Bacterial phylogenetic clusters revealed by genome structure.

Current bacterial taxonomy is mostly based on phenotypic criteria, which may yield misleading interpretations in classification and identification. As a result, bacteria not closely related may be grouped together as a genus or species. For pathogenic bacteria, incorrect classification or misidentification could be disastrous. There is therefore an urgent need for appropriate methodologies to classify bacteria according to phylogeny and corresponding new approaches that permit their rapid and accurate identification. For this purpose, we have devised a strategy enabling us to resolve phylogenetic clusters of bacteria by comparing their genome structures. These structures were revealed by cleaving genomic DNA with the endonuclease I-CeuI, which cuts within the 23S ribosomal DNA (rDNA) sequences, and by mapping the resulting large DNA fragments with pulsed-field gel electrophoresis. We tested this experimental system on two representative bacterial genera: Salmonella and Pasteurella. Among Salmonella spp., I-CeuI mapping revealed virtually indistinguishable genome structures, demonstrating a high degree of structural conservation. Consistent with this, 16S rDNA sequences are also highly conserved among the Salmonella spp. In marked contrast, the Pasteurella strains have very different genome structures among and even within individual species. The divergence of Pasteurella was also reflected in 16S rDNA sequences and far exceeded that seen between Escherichia and Salmonella. Based on this diversity, the Pasteurella haemolytica strains we analyzed could be divided into 14 phylogenetic groups and the Pasteurella multocida strains could be divided into 9 groups. If criteria for defining bacterial species or genera similar to those used for Salmonella and Escherichia coli were applied, the striking phylogenetic diversity would allow bacteria in the currently recognized species of P. multocida and P. haemolytica to be divided into different species, genera, or even higher ranks. On the other hand, strains of Pasteurella ureae and Pasteurella pneumotropica are very similar to those of P. multocida in both genome structure and 16S rDNA sequence and should be regarded as strains within this species. We conclude that large-scale genome structure can be a sensitive indicator of phylogenetic relationships and that, therefore, I-CeuI-based genomic mapping is an efficient tool for probing the phylogenetic status of bacteria.     

A unique DNA repair and recombination gene (recN) sequence for identification and intraspecific molecular typing of bacterial wilt pathogen Ralstonia solanacearum and its comparative analysis with ribosomal DNA sequences

Ribosomal gene sequences are a popular choice for identification of bacterial species and, often, for making phylogenetic interpretations. Although very popular, the sequences of 16S rDNA and 16-23S intergenic sequences often fail to differentiate closely related species of bacteria. The availability of complete genome sequences of bacteria, in the recent years, has accelerated the search for new genome targets for phylogenetic interpretations. The recently published full genome data of nine strains of R. solanacearum, which causes bacterial wilt of crop plants, has provided enormous genomic choices for phylogenetic analysis in this globally important plant pathogen. We have compared a gene candidate recN, which codes for DNA repair and recombination function, with 16S rDNA/16-23S intergenic ribosomal gene sequences for identification and intraspecific phylogenetic interpretations in R. solanacearum. recN gene sequence analysis of R. solanacearum revealed subgroups within phylotypes (or newly proposed species within plant pathogenic genus, Ralstonia), indicating its usefulness for intraspecific genotyping. The taxonomic discriminatory power of recN gene sequence was found to be superior to ribosomal DNA sequences. In all, the recN-sequence-based phylogenetic tree generated with the Bayesian model depicted 21 haplotypes against 15 and 13 haplotypes obtained with 16S rDNA and 16-23S rDNA intergenic sequences, respectively. Besides this, we have observed high percentage of polymorphic sites (S 23.04%), high rate of mutations (Eta 276) and high codon bias index (CBI 0.60), which makes the recN an ideal gene candidate for intraspecific molecular typing of this important plant pathogen.     

