Swarmer Cells Research Articles

Potential Role of a Bistable Histidine Kinase Switch in the Asymmetric Division Cycle of Caulobacter crescentus

The free-living aquatic bacterium, Caulobacter crescentus, exhibits two different morphologies during its life cycle. The morphological change from swarmer cell to stalked cell is a result of changes of function of two bi-functional histidine kinases, PleC and CckA. Here, we describe a detailed molecular mechanism by which the function of PleC changes between phosphatase and kinase state. By mathematical modeling of our proposed molecular interactions, we derive conditions under which PleC, CckA and its response regulators exhibit bistable behavior, thus providing a scenario for robust switching between swarmer and stalked states. Our simulations are in reasonable agreement with in vitro and in vivo experimental observations of wild type and mutant phenotypes. According to our model, the kinase form of PleC is essential for the swarmer-to-stalked transition and to prevent premature development of the swarmer pole. Based on our results, we reconcile some published experimental observations and suggest novel mutants to test our predictions.

Effect of Nitroxides on Swarming Motility and Biofilm Formation, Multicellular Behaviors in Pseudomonas aeruginosa

The ability of nitric oxide (NO) to induce biofilm dispersion has been well established. Here, we investigated the effect of nitroxides (sterically hindered nitric oxide analogues) on biofilm formation and swarming motility in Pseudomonas aeruginosa. A transposon mutant unable to produce nitric oxide endogenously (nirS) was deficient in swarming motility relative to the wild type and the complemented strain. Moreover, expression of the nirS gene was upregulated by 9.65-fold in wild-type swarming cells compared to planktonic cells. Wild-type swarming levels were substantially restored upon the exogenous addition of nitroxide containing compounds, a finding consistent with the hypothesis that NO is necessary for swarming motility. Here, we showed that nitroxides not only mimicked the dispersal activity of NO but also prevented biofilms from forming in flow cell chambers. In addition, a nirS transposon mutant was deficient in biofilm formation relative to the wild type and the complemented strain, thus implicating NO in the formation of biofilms. Intriguingly, despite its stand-alone action in inhibiting biofilm formation and promoting dispersal, a nitroxide partially restored the ability of a nirS mutant to form biofilms.

Molecular Adsorption Steers Bacterial Swimming at the Air/Water Interface

Microbes inhabiting Earth have adapted to diverse environments of water, air, soil, and often at the interfaces of multiple media. In this study, we focus on the behavior of Caulobacter crescentus, a singly flagellated bacterium, at the air/water interface. Forward swimming C. crescentus swarmer cells tend to get physically trapped at the surface when swimming in nutrient-rich growth medium but not in minimal salt motility medium. Trapped cells move in tight, clockwise circles when viewed from the air with slightly reduced speed. Trace amounts of Triton X100, a nonionic surfactant, release the trapped cells from these circular trajectories. We show, by tracing the motion of positively charged colloidal beads near the interface that organic molecules in the growth medium adsorb at the interface, creating a high viscosity film. Consequently, the air/water interface no longer acts as a free surface and forward swimming cells become hydrodynamically trapped. Added surfactants efficiently partition to the surface, replacing the viscous layer of molecules and reestablishing free surface behavior. These findings help explain recent similar studies on Escherichia coli, showing trajectories of variable handedness depending on media chemistry. The consistent behavior of these two distinct microbial species provides insights on how microbes have evolved to cope with challenging interfacial environments.

English

We described in detail the spore germination events in Physarum globuliferum. Observations were made using depression slides under a computer-supported video microscope. When a spore germinated, one protoplast, one myxamoeba or one swarm cell emerged; not more than one of these was observed. Under most conditions, the release of the spore protoplast was accomplished by the splitting of the spore wall into what appeared to be a V-shaped opening. Sometimes, the spores germinated through a minute pore dissolved in the spore wall and a myxamoeba or a swarm cell emerges. Under unfavorable conditions, myxamoeba may round up to form microcysts. Swarm cells are peanut-like, with one anteriorly attached flagellum. The pseudopodium of the myxamoeba is a typical lobopodium containing endoplasm only.   Key words: Myxomycete, protoplast, swarm cell, myxamoeba, microcyst.

