Entamoeba histolytica: an explanation for the reported conversion of "nonpathogenic" amebae to the "pathogenic" form

The reported conversion of "nonpathogenic" Entamoeba histolytica isolates to the "pathogenic" form during attempted axenization of the amebae is highly controversial. After failing to obtain conversions ourselves we concluded that the simplest explanation for the published observations would be contamination of nonpathogenic cultures with pathogenic amebae. To address this possibility we used a method based on analysis of stable DNA polymorphisms that allows the positive identification of individual pathogenic isolates. The DNA patterns obtained using the "converted" amebae proved to be identical to those of reference isolates present in the laboratories at the time of conversion. We also found that very few cells need be transferred for a pathogenic contaminant to become established in a nonpathogenic culture. Cross-contamination fully explains the conversion phenomenon and thus recognition of nonpathogenic and pathogenic amebae as the distinct species Entamoeba dispar Brumpt 1925 and E. histolytica Schaudinn 1903 (Emend. Walker 1911), respectively, is upheld.