Phenotypic characterization of Astragalus glycyphyllos symbionts and their phylogeny based on the 16S rDNA sequences and RFLP of 16S rRNA gene

In this study, the nitrogen fixing Astragalus glycyphyllos symbionts were characterized by phenotypic properties, restriction fragment length polymorphism (RFLP), and sequences of 16S rDNA. The generation time of A. glycyphyllos rhizobia in yeast extract mannitol medium was in the range 4–6 h. The studied isolates exhibited a low resistance to antibiotics, a moderate tolerance to NaCl, assimilated di- and trisaccharides, and produced acid in medium containing mannitol as a sole carbon source. In the cluster analysis, based on 86 phenotypic properties of A. glycyphyllos symbionts and the reference rhizobia, examined isolates and the genus Mesorhizobium strains were placed on a single branch, clearly distinct from other lineages of rhizobial genera. By the comparative analysis of 16S rRNA gene sequences and 16S rDNA–RFLP, A. glycyphyllos nodulators were also identified as the members of the genus Mesorhizobium. On the 16S rDNA sequence phylogram, the representatives of A. glycyphyllos nodule isolates formed a robust, monophyletic cluster together with the Mesorhizobium species at 16S rDNA sequence similarity of these bacteria between 95 and 99 %. Similarly, the cluster analysis of the combined RFLP–16S rDNA patterns, obtained with seven restriction endonucleases, showed that A. glycyphyllos rhizobia are closely related to the genus Mesorhizobium bacteria. The taxonomic approaches used in this paper allowed us to classify the studied bacteria into the genus Mesorhizobium.     

Phylogenetic analysis of rhizosphere-associated beta-subclass proteobacterial ammonia oxidizers in a municipal wastewater treatment plant based on rhizoremediation technology

In wastewater treatment plants based on the rhizosphere zone (rhizoremediation technology), ammonia-oxidizing bacteria (AOB) play an important role in the removal of fixed nitrogen. However, the diversity of these bacteria in rhizoremediation wastewater treatment plants is largely unknown. We employed direct PCR amplification and cloning of 16S rRNA genes to determine the phylogenetic affiliation of AOB occurring in root and soil samples of a wastewater treatment plant (Merzdorf plant, Brandenburg, Germany). 16S rDNA clone libraries were screened by hybridization using an oligonucleotide probe specific for AOB of the beta subclass of proteobacteria. Comparative sequence analysis of all hybridization-positive clones revealed that the majority of rDNA sequences was affiliated to members of the genus Nitrosospira and formed a novel subcluster (SM cluster), whereas only three sequences were most closely related to Nitrosomonas species. Affiliation of the novel Nitrosospira-like sequences with those of isolates from soil and rhizosphere suggests that phylogenetic clusters reflect physiological differences between members of this genus.     

Use of the genomic signature in bacterial classification and identification

In this study we investigated the correlation between dinucleotide relative abundance values (the genomic signature) obtained from bacterial whole-genome sequences and two parameters widely used for bacterial classification, 16S rDNA sequence similarity and DNA-DNA hybridisation values. Twenty-eight completely sequenced bacterial genomes were included in the study. The correlation between the genomic signature and DNA-DNA hybridisation values was high and taxa that showed less than 30% DNA-DNA binding will in general not have dinucleotide relative abundance dissimilarity (delta*) values below 40. On the other hand, taxa showing more than 50% DNA-DNA binding will not have delta* values higher than 17. Our data indicate that the overall correlation between genomic signature and 16S rDNA sequence similarity is low, except for closely related organisms (16S rDNA similarity >94%). Statistical analysis of delta* values between different subgroups of the Proteobacteria indicate that the beta- and gamma-Proteobacteria are more closely related to each other than to the other subgroups of the Proteobacteria and that the alpha- and epsilon-Proteobacteria form clearly separate subgroups. Using the genomic signature we have also predicted DNA-DNA binding values for fastidious or unculturable endosymbionts belonging to the genera Rickettsia, Wigglesworthia and Buchnera.     

In Situ Detection of Novel Bacterial Endosymbionts of Acanthamoeba spp. Phylogenetically Related to Members of the Order Rickettsiales