DNA Binding of the Cell Cycle Transcriptional Regulator GcrA Depends on N6-Adenosine Methylation in Caulobacter crescentus and Other Alphaproteobacteria

Several regulators are involved in the control of cell cycle progression in the bacterial model system Caulobacter crescentus, which divides asymmetrically into a vegetative G1-phase (swarmer) cell and a replicative S-phase (stalked) cell. Here we report a novel functional interaction between the enigmatic cell cycle regulator GcrA and the N6-adenosine methyltransferase CcrM, both highly conserved proteins among Alphaproteobacteria, that are activated early and at the end of S-phase, respectively. As no direct biochemical and regulatory relationship between GcrA and CcrM were known, we used a combination of ChIP (chromatin-immunoprecipitation), biochemical and biophysical experimentation, and genetics to show that GcrA is a dimeric DNA–binding protein that preferentially targets promoters harbouring CcrM methylation sites. After tracing CcrM-dependent N6-methyl-adenosine promoter marks at a genome-wide scale, we show that these marks recruit GcrA in vitro and in vivo. Moreover, we found that, in the presence of a methylated target, GcrA recruits the RNA polymerase to the promoter, consistent with its role in transcriptional activation. Since methylation-dependent DNA binding is also observed with GcrA orthologs from other Alphaproteobacteria, we conclude that GcrA is the founding member of a new and conserved class of transcriptional regulators that function as molecular effectors of a methylation-dependent (non-heritable) epigenetic switch that regulates gene expression during the cell cycle.

Putrescine Importer PlaP Contributes to Swarming Motility and Urothelial Cell Invasion in Proteus mirabilis

Previously, we reported that the speA gene, encoding arginine decarboxylase, is required for swarming in the urinary tract pathogen Proteus mirabilis. In addition, this previous study suggested that putrescine may act as a cell-to-cell signaling molecule (Sturgill, G., and Rather, P. N. (2004) Mol. Microbiol. 51, 437-446). In this new study, PlaP, a putative putrescine importer, was characterized in P. mirabilis. In a wild-type background, a plaP null mutation resulted in a modest swarming defect and slightly decreased levels of intracellular putrescine. In a P. mirabilis speA mutant with greatly reduced levels of intracellular putrescine, plaP was required for the putrescine-dependent rescue of swarming motility. When a speA/plaP double mutant was grown in the presence of extracellular putrescine, the intracellular levels of putrescine were greatly reduced compared with the speA mutant alone, indicating that PlaP functioned as the primary putrescine importer. In urothelial cell invasion assays, a speA mutant exhibited a 50% reduction in invasion when compared with wild type, and this defect could be restored by putrescine in a PlaP-dependent manner. The putrescine analog Triamide-44 partially inhibited the uptake of putrescine by PlaP and decreased both putrescine stimulated swarming and urothelial cell invasion in a speA mutant.

English

Our previous studies showed that urea in acidic broth induced swarmer morphotypes in Proteus mirabilis, but the effects to other bacterial species remain unsolved. Here we report effects of urea on morphology and size distribution of urease-negativeEscherichia coli and Salmonella enterica serovar Abony grown in urea-containing broth. Statistical analysis indicated lognormal distribution of the cellular lengths below a threshold level, suggesting that the growth process of bacterial cells obeys some random multiplicative process below a threshold value, despite any other factors affecting the process above the threshold value, to produce elongated cells. In urea broth, the distribution shifted to larger size and enormously elongated cells appeared. Morphological studies in urea broth revealed that filamentous cells of E. coli and S. Abony, accompanied with incomplete chromosomal segregation without forming hyperflagellates were induced at logarithmic growth phase, unlike swarmer cells in P. mirabilis. Since cell division protein FtsZ (filamenting temperature-sensitive mutant Z) assembled round the chromosome segregated point and cells were divided into short rods after cell counts had reached to a threshold level, urea in broth was responsible for delay of chromosomal segregation.   Key words: Chromosomal segregation, enterobacteria, filamentous cells, urea.

