A Taiwanese research group has proposed that the pear-colonizing genotype of Xylella fastidiosa, so far only found in Taiwan, be assigned to a new species, Xylella taiwanensis. This is now the second species of Xylella. How much more diversity remains to be described?
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TITLE: Xylella taiwanensis sp. nov. cause of pear leaf scorch disease in Taiwan
AUTHORS: C.-C Su, W. -L. Deng, F.-J. Jan, C. -J. Chang, H. Huang, H.-T Shih, J Chen
ABSTRACT: A Gram-negative, nutritionally fastidious bacterium (PLS229T) causing pear leaf scorch was identified in Taiwan and previously grouped into Xylella fastidiosa. Yet, significant variations between PLS229T and X. fastidiosa were noted. In this study, PLS229T was phenotypically and genotypically evaluated against representative strains of X. fastidiosa, including strains of the currently known subspp. fastidiosa, multiplex and “pauca”. Because of the difficulty in in vitro culture characterization, emphases were made to utilize the available whole genome sequence information. The average nucleotide identity (ANI) values, an alternative for DNA-DNA hybridization relatedness, between PLS229T and X. fastidiosa were 83.4-83.9%, significantly lower than the bacterial species threshold of 95%. In contrast, sequence similarity of 16S rRNA genes was above 98%, larger than the 97% threshold to justify if two bacterial strains belong to different species. The uniqueness of PLS229T also was evident by observing only about 87% similarity in 16S-23S internal transcribed spacer (ITS) between PLS229T and X. fastidiosa strains, discovering significant single nucleotide polymorphisms at 18 randomly selected housekeeping gene loci, observing distinct fatty acid profile for PLS229T compared to X. fastidiosa, and PLS229T having different observable phenotypes, such as different susceptibility to antibiotics. A phylogenetic tree derived from 16S rRNA gene sequences showed a distinct PLS229T phyletic lineage positioning between X. fastidiosa and Xanthomonas. On the base of these data, a new species, Xylella taiwanensis sp. nov