The phloem-limited bacterium of greening disease of citrus is a member of the α subdivision of the Proteobacteria.
The 16S ribosomal DNAs (rDNAs) of an Asian strain and an African strain of the uncultured, Gram-negative, walled, phloem-limited bacterium-like organism (BLO) associated with citrus greening disease was amplified using PCR. Coamplification of chloroplast 16S rDNA was evaded by using restriction enzymes; the chloroplast 16S rDNA was sensitive to BclI digestion and resistant to EcoRI digestion, while the 16S rDNA of the BLO was resistant to BclI digestion and sensitive to EcoRI digestion. The 16S rDNA of the African BLO strain was amplified from BclI-digested DNA extracted from infected periwinkle leaf midribs. The Asian strain was isolated from plant extract using a specific monoclonal antibody coated onto the surface of a PCR tube. The 16S rDNAs of the 2 BLO strains were cloned and sequenced. Comparisons with sequences of 16S rDNAs obtained from the GenBank database revealed that the 2 citrus greening disease BLOs belong to the alpha subdivision of the class Proteobacteria. Even though their closest relatives are members of the alpha-2 subgroup, these BLOs are distinct from this subgroup as only 87.5% homology was observed between the 16S rDNAs examined. It is suggested that the 2 BLOs studied are probably members of a new lineage in the α subdivision of the Proteobacteria. The trivial name "liberobacter" is provisionally proposed for this new group of bacteria and a more formal name will be proposed when additional characteristics have been determined.