Swarmer cells of Proteus mirabilis migrate faster than swimmer cells in viscous environments

It has been suggested that swarming, a type of surface motility that allows rapid migration, is involved in urinary tract colonization. During swarming, cells of P. mirabilis undergo a morphological change from ~5 μm long rods with 5–15 flagella to ~10–50 μm long rods with 50 to more than 100 flagella. Prior work has shown that increasing the number of flagella on a cell does not yield the expected increase in torque – i.e., a four‐fold increase in the number of flagella yields less than a two‐fold increase in torque. This result suggests that the dense flagellation of swarmer cells should not convey a motility advantage. However, mutant cells of Bacillus pseudiformis that express an increased number of flagella do swim faster than their wild‐type counterparts in viscous solutions, and it has been proposed that this increase in speed is due to altered flagellar bundling. We are examining whether this phenomenon also occurs with the different morphological forms of P. mirabilis. We have found that swarmer cells of wild‐type P. mirabilis are significantly faster (P<0.001) than all other cell types in media with a viscosity of 9 – 830 cP. In media with a viscosity of 8340 cP, swarmer cells are motile while all other cell types are amotile. These results suggest a possible role for the swarm cell morphology: allowing migration through viscous mucosal layers during infection.

RK2 plasmid dynamics in Caulobacter crescentus cells – two modes of DNA replication initiation

Undisturbed plasmid dynamics is required for the stable maintenance of plasmid DNA in bacterial cells. In this work, we analysed subcellular localization, DNA synthesis and nucleoprotein complex formation of plasmid RK2 during the cell cycle of Caulobacter crescentus. Our microscopic observations showed asymmetrical distribution of plasmid RK2 foci between the two compartments of Caulobacter predivisional cells, resulting in asymmetrical allocation of plasmids to progeny cells. Moreover, using a quantitative PCR (qPCR) method, we estimated that multiple plasmid particles form a single fluorescent focus and that the number of plasmids per focus is approximately equal in both swarmer and predivisional Caulobacter cells. Analysis of the dynamics of TrfA-oriV complex formation during the Caulobacter cell cycle revealed that TrfA binds oriV primarily during the G1 phase, however, plasmid DNA synthesis occurs during the S and G2 phases of the Caulobacter cell cycle. Both in vitro and in vivo analysis of RK2 replication initiation in C. crescentus cells demonstrated that it is independent of the Caulobacter DnaA protein in the presence of the longer version of TrfA protein, TrfA-44. However, in vivo stability tests of plasmid RK2 derivatives suggested that a DnaA-dependent mode of plasmid replication initiation is also possible.

Prosthecate sphingomonads: proposal of Sphingomonas canadensis sp. nov.

Two stalked, aerobic, catalase- and oxidase-positive rod-shaped isolates, VKM B-1508 ( = CB 258) and FWC47(T), were analysed using a polyphasic approach. While the morphology and the 16S rRNA gene sequence of strain VKM B-1508 were 100% identical to the ones of Sphingomonas leidyi DSM 4733(T), the morphology of FWC47(T) was different, and the closest recognized species were Sphingomonas oligophenolica S213(T) ( = DSM 17107(T)) and Sphingomonas leidyi DSM 4733(T) with 97.2% and 97.0% 16S rRNA gene sequence similarity, respectively. DNA-DNA hybridization studies supported the differentiation of strain FWC47(T) from S. oligophenolica and S. leidyi. Strain FWC47(T) grew optimally at 28-30 °C, and pH 6.0-8.0. The major respiratory quinone was Q10 and the major polyamine was sym-homospermidine. The major fatty acids were C(17:1)ω6c and C(18:1)ω7c and C(15:0) 2-OH was the major 2-hydroxy fatty acid. The major polar lipids were phosphatidylglycerol, diphosphatidylglycerol, phosphatidyldimethylethylamine and unidentified sphingoglycolipids. The G+C content of the genomic DNA of strain FWC47(T) was 67.1 mol%. Strain FWC47(T) differed from S. leidyi by its ability to assimilate l-alanine, maltose and sucrose, by the presence of β-galactosidase and α-chymotrypsin, and the lack of valine arylamidase and β-glucosidase activities. Contrary to S. leidyi, FWC47(T) did not reduce nitrate and could not use fructose, acetate and N-acetyl-glusosamine. In the genus Sphingomonas, the dimorphic life cycle involving a prosthecate sessile and a flagellated swarmer cell was hitherto only known from Sphingomonas leidyi. Therefore, strain FWC47(T) represents an additional distinct prosthecate species in this genus for which the name Sphingomonas canadensis is proposed. The type strain is FWC47(T) ( =LMG 27141(T) =CCUG 62982(T)).

