SUBGENUS SYSTEMATICS OF ACANTHAMOEBA - 4 NUCLEAR 18S RDNA SEQUENCE TYPES

Classification of Acanthamaeba at the subgenus level has been problema tic, but increasing reports of Acanthamoeba as an opportunistic human pathogen have generated an interest in finding a more consistent basis for classification. Thus, we are developing a classification scheme b ased on RNA gene sequences. This first report is based on analysis of complete sequences of nuclear small ribosomal subunit RNA genes (Rns) from 18 strains. Sequence variation was localized in 12 highly variabl e regions. Four distinct sequence types were identified based on parsi mony and distance analyses. Three were obtained from single strains: T ype T1 from Acanthamoeba castellanii V006, T2 from Acanthamoeba palest inensis Reich, and T3 from Acanthamoeba griffini S-7. T4, the fourth s equence type, included 15 isolates classified as A. castellanii, Acant hamoeba polyphaga, Acanthamoeba rhysodes, or Acanthamoeba sp., and inc luded all 10 Acanthamoeba keratitis isolates. Interstrain sequence dif ferences within T4 were 0%-4.3%, whereas differences among sequence ty pes were 6%-12%. Branching orders obtained by parsimony and distance a nalyses were inconsistent with the current classification of T4 strain s and provided further evidence of a need to reevaluate criteria for c lassification in this genus. Based on this report and others in prepar ation, we propose that Rns sequence types provide the consistent quant ititive basis for classification that is needed.