Emended phenotypic characterization of Prototheca zopfii: a proposal for three biotypes and standards for their identification.

A representative selection of Prototheca zopfii strains isolated from different environmental habitats or clinical cases was characterized in a polyphasic approach in order to assess their intraspecies taxonomic position. Recently, the recognition of distinct phenotypic clusters has been reported as the assignment of 'variants'. In the present study, 11 strains were compared by a number of phenotypic and genetic criteria, including growth characteristics, biochemical reactions and serotyping results. Based on emended standards for biotype identification, P. zopfii strains showed auxanographic differences and distinct assimilation patterns with respect to utilization of amino acids and glycerol. Serotyping by means of immunoblotting revealed that all isolates of variant II obtained from clinical cases, i.e. isolates from bovine mastitis or from human enteropathia, showed specific antigen patterns. They were found to be different from strains assigned to the other two variants with respect to their immunogenic antigens. Furthermore, comparison of partial 18S rDNA sequences confirmed distinct differences between the former variants. Based on these results, it is proposed that P. zopfii merits classification as a species comprising three biotypes.