Choice of Pore Size Can Introduce Artefacts when Filtering Picoeukaryotes for Molecular Biodiversity Studies

Published results of studies based on samples size fractionated by sequential filtration (e.g. 0.2-3 μm) indicate that many ciliate, dinoflagellate and rhizarian phylotypes are found among marine picoeukaryotes. This is somewhat surprising as these protists are typically known as being large organisms (often >10 μm) and no picoplanktonic species have so far been identified. Here, the abundances of ciliate and dinoflagellate phylotypes in published molecular studies of picoeukaryotes are shown to correlate negatively with the pore size chosen for the end filter in the sequential filtrations (i.e. the filter used to collect the microbial biomass). This suggests that extracellular DNA adhering to small particles may be the source of ciliate and dinoflagellate phylotypes in picoplanktonic size fractions. This hypothesis was confirmed using real-time qPCR, which revealed significantly less dinoflagellate 18S rDNA in a 0.8-3-μm size fraction compared to 0.2-3 μm. On average, the abundance of putative extracellular phylotypes decreased by 84-89 % when a 0.8- μm end filter was used rather than a 0.2-μm end filter. A 0.8-μm filter is, however, not sufficient to retain all picoeukaryotic cells. Thus, selection of filter pore size involves a trade-off between avoiding artefacts generated by extracellular DNA and sampling the entire picoeukaryotic community. In contrast to ciliate and dinoflagellate phylotypes, rhizarian phylotypes in the picoplankton size range do not display a pattern consistent with an extracellular origin. This is likely due to the documented existence of picoplanktonic swarmer cells within this group.

Initiation of Swarming Motility by Proteus mirabilis Occurs in Response to Specific Cues Present in Urine and Requires Excess L-Glutamine

Proteus mirabilis, a leading cause of catheter-associated urinary tract infection (CaUTI), differentiates into swarm cells that migrate across catheter surfaces and medium solidified with 1.5% agar. While many genes and nutrient requirements involved in the swarming process have been identified, few studies have addressed the signals that promote initiation of swarming following initial contact with a surface. In this study, we show that P. mirabilis CaUTI isolates initiate swarming in response to specific nutrients and environmental cues. Thirty-three compounds, including amino acids, polyamines, fatty acids, and tricarboxylic acid (TCA) cycle intermediates, were tested for the ability to promote swarming when added to normally nonpermissive media. L-Arginine, L-glutamine, DL-histidine, malate, and DL-ornithine promoted swarming on several types of media without enhancing swimming motility or growth rate. Testing of isogenic mutants revealed that swarming in response to the cues required putrescine biosynthesis and pathways involved in amino acid metabolism. Furthermore, excess glutamine was found to be a strict requirement for swarming on normal swarm agar in addition to being a swarming cue under normally nonpermissive conditions. We thus conclude that initiation of swarming occurs in response to specific cues and that manipulating concentrations of key nutrient cues can signal whether or not a particular environment is permissive for swarming.

Mechanics of swimming of multi-body bacterial swarmers using non-labeled cell tracking algorithm

To better understand the survival strategy of bacterial swarmers and the mechanical advantages offered by the linear chain (head-tail) attachment of the multiple bacterial bodies in an individual swarmer cell at low Reynolds number, a non-labeled cell tracking algorithm was used to quantify the mechanics of multi-body flagellated bacteria, Serratia marcescens, swimming in a motility buffer that originally exhibited the swarming motility. Swarming is a type of bacterial motility that is characterized by the collective coordinated motion of differentiated swarmer cells on a two-dimensional surface such as agar. In this study, the bacterial swarmers with multiple cell bodies (2, 3, and 4) were extracted from the swarm plate, and then tracked individually after resuspending in the motility medium. Their motion was investigated and compared with individual undifferentiated swimming bacterial cells. The swarmers when released into the motility buffer swam actively without tumbles. Their speeds, orientations, and the diffusive properties were studied by tracking the individual cell trajectories over a short distance in two-dimensional field when the cells are swimming at a constant depth in a bulk aqueous environment. At short time scales, the ballistic trajectory was dominant for both multi-body swarmers and undifferentiated cells.

From swimming to swarming: Escherichia coli cell motility in two-dimensions

Escherichia coli swarmer cells coordinate their movement when confined in thin layers of fluid on agar surfaces. The motion and dynamics of cells, pairs of cells, and packs of cells can be recapitulated and studied in polymer microfluidic systems that are designed to constrain swarmer cell movement in thin layers of fluid between no-slip surfaces. The motion of elongated, smooth swimming E. coli cells in these environments reproduces the behavior of packs of cells observed at the leading edge of swarming communities and demonstrates the delicate balance between the physical dimensions of fluids and bacterial cell behavior.

