IMSS strain. The library was immunologically screened with monoclonal antibody 4G6, which is specific for the 30,000-M(r) antigen of pathogenic isolates. A 0.7-kb clone was isolated, and its nucleotide sequence was determined. To examine whether this gene was specific for pathogenic isolates, a polymerase chain reaction was performed by using four sets of primers and the genomic DNA of pathogenic and nonpathogenic isolates as templates. Amplified DNAs were detected not only in pathogenic isolates but also in nonpathogenic isolates. However, when sequences of amplified DNA of these isolates were compared, minor differences were observed. By considering the presence or absence of recognition sites of some endonucleases, it was possible to distinguish between the pathogenic and nonpathogenic isolates. When various isolates with different zymodemes were examined by polymerase chain reaction and enzyme digestion, the results of typing were entirely in accord with those of zymodeme analysis. These results indicate that there is dimorphism in the genomic DNA coding the 30,000-M(r) antigen of E. histolytica and that the combined use of the polymerase chain reaction and enzyme digestion is a useful strategy for identification of species and determination of pathogenicity.
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