Refining the taxonomy of the order Hyphomicrobiales (Rhizobiales) based on whole genome comparisons of over 130 type strains

The alphaproteobacterial order Hyphomicrobiales consists of 38 families comprising at least 152 validly published genera as of January 2024. The order Hyphomicrobiales was first described in 1957 and underwent important revisions in 2020. However, we show that several inconsistencies in the taxonomy of this order remain and we argue that there is a need for a consistent framework for defining families within the order. We propose a common genome-based framework for defining families within the order Hyphomicrobiales, suggesting that families represent monophyletic groups in core-genome phylogenies that share pairwise average amino acid identity values above ~75 % when calculated from a core set of 59 proteins. Applying this framework, we propose the formation of four new families and to reassign the genera Salaquimonas, Rhodoblastus, and Rhodoligotrophos into Salaquimonadaceae fam. nov., Rhodoblastaceae fam. nov., and Rhodoligotrophaceae fam. nov., respectively, and the genera Albibacter, Chenggangzhangella, Hansschlegelia, and Methylopila into Methylopilaceae fam. nov. We further propose to unify the families Bartonellaceae, Brucellaceae, Phyllobacteriaceae, and Notoacmeibacteraceae as Bartonellaceae; the families Segnochrobactraceae and Pseudoxanthobacteraceae as Segnochrobactraceae; the families Lichenihabitantaceae and Lichenibacteriaceae as Lichenihabitantaceae; and the families Breoghaniaceae and Stappiaceae as Stappiaceae. Lastly, we propose to reassign several genera to existing families. Specifically, we propose to reassign the genus Pseudohoeflea to the family Rhizobiaceae; the genera Oricola, Roseitalea, and Oceaniradius to the family Ahrensiaceae; the genus Limoniibacter to the emended family Bartonellaceae; the genus Faunimonas to the family Afifellaceae; and the genus Pseudochelatococcus to the family Chelatococcaceae. Our data also support the recent proposal to reassign the genus Prosthecomicrobium to the family Kaistiaceae.

Phyllobacteriaceae: a family of ecologically and metabolically diverse bacteria with the potential for different applications

The family Phyllobacteriaceae is a heterogeneous assemblage of more than 146 species of bacteria assigned to its existing 18 genera. Phylogenetic analyses have shown great phylogenetic diversity and also suggested about incorrect classification of several species that need to be reassessed for their proper phylogenetic classification. However, almost 50% of the family members belong to the genus Mesorhizobium only, of which the majority are symbiotic nitrogen fixers associated with different legumes. Other major genera are Phyllobacterium, Nitratireductor, Aquamicrobium, and Aminobacter. Nitrogen-fixing, legume nodulating members are present in Aminobacter and Phyllobacterium as well. Aquamicrobium spp. can degrade environmental pollutants, like 2,4-dichlorophenol, 4-chloro-2-methylphenol, and 4-chlorophenol. Chelativorans, Pseudaminobacter, Aquibium, and Oricola are the other genera that contain multiple species having diverse metabolic capacities, the rest being single-membered genera isolated from varied environments. In addition, heavy metal and antibiotic resistance, chemolithoautotrophy, poly-β-hydroxybutyrate storage, cellulase production, etc., are the other notable characteristics of some of the family members. In this report, we have comprehensively reviewed each of the species of the family Phyllobacteriaceae in their eco-physiological aspects and found that the family is rich with ecologically and metabolically highly diverse bacteria having great potential for human welfare and environmental clean-up.

Polyphasic taxonomic analysis of Nitratireductor arenosus sp. nov., isolated from sea sand

A novel proteobacterial bacterium, designated strain CAU 1489T, was isolated from Jeju Island, Republic of Korea. Cells were strictly anaerobic, Gram stain-negative, cream-pigmented, non-spore-forming, motile and short rod-shaped. Strain CAU 1489T exhibited the highest 16S rRNA gene sequence similarity (98.2%) to Nitratireductor mangrovi SY7T. Multilocus sequence analysis of 16S rRNA and four housekeeping genes (rpoB, rpoC, gyrB and dnaK) indicated that CAU 1489T represents a distinct branch within Nitratireductor. The whole genome was 4.8 Mb with a G + C content of 64.7 mol%, including protein-coding genes related to the function terms amino acids and derivatives, nucleotides and nucleosides, protein metabolism, carbohydrates and cofactors, vitamins, prosthetic groups and pigments. The major fatty acids were 11-methyl C18:1ω7c, cyclo- C19:0ω8c, iso-C17:0 and summed feature 8 (C18:1ω6c and/or C18:1ω7c), and the predominant respiratory quinone was Q-10. The polar lipids comprised diphosphatidylglycerol, phosphatidylglycerol, phosphatidylethanolamine and two unidentified phospholipids. Digital DNA-DNA hybridization and average nucleotide identity values were 19.4-22.0% and 72.4-79.1%, respectively. On the basis of taxonomic characterization, strain CAU 1489T constitutes a novel species, for which the name Nitratireductor arenosus sp. nov. is proposed. The type strain is CAU 1489T ( = KCTC 62997T = NBRC 113694T).