More than Motility: Salmonella Flagella Contribute to Overriding Friction and Facilitating Colony Hydration during Swarming

We show in this study that Salmonella cells, which do not upregulate flagellar gene expression during swarming, also do not increase flagellar numbers per μm of cell length as determined by systematic counting of both flagellar filaments and hooks. Instead, doubling of the average length of a swarmer cell by suppression of cell division effectively doubles the number of flagella per cell. The highest agar concentration at which Salmonella cells swarmed increased from the normal 0.5% to 1%, either when flagella were overproduced or when expression of the FliL protein was enhanced in conjunction with stator proteins MotAB. We surmise that bacteria use the resulting increase in motor power to overcome the higher friction associated with harder agar. Higher flagellar numbers also suppress the swarming defect of mutants with changes in the chemotaxis pathway that were previously shown to be defective in hydrating their colonies. Here we show that the swarming defect of these mutants can also be suppressed by application of osmolytes to the surface of swarm agar. The "dry" colony morphology displayed by che mutants was also observed with other mutants that do not actively rotate their flagella. The flagellum/motor thus participates in two functions critical for swarming, enabling hydration and overriding surface friction. We consider some ideas for how the flagellum might help attract water to the agar surface, where there is no free water.

Proteus sp. – an opportunistic bacterial pathogen – classification, swarming growth, clinical significance and virulence factors

The genus Proteus belongs to the Enterobacteriaceae family, where it is placed in the tribe Proteeae, together with the genera Morganella and Providencia. Currently, the genus Proteus consists of five species: P. mirabilis, P. vulgaris, P. penneri, P. hauseri and P. myxofaciens, as well as three unnamed Proteus genomospecies. The most defining characteristic of Proteus bacteria is a swarming phenomenon, a multicellular differentiation process of short rods to elongated swarmer cells. It allows population of bacteria to migrate on solid surface. Proteus bacteria inhabit the environment and are also present in the intestines of humans and animals. These microorganisms under favorable conditions cause a number of infections including urinary tract infections (UTIs), wound infections, meningitis in neonates or infants and rheumatoid arthritis. Therefore, Proteus is known as a bacterial opportunistic pathogen. It causes complicated UTIs with a higher frequency, compared to other uropathogens. Proteus infections are accompanied by a formation of urinary stones, containing struvite and carbonate apatite. The virulence of Proteus rods has been related to several factors including fimbriae, flagella, enzymes (urease - hydrolyzing urea to CO2 and NH3, proteases degrading antibodies, tissue matrix proteins and proteins of the complement system), iron acqusition systems and toxins: hemolysins, Proteus toxin agglutinin (Pta), as well as an endotoxin - lipopolysaccharide (LPS). Proteus rods form biofilm, particularly on the surface of urinary catheters, which can lead to serious consequences for patients. In this review we present factors involved in the regulation of swarming phenomenon, discuss the role of particular pathogenic features of Proteus spp., and characterize biofilm formation by these bacteria.

Activity of Proteus mirabilis FliL Is Viscosity Dependent and Requires Extragenic DNA

Proteus mirabilis is a urinary tract pathogen and well known for its ability to move over agar surfaces by flagellum-dependent swarming motility. When P. mirabilis encounters a highly viscous environment, e.g., an agar surface, it differentiates from short rods with few flagella to elongated, highly flagellated cells that lack septa and contain multiple nucleoids. The bacteria detect a surface by monitoring the rotation of their flagellar motors. This process involves an enigmatic flagellar protein called FliL, the first gene in an operon (fliLMNOPQR) that encodes proteins of the flagellar rotor switch complex and flagellar export apparatus. We used a fliL knockout mutant to gain further insight into the function of FliL. Loss of FliL results in cells that cannot swarm (Swr(-)) but do swim (Swm(+)) and produces cells that look like wild-type swarmer cells, termed "pseudoswarmer cells," that are elongated, contain multiple nucleoids, and lack septa. Unlike swarmer cells, pseudoswarmer cells are not hyperflagellated due to reduced expression of flaA (the gene encoding flagellin), despite an increased transcription of both flhD and fliA, two positive regulators of flagellar gene expression. We found that defects in fliL prevent viscosity-dependent sensing of a surface and viscosity-dependent induction of flaA transcription. Studies with fliL cells unexpectedly revealed that the fliL promoter, fliL coding region, and a portion of fliM DNA are needed to complement the Swr(-) phenotype. The data support a dual role for FliL as a critical link in sensing a surface and in the maintenance of flagellar rod integrity.

