Acanthamoeba misidentification and multiple labels: redefining genotypes T16, T19, and T20 and proposal for Acanthamoeba micheli sp nov (genotype T19)

Acanthamoeba species are ubiquitous amoebae able to cause important infections in humans and other vertebrates. The full/near-full sequences (> 2000 bp) of the small subunit ribosomal RNA gene (SSU rDNA or 18S rDNA) are used to cluster Acanthamoeba as genotypes, labeled T1 to T20. Genotype T15 remains an exception, being described only partially on a 1500-bp fragment. Strains are thus usually identified based on their 18S identity matches with reference strains, often using shorter (< 500 bp) diagnostic fragments of the gene. Nevertheless, short fragments (< 1000 bp) have been used to propose genotypes. This has been criticized, and doubts arise therefore on possible confusion leading to classify distinct partial sequences with a same label(s). We demonstrate herein that several partial sequences misassigned either to T16 or to T4, actually belong to at least two separate and distinct genotypes. We obtained the full 18S rDNA of a strain previously typed as T16 on the basis of a small fragment and demonstrated that it actually belongs to the recently described T19. We propose the name Acanthamoeba micheli sp. nov., for this strain. Furthermore, partial molecular phylogenies were performed to show that several other misassigned T16 partial sequences belong to a new genotype. This latter includes also misassigned T4 partial sequences, only recently available as full sequences and labeled as T20. We thus reassign these partial sequences to the genotype T20. Longer sequences, ideally at least 90 % of the total gene length, should be obtained from strains to ensure reliable diagnostic and phylogenetic results.