Acanthamoebae are ubiquitous soil and water bactivores which may serve as amplification vehicles for a variety of pathogenic facultative bacteria and as hosts to other, presently uncultured bacterial endosymbionts. The spectrum of uncultured endosymbionts includes gram-negative rods and gram-variable cocci, the latter recently shown to be members of the Chlamydiales. We report here the isolation from corneal scrapings of two Acanthamoeba strains that harbor gram-negative rod endosymbionts that could not be cultured by standard techniques. These bacteria were phylogenetically characterized following amplification and sequencing of the near-full-length 16S rRNA gene. We used two fluorescently labelled oligonucleotide probes targeting signature regions within the retrieved sequences to detect these organisms in situ. Phylogenetic analyses demonstrated that they displayed 99.6% sequence similarity and formed an independent and well-separated lineage within the Rickettsiales branch of the alpha subdivision of the Proteobacteria. Nearest relatives included members of the genus Rickettsia, with sequence similarities of approximately 85 to 86%, suggesting that these symbionts are representatives of a new genus and, perhaps, family. Distance matrix, parsimony, and maximum-likelihood tree-generating methods all consistently supported deep branching of the 16S rDNA sequences within the Rickettsiales. The oligonucleotide probes displayed at least three mismatches to all other available 16S rDNA sequences, and they both readily permitted the unambiguous detection of rod-shaped bacteria within intact acanthamoebae by confocal laser-scanning microscopy. Considering the long-standing relationship of most Rickettsiales with arthropods, the finding of a related lineage of endosymbionts in protozoan hosts was unexpected and may have implications for the preadaptation and/or recruitment of rickettsia-like bacteria to metazoan hosts.     

16S rRNA sequencing in routine bacterial identification: a 30-month experiment

Accurate identification of bacterial isolates is an essential task in clinical microbiology. Phenotypic methods are time-consuming and either fail to identify some bacteria such as Gram-positive rods entirely or at least fail to do so in some clinical situations. 16S rDNA sequencing is a recent method of identification which offers a useful alternative. In this study, we investigate the usefulness of this method for identifying a range of bacteria in a clinical laboratory under routine conditions. Over a period of 30 months, 683 isolates were obtained from clinical specimens, sequenced and analysed. For 568 of these isolates (83.1%), the sequence provided species level identification. For 108 isolates (15.8%), the identification was limited to the genus level, and for 7 isolates (1%), the sequence remained unidentifiable by 16S rDNA sequence analysis. For the isolates identified only to the genus level, the 16S rDNA approach failed to identify bacteria to the taxonomic level for 3 reasons: failure to differentiate between species in 72 isolates (66%), the lack of any closely related sequence in the database for 15 isolates (13.8%) and the presence of more than 1% of undetermined position in the sequence for 13 isolates (12%).     

Molecular evidence suggesting species in the zoanthid genera Palythoa and Protopalythoa (Anthozoa: Hexacorallia) are congeneric

Taxonomic status of the zoanthid genera Palythoa and Protopalythoa has been in question for almost a century. Separation of the two genera has been based on traditional morphological methods (colony and polyp form, nematocyst size and form, and number of septa), with Palythoa polyps embedded in a well developed coenenchyme and Protopalythoa polyps standing free and clear of the coenenchyme. Here we sequenced two mitochondrial regions, the cytochrome oxidase I (COI) gene and 16S ribosomal DNA (16S rDNA) genes, from Palythoa and Protopalythoa samples from various parts of the world and performed phylogenetic analyses of the sequence data. The phylogenetic trees for both COI and 16S rDNA from Palythoa and Protopalythoa show four monophyletic groups (designated Palythoa tuberculosa, Palythoa heliodiscus, Palythoa mutuki 1, and Palythoa mutuki 2), with levels of sequence divergence (COI and 16S rDNA divergence approximately 0.0 approximately 1.1%) similar to or lower than that previously found among congeneric species within the closely related genus Zoanthus. Surprisingly, sequence differences among Palythoa tuberculosa, Palythoa mutuki 1, and Palythoa mutuki 2 were negligible (0.0 approximately 0.2% for both COI and 16S rDNA), potentially indicating relationships below the species level. Our sequences align well with the few Palythoa and Protopalythoa sequences reported to date. These findings strongly indicate that our samples represent a minimum of two and possibly up to four species (the Palythoa tuberculosa - P. mutuki 1 - P. mutuki 2 group, and P. heliodiscus) within the genus Palythoa, and that the genus Protopalythoa is erroneous nomenclature.     

