Xylella taiwanensis sp nov., causing pear leaf scorch disease
A Gram-stain-negative, nutritionally fastidious bacterium (PLS229(T)) causing pear leaf scorch was identified in Taiwan and previously grouped into Xylella fastidiosa. Yet, significant variations between PLS229(T) and Xylella fastidiosa were noted. In this study, PLS229(T) was evaluated phenotypically and genotypically against representative strains of Xylella fastidiosa, including strains of the currently known subspecies of Xylella fastidiosa, Xylella fastidiosa subsp. multiplex and 'Xylella fastidiosa subsp. pauca'. Because of the difficulty of 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 PLS229(T) and Xylella fastidiosa were 83.4-83.9%, significantly lower than the bacterial species threshold of 95 %. In contrast, sequence similarity of 16S rRNA genes was greater than 98 %, higher than the 97% threshold to justify if two bacterial strains belong to different species. The uniqueness of PLS229(T) was also evident by observing only about 87% similarity in the sequence of the 16S-23S internal transcribed spacer (ITS) between PLS229(T) and strains of Xylella fastidiosa, discovering significant single nucleotide polymorphisms at 18 randomly selected housekeeping gene loci, observing a distinct fatty acid profile for PLS229(T) compared with Xylella fastidiosa, and PLS229(T) having different observable phenotypes, such as different susceptibility to antibiotics. A phylogenetic tree derived from 16S rRNA gene sequences showed a distinct PLS229(T) phyletic lineage positioning it between Xylella fastidiosa and members of the genus Xanthomonas. On the basis of these data, a novel species, Xylella taiwanensis sp. nov. is proposed. The type strain is PLS229(T) (=BCRC 80915(T) = JCM 31187(T)).