Oricola thermophila sp. nov., a marine bacterium isolated from tidal flat sediment and emended description of the genus Oricola Hameed et al. 2015

A Gram-stain-negative, facultatively anaerobic, rod-shaped (1.8-4.4×0.5-0.7 µm) and motile marine bacterium, designated as MEBiC13590^T, was isolated from tidal flat sediment sampled at Incheon City, on the west coast of the Republic of Korea. The 16S rRNA gene sequence analysis revealed that strain MEBiC13590^T showed high similarity to Oricola cellulosilytica CC-AMH-0^T (98.2 %), followed by Oceaniradius stylonematis StC1^T (97.5 %); however, it clustered with Oricola cellulosilytica. The phylogenomic tree inferred by the up-to-date bacterial core gene set suggested that strain MEBiC13590^T shared a phyletic line with Oricola cellulosilytica. Average nucleotide identity and digital DNA-DNA hybridization values (75.0 and 19.3 %, respectively) between strain MEBiC13590^T and Oricola cellulosilytica CC-AMH-0^T were below the respective species delineation cutoffs. Growth was observed at 22-50 °C (optimum, 45 °C), at pH 5-9 (optimum, pH 7) and with 1-6 % (optimum, 3 %) NaCl. The predominant cellular fatty acids were C_16 : 0 (7.6 %), C_18 : 0 (12.2 %), 11-methyl C_18 : 1 ω7c (5.7 %), C_19 : 0 cyclo ω6c and summed feature 8 (comprising C_18 : 1 ω7c and/or C_18 : 1 ω6c; 38 %). The DNA G+C content was 63.5 mol%. The major respiratory quinone was Q-10. Several phenotypic characteristics such as growth temperature, oxygen requirement, enzyme activities of urease, gelatinase, lipase (C₁₄), α-chymotrypsin, acid phosphatase, β-galactosidase, β-glucosidase etc. differentiate strain MEBiC13590^T from Oricola cellulosilytica CC-AMH-0^T. Based on this polyphasic taxonomic data, strain MEBiC13590^T should be classified as representing a novel species in the genus Oricola for which the name Oricola thermophila sp. nov. is proposed . The type strain is MEBiC13590^T (=KCCM 43313^T=JCM 33661^T).

nov., a novel bacterium within the family Phyllobacteriaceae

A novel aerobic, Gram-negative bacterial strain, RR3-28^T, was isolated from a seawater recirculating aquaculture system in Busan, Republic of Korea. Cells were rod-shaped, non-motile, oxidase-positive, catalase-negative and grew optimally at 25-30 °C, pH 8.5 and 3 % (w/v) NaCl. Based on the results of phylogenetic analysis, strain RR3-28^T was most closely related to Zhengella mangrovi X9-2-2^T within the family Phyllobacteriaceae with 95.97 % 16S rRNA gene sequence similarity. The major cellular fatty acids were summed feature 8 (C_18 : 1ω7c and/or C_18 : 1ω6c, 71.1 %) and 11-methyl C_18 : 1ω7c (14.4 %). The major polar lipids were phosphatidylglycerol, phosphatidylethanolamine, phosphatidylmonomethylethanolamine, phosphatidylcholine and unidentified aminolipids. The predominant quinone was ubiquinone Q-10 and the DNA G+C content was 58.6 mol%. On the basis of its phenotypic and genotypic characteristics, strain RR3-28^T represents a novel genus and species belonging to the family Phyllobacteriaceae, for which the name Salaquimonas pukyongi gen. nov., sp. nov. is proposed. The type strain of the species is RR3-28^T (=KCTC 52649^T=DSM 107947^T).