Bacterial diversity in deep-sea sediments from different depths

Seven sediment samples have been examined, taken from different depths of the deep-sea in the range of 1159m to 6482m. A total of 75 different 16S rDNA sequences (149 clones) analyzed clustered into the Proteobacteria, Gram-positive bacteria, Cytophaga, Planctomyces, and Actinomycetes and many sequences were from microorganisms that showed no phylogenetic affiliation with known bacteria. Clones identical to 16S rDNA sequences of members of the genus Pseudomonas were observed in all of the sediments examined. The second group of common sequences cloned from six sediment samples was related to the 16S rDNA sequence of a chemoautotrophic bacterium, the Solemya velum symbiont. Five 16S rDNA sequences from three sediments were related to those of the Alvinella pompejana epibiont which is a member of the -Proteobacteria. Only one sequence was obtained that was closely related to the 16S rDNA of the barophilic bacterium, Shewanella benthica, which might be a minor population in the deeper sediments. -Proteobacteria-related sequences were cloned from sediments obtained from sites near man-made garbage deposits and a Calyptogena community. These environments obviously would be richer in nutrients than other sites, and might be expected to show more types of bacteria than other deep-sea sediments. A large number of cloned sequences in this study showed very low identity to known sequences. These sequences may represent communities of as-yet-uncultivated microorganisms in the sediments.     

烟草可培养内生细菌的分离及多样性分析

采用稀释平板法, 从健康烟草的根、茎、叶组织中分离到267株内生细菌。利用细菌菌落表征性状和16S rRNA序列对这些分离物进行了多样性分析。通过数值分析比较了8个菌落形态表征性状, 以类平均连锁聚类法的方式进行聚类分析, 在2.15的水平上可分成5个聚类群和56个亚群。16S rDNA序列系统发育分析表明267株分离物可分为21个类群。研究表明同一菌落形态类型的菌株在系统发育树中不一定聚为一类, 菌落形态分类与分子生物学方法分类结果不完全一致。经克隆测序分析表明, 这267株分离物分别与GenBank中6类细菌中的21个已知种相似性达到98%?99%。其中芽孢杆菌属(Bacillus)细菌是烟草可培养内生细菌的优势种群。     

A preliminary report of phylogenetic diversity of bacterial strains isolated from marine creatures

Bacterial diversity among marine creatures, especially molluscs, as a source for searching out novel lineages of bacteria, was studied. Marine creatures were collected at the coasts of the Kanto area in Japan. A total of 116 strains of bacteria were isolated from the intestines of 19 species of marine creatures includings molluscs, pisces and protochordata. Partial sequencing of 16S rDNA revealed that most of the isolates belonged to the gamma subclass of the Proteobacteria and Cytophaga-Flavobacterium-Bacteroides group. The BLAST searches revealed that the complete 16S rDNA sequence of 17 strains out of 116 isolates showed less than 94% similarity with 16S rDNA sequences deposited in the database. Four strains out of the 17 isolates belonged to the Rhodobacter group, 8 strains to the Alteromonas group, and the remaining 5 strains to the Cytophaga-Flavobacterium-Bacteroides group. Phylogenetic positions of 6 strains belonging to the Alteromonas group, which were isolated from different marine creatures, were close to each other, and represented a novel 16S rDNA lineage within the gamma subclass of Proteobacteria. Therefore, it may be inferred that these 6 strains belong to a new genus of Proteobacteria. Phylogenetic positions of the other strains are also independent from neighboring taxa, and they were suggested to respectively form a novel lineage. From these results, it is clear that the biodiversity of bacteria in marine creatures is much wider than was previously thought, and unknown microbiological resources are buried in these organisms.     

Detection of Desulfotomaculum in an Italian rice paddy soil by 16S ribosomal nucleic acid analyses

Two specific primers were developed for the amplification of 16S rRNA genes of Desulfotomaculum lineage 1 to detect members of the genus Desulfotomaculum in rice field soil. The combination of both primers in PCR allowed the specific amplification and cloning of ten 16S rDNA sequences of this group from rice paddy soil DNA extracts. The phylogenetic analysis showed that these sequences formed a deeply branching cluster within Desulfotomaculum lineage 1, together with two sequences from the database and two sequences from a hydrocarbon-contaminated aquifer. Dissimilarity values to validly described species, including recently isolated strains of Desulfotomaculum from rice paddy microcosms, were higher than 12%. Within the new cluster the cloned sequences formed three separate groups which were each represented by at least two sequences with identities of >/=99% while one sequence represented an additional group. The sequences should represent sulfate-reducing organisms because they clearly fell into the physiologically coherent group of Gram-positive sulfate reducers. The relative abundance of bacteria of the Desulfotomaculum lineage 1 in rice paddy soil and root samples was estimated with rRNA dot blot hybridizations of extracted RNA. The relative RNA content of Desulfotomaculum lineage 1 was 0.55% in the bulk soil and 1% in the rice root samples, respectively, of the total 16S rRNA content (probe Eub338). Hybridization of rRNA with a probe targeting the new cluster represented by the cloned sequences confirmed the high abundance of 16S rRNA sequences from this cluster in the rice paddy field samples. Another hybridization probe detecting Desulfotomaculum acetoxidans and two closely related Desulfotomaculum isolates from rice paddy soil indicated that these bacteria were less abundant.     

