Publications

4474

ABSTRACT “ Candidatus Liberibacter asiaticus” is a psyllid-transmitted, phloem-limited alphaproteobacterium and the most prevalent species of “ Ca . Liberibacter” associated with a devastating worldwide citrus disease known as huanglongbing (HLB). Two related and hypervariable genes ( hyv I and hyv II ) were identified in the prophage regions of the Psy62 “ Ca. Liberibacter asiaticus” genome. Sequence analyses of the hyv I and hyv II genes in 35 “ Ca. Liberibacter asiaticus” DNA isolates collected globally revealed that the hyv I gene contains up to 12 nearly identical tandem repeats (NITRs, 132 bp) and 4 partial repeats, while hyv II contains up to 2 NITRs and 4 partial repeats and shares homology with hyv I . Frequent deletions or insertions of these repeats within the hyv I and hyv II genes were observed, none of which disrupted the open reading frames. Sequence conservation within the individual repeats but an extensive variation in repeat numbers, rearrangement, and the sequences flanking the repeat region indicate the diversity and plasticity of “ Ca. Liberibacter asiaticus” bacterial populations in the world. These differences were found not only in samples of distinct geographical origins but also in samples from a single origin and even from a single “ Ca. Liberibacter asiaticus”-infected sample. This is the first evidence of different “ Ca. Liberibacter asiaticus” populations coexisting in a single HLB-affected sample. The Florida “ Ca. Liberibacter asiaticus” isolates contain both hyv I and hyv II , while all other global “ Ca. Liberibacter asiaticus” isolates contain either one or the other. Interclade assignments of the putative Hyv I and Hyv II proteins from Florida isolates with other global isolates in phylogenetic trees imply multiple “ Ca. Liberibacter asiaticus” populations in the world and a multisource introduction of the “ Ca. Liberibacter asiaticus” bacterium into Florida.

Summary ‘ Candidatus Accumulibacter’ is a biotechnologically important bacterial group that can accumulate large amounts of intracellular polyphosphate, contributing to biological phosphorus removal in wastewater treatment. Since its first molecular identification more than a decade ago, this bacterial group has drawn significant research attention due to its high abundance in many biological phosphorus removal systems. In the past 6 years, our understanding of Accumulibacter microbiology and ecophysiology has advanced rapidly, largely owing to genomic information obtained through shotgun metagenomic sequencing efforts. In this review, we focus on the metabolism, physiology, fine‐scale population structure and ecological distribution of Accumulibacter, aiming to integrate the information learned so far and to present a more complete picture of the microbiology of this important bacterial group.

A novel bacterium of the genusPasteuriawas discovered parasitizing bacterivorous nematodes of the genusBursilla, in selected bermudagrass (Cynodon) field plots in Davie, FL, USA. Soil containing this bacterium was sampled and supplied with bi-weekly inoculations of cultured species of the genusBursillain order to build and maintain a source of endospores for continuousin vivoconservation of the bacteria for further study and characterization. 16S rRNA gene sequence similarities supported its congeneric ranking with other members of the genusPasteuriathat have been identified from nematodes and cladocerans. There were, however, no clear sister candidates for this organism, which supported the evidence of endospore ultrastructure and host-range studies, suggesting it belonged to a novel taxon. Because members of the genusPasteuriacannot yet be isolated, definitive type strains could not be maintained; therefore, the name ‘CandidatusPasteuria aldrichii’ is proposed for this organism.

In addition to the grapevine flavescence dorée phytoplasmas, other members of taxonomic group 16SrV phytoplasmas infect grapevines, alders and species of the genera Clematis and Rubus in Europe. In order to investigate which phytoplasmas constitute discrete, species-level taxa, several strains were analysed by comparing their 16S rRNA gene sequences and a set of five housekeeping genes. Whereas 16S rRNA gene sequence similarity values were >97.5 %, the proposed threshold to distinguish two ‘Candidatus Phytoplasma’ taxa, phylogenetic analysis of the combined sequences of the tuf, rplV-rpsC, rplF-rplR, map and uvrB-degV genetic loci showed that two discrete phylogenetic clusters could be clearly distinguished. The first cluster grouped flavescence dorée (FD) phytoplasmas, alder yellows (AldY) phytoplasmas, Clematis (CL) phytoplasmas and the Palatinate grapevine yellows (PGY) phytoplasmas. The second cluster comprised Rubus stunt (RS) phytoplasmas. In addition to the specificity of the insect vector, the Rubus stunt phytoplasma contained specific sequences in the 16S rRNA gene. Hence, the Rubus stunt phytoplasma 16S rRNA gene was sufficiently differentiated to represent a novel putative taxon: ‘Candidatus Phytoplasma rubi’.