Mesorhizobium composti sp. nov., isolated from compost

A polyphasic taxonomic approach was used to characterize a presumptively novel diazotrophic bacterium, designated strain CC-YTH430^T, isolated from a compost sample in Taiwan. Cells of strain CC-YTH430^T were found to be Gram-stain negative, facultative anaerobic rods that formed yellow-colored colonies on nutrient agar. Cell growth occurred at 15-40 °C, pH 5.0-9.0 and in the presence of 0-2% NaCl. Strain CC-YTH430^T resembled Mesorhizobium species while sharing high pair-wise 16S rRNA gene sequence similarities with Mesorhizobium silamurunense, Mesorhizobium thiogangeticum, Mesorhizobium plurifarium, Mesorhizobium tamadayense, Mesorhizobium amorphae (96.9% each), Mesorhizobium sediminum (96.8%), and Mesorhizobium soli (96.5%) and < 96.5% similarity to other species. Strain CC-YTH430^T showed 78.8-79.7% average nucleotide identity compared to the type strains of M. amorphae, M. plurifarium, M. soli, M. tamadayense and M. wenxiniae. The N₂-fixing activity of strain CC-YTH430^T was 0.2 nmol ethylene h^-1 at 30 °C. The respiratory system was ubiquinone 10 (Q-10) and the DNA G+C content was 62.0 ± 0.2 mol%. The major fatty acids (> 5%) were C_16:0, C_17:0 cyclo, C_19:0 cyclo ω8c, C_14:0 3OH/C_16:1 iso I and C_18:1ω7c/C_18:1ω6c. The polar lipid profile contained diphosphatidylglycerol, phosphatidylglycerol, phosphatidylcholine, phosphatidylmonomethylethanolamine and an unidentified aminolipid in major amounts. In addition, phosphatidylethanolamine, an unidentified lipid and several unidentified polar lipids were also found in moderate-to-trace amounts. Based on the phylogenetic, phenotypic and chemotaxonomic features, strain CC-YTH430^T is proposed to represent a novel Mesorhizobium species, for which the name Mesorhizobium composti sp. nov. (type strain CC-YTH430^T = BCRC 81024^T = JCM 31762^T) is proposed.

nov., a member of the family Phyllobacteriaceae, isolated from bark of Populus×euramericana

Two Gram-staining-negative, aerobic, motile, slimy, glossy bacterial strains were isolated from bark tissue of Populus×euramericana. The bacteria grew at 10-37 °C, pH 5-10, with optimal growth at 28-30 °C, pH 6.0-8.0. Both strains grew with 0-3 % (w/v) NaCl. In the maximum-likelihood phylogenetic tree, the two isolates formed a distinct branch within the family Phyllobacteriaceae, and they were not closely related to any of the genera within the family Phyllobacteriaceae. The two novel isolates werepositive for oxidase andcatalase activity. The polar lipids profile revealed diphosphatidylglycerol, phosphatidylcholine, phospholipid, phosphatidylethanolamine, phosphatidylglycerol and five unknown lipids. The major fatty acids were C18 : 1ω7c and C16 : 0. The DNA G+C content was 56.4 mol%. On the basis of phylogenetic, chemotaxonomic and phenotypic data, the two strains represent a novel species belonging to a novel genus of the family Phyllobacteriaceae, for which the name Corticibacterium populi gen. nov., sp. nov. is proposed. The type strain of the type species is 16B10-2-7^T (=CFCC 12884^T=KCTC 42249^T).

Mixed group of Rhizobiales microbes in lung and blood of a patient with fatal pulmonary illness

We examined the microbial composition in the diseased lung and early-phase microbial cultures from the blood of a patient with a rapidly progressing fatal pulmonary illness. Although no microbes could be isolated from such cultures during the initial study, the HTS-microbiome study revealed the presence of a unique mixture of alphaproteobacteria, composed mainly of different families of Rhizobiales microbes. Microbial 16S rDNA sequences matching closely to Afipia cberi were identified mainly in the patient's diseased lung tissue, but only rarely in the early-phase blood cultures. Conversely, the high abundance of sequences found in early-phase blood cultures of different broth media matched closely with those of the families Methylobacteriaceae, Phyllobacteriaceae and Sphingomonadaceae. The two species that successfully adapted to grow in a laboratory culture system were A. cberi and Mesorhizobium hominis, which eventually were isolated from a previously cryopreserved blood culture of SP4 broth. Many other species, including members of the Bradyrhizobiaceae and Phyllobacteriaceae families, and all members of the Methylobacteriaceae and Sphingomonadaceae families identified by HTS remained non-cultivated. We developed specific PCR primers and FISH probes, which detected the target Rhizobiales microbes in former blood cultures and autopsy lung tissues. It is unclear what role these Rhizobiales microbes might have played in the patient's complex disease process. However, the above mentioned assays should help in rapidly detecting and identifying these previously unrecognized Rhizobiales microbes in patients.

Genome of the marine alphaproteobacterium Hoeflea phototrophica type strain (DFL-43(T))

Hoeflea phototrophica Biebl et al. 2006 is a member of the family Phyllobacteriaceae in the order Rhizobiales, which is thus far only partially characterized at the genome level. This marine bacterium contains the photosynthesis reaction-center genes pufL and pufM and is of interest because it lives in close association with toxic dinoflagellates such as Prorocentrum lima. The 4,467,792 bp genome (permanent draft sequence) with its 4,296 protein-coding and 69 RNA genes is a part of the Marine Microbial Initiative.