CORE: a phylogenetically-curated 16S rDNA database of the core oral microbiome

Comparing bacterial 16S rDNA sequences to GenBank and other large public databases via BLAST often provides results of little use for identification and taxonomic assignment of the organisms of interest. The human microbiome, and in particular the oral microbiome, includes many taxa, and accurate identification of sequence data is essential for studies of these communities. For this purpose, a phylogenetically curated 16S rDNA database of the core oral microbiome, CORE, was developed. The goal was to include a comprehensive and minimally redundant representation of the bacteria that regularly reside in the human oral cavity with computationally robust classification at the level of species and genus. Clades of cultivated and uncultivated taxa were formed based on sequence analyses using multiple criteria, including maximum-likelihood-based topology and bootstrap support, genetic distance, and previous naming. A number of classification inconsistencies for previously named species, especially at the level of genus, were resolved. The performance of the CORE database for identifying clinical sequences was compared to that of three publicly available databases, GenBank nr/nt, RDP and HOMD, using a set of sequencing reads that had not been used in creation of the database. CORE offered improved performance compared to other public databases for identification of human oral bacterial 16S sequences by a number of criteria. In addition, the CORE database and phylogenetic tree provide a framework for measures of community divergence, and the focused size of the database offers advantages of efficiency for BLAST searching of large datasets. The CORE database is available as a searchable interface and for download at http://microbiome.osu.edu.     

Microbial diversity in hot synthetic compost as revealed by PCR-amplified rRNA sequences from cultivated isolates and extracted DNA

High-temperature (>/=60 degrees C) synthetic food waste compost was examined by cultivation-dependent and -independent methods to determine predominant microbial populations. Fluorescent direct counts totaled 6.4 (+/-2.5)x10(10) cells gdw(-1) in a freeze-dried 74 degrees C compost sample, while plate counts for thermophilic heterotrophic aerobes averaged 2.6 (+/-1.0)x10(8) CFU gdw(-1). A pre-lysis cell fractionation method was developed to obtain community DNA and a suite of 16S and 18S rDNA-targeted PCR primers was used to examine the presence of Bacteria, Archaea and fungi. Bacterial 16S rDNA, including a domain-specific 1500-bp fragment and a 300-bp fragment specific for Actinobacteria, was amplified by PCR from all compost samples tested. Archaeal rDNA was not amplified in any sample. Fungal 18S rDNA was only amplified from a separate dairy manure compost that reached a peak temperature of 50 degrees C. Amplified rDNA restriction analysis (ARDRA) was used to screen isolated thermophilic bacteria and a clone library of full-length rDNA fragments. ARDRA screening revealed 14 unique patterns among 63 isolates, with one pattern accounting for 31 of the isolates. In the clone library, 52 unique patterns were detected among 70 clones, indicating high diversity of uncultivated bacteria in hot compost. Phylogenetic analysis revealed that the two most abundant isolates belonged in the genera Aneurinibacillus and Brevibacillus, which are not commonly associated with hot compost. With the exception of one Lactobacillus-type sequence, the clone library contained only sequences that clustered within the genus Bacillus. None of the isolates or cloned sequences could be assigned to the group of obligate thermophilic Bacillus spp. represented by B. stearothermophilus, commonly believed to dominate high-temperature compost. Amplified partial fragments from Actinobacteria, spanning the V3 variable region (Neefs et al. (1990) Nucleic Acids Res. 18, 2237-2242), included sequences related to the genera Saccharomonospora, Gordonia, Rhodococcus and Corynebacterium, although none of these organisms were detected among the isolates or full-length cloned rDNA sequences. All of the thermophilic isolates and sequenced rDNA fragments examined in this study were from Gram-positive organisms.     

Specific Ribosomal DNA Sequences from Diverse Environmental Settings Correlate with Experimental Contaminants

Phylogenetic analysis of 16S ribosomal DNA (rDNA) clones obtained by PCR from uncultured bacteria inhabiting a wide range of environments has increased our knowledge of bacterial diversity. One possible problem in the assessment of bacterial diversity based on sequence information is that PCR is exquisitely sensitive to contaminating 16S rDNA. This raises the possibility that some putative environmental rRNA sequences in fact correspond to contaminant sequences. To document potential contaminants, we cloned and sequenced PCR-amplified 16S rDNA fragments obtained at low levels in the absence of added template DNA. 16S rDNA sequences closely related to the genera Duganella (formerly Zoogloea), Acinetobacter, Stenotrophomonas, Escherichia, Leptothrix, and Herbaspirillum were identified in contaminant libraries and in clone libraries from diverse, generally low-biomass habitats. The rRNA sequences detected possibly are common contaminants in reagents used to prepare genomic DNA. Consequently, their detection in processed environmental samples may not reflect environmentally relevant organisms.     

Phylogenetic position of the spirochetal genus Cristispira.

Comparative sequence analysis of 16S rRNA genes was used to determine the phylogenetic relationship of the genus Cristispira to other spirochetes. Since Cristispira organisms cannot presently be grown in vitro, 16S rRNA genes were amplified directly from bacterial DNA isolated from Cristispira cell-laden crystalline styles of the oyster Crassostrea virginica. The amplified products were then cloned into Escherichia coli plasmids. Sequence comparisons of the gene coding for 16S rRNA (rDNA) insert of one clone, designated CP1, indicated that it was spirochetal. The sequence of the 16S rDNA insert of another clone was mycoplasmal. The CP1 sequence possessed most of the individual base signatures that are unique to 16S rRNA (or rDNA) sequences of known spirochetes. CP1 branched deeply among other spirochetal genera within the family Spirochaetaceae, and accordingly, it represents a separate genus within this family. A fluorescently labeled DNA probe designed from the CP1 sequence was used for in situ hybridization experiments to verify that the sequence obtained was derived from the observed Cristispira cells.     

Two Intracellular Symbiotic Bacteria from the Mulberry Psyllid Anomoneura mori (Insecta, Homoptera)

We characterized the intracellular symbiotic bacteria of the mulberry psyllid Anomoneura mori by performing a molecular phylogenetic analysis combined with in situ hybridization. In its abdomen, the psyllid has a large, yellow, bilobed mycetome (or bacteriome) which consists of many round uninucleated mycetocytes (or bacteriocytes) enclosing syncytial tissue. The mycetocytes and syncytium harbor specific intracellular bacteria, the X-symbionts and Y-symbionts, respectively. Almost the entire length of the bacterial 16S ribosomal DNA (rDNA) was amplified and cloned from the whole DNA of A. mori, and two clones, the A-type and B-type clones, were identified by restriction fragment length polymorphism analysis. In situ hybridization with specific oligonucleotide probes demonstrated that the A-type and B-type 16S rDNAs were derived from the X-symbionts and Y-symbionts, respectively. Molecular phylogenetic analyses of the 16S rDNA sequences showed that these symbionts belong to distinct lineages in the γ subdivision of the Proteobacteria. No 16S rDNA sequences in the databases were closely related to the 16S rDNA sequences of the X- and Y-symbionts. However, the sequences that were relatively closely related to them were the sequences of endosymbionts of other insects. The nucleotide compositions of the 16S rDNAs of the X- and Y-symbionts were highly AT biased, and the sequence of the X-symbiont was the most AT-rich bacterial 16S rDNA sequence reported so far.     

Evidence for conserved tRNA genes in the 16S-23S rDNA spacer sequence and two rrn operons of Xylella fastidiosa

The 16S-23S rDNA spacer of the type strain (ATCC 35879) of Xylella fastidiosa was amplified by PCR, cloned, and sequenced. The spacer sequence (455 bp) contains two tRNA (tRNA(ala) and tRNA(ile)) genes. Identical tRNA genes were also found in the 16S-23S spacer sequences of all the 51 strains of X. fastidiosa retrieved from the GenBank database. At this particular locus, the gene order of tRNA(ala)-tRNA(ile) is conserved among all the studied strains of Xylella and Xanthomonas, and different from those of other bacteria. Sequence analysis showed that Xanthomonas is the most closely related genus. Results from restriction endonuclease analysis suggested the presence of two rrn operons in the genome of a Xylella fastidiosa Pierce's disease strain.