Flagellum Density Regulates Proteus mirabilis Swarmer Cell Motility in Viscous Environments

Proteus mirabilis is an opportunistic pathogen that is frequently associated with urinary tract infections. In the lab, P. mirabilis cells become long and multinucleate and increase their number of flagella as they colonize agar surfaces during swarming. Swarming has been implicated in pathogenesis; however, it is unclear how energetically costly changes in P. mirabilis cell morphology translate into an advantage for adapting to environmental changes. We investigated two morphological changes that occur during swarming--increases in cell length and flagellum density--and discovered that an increase in the surface density of flagella enabled cells to translate rapidly through fluids of increasing viscosity; in contrast, cell length had a small effect on motility. We found that swarm cells had a surface density of flagella that was ∼5 times larger than that of vegetative cells and were motile in fluids with a viscosity that inhibits vegetative cell motility. To test the relationship between flagellum density and velocity, we overexpressed FlhD(4)C(2), the master regulator of the flagellar operon, in vegetative cells of P. mirabilis and found that increased flagellum density produced an increase in cell velocity. Our results establish a relationship between P. mirabilis flagellum density and cell motility in viscous environments that may be relevant to its adaptation during the infection of mammalian urinary tracts and movement in contact with indwelling catheters.

Anaerobic Respiration Using a Complete Oxidative TCA Cycle Drives Multicellular Swarming in Proteus mirabilis

ABSTRACTProteus mirabilis rapidly migrates across surfaces using a periodic developmental process of differentiation alternating between short swimmer cells and elongated hyperflagellated swarmer cells. To undergo this vigorous flagellum-mediated motility, bacteria must generate a substantial proton gradient across their cytoplasmic membranes by using available energy pathways. We sought to identify the link between energy pathways and swarming differentiation by examining the behavior of defined central metabolism mutants. Mutations in the tricarboxylic acid (TCA) cycle (fumC and sdhB mutants) caused altered patterns of swarming periodicity, suggesting an aerobic pathway. Surprisingly, the wild-type strain swarmed on agar containing sodium azide, which poisons aerobic respiration; the fumC TCA cycle mutant, however, was unable to swarm on azide. To identify other contributing energy pathways, we screened transposon mutants for loss of swarming on sodium azide and found insertions in the following genes that involved fumarate metabolism or respiration: hybB, encoding hydrogenase; fumC, encoding fumarase; argH, encoding argininosuccinate lyase (generates fumarate); and a quinone hydroxylase gene. These findings validated the screen and suggested involvement of anaerobic electron transport chain components. Abnormal swarming periodicity of fumC and sdhB mutants was associated with the excretion of reduced acidic fermentation end products. Bacteria lacking SdhB were rescued to wild-type pH and periodicity by providing fumarate, independent of carbon source but dependent on oxygen, while fumC mutants were rescued by glycerol, independent of fumarate only under anaerobic conditions. These findings link multicellular swarming patterns with fumarate metabolism and membrane electron transport using a previously unappreciated configuration of both aerobic and anaerobic respiratory chain components.

Brevundimonas abyssalis sp. nov., a dimorphic prosthecate bacterium isolated from deep-subsea floor sediment

A novel Gram-negative, aerobic, psychrotolerant, alkali-tolerant, heterotrophic and dimorphic prosthecate bacterium, designated strain TAR-001(T), was isolated from deep-sea floor sediment in Japan. Cells of this strain had a dimorphic life cycle and developed an adhesive stalk at a site not coincident with the centre of the cell pole, and the other type of cell, a swarm cell, had a polar flagellum. Colonies were glossy, viscous and yellowish-white in colour. The temperature, pH and salt concentration range for growth were 2-41 °C, pH 6.5-10.0 and 1-4% (w/v) NaCl. Phylogenetic analysis based on 16S rRNA gene sequences confirmed that strain TAR-001(T) belongs to the family Caulobacteraceae of the class Alphaproteobacteria, and lies between the genus Brevundimonas and the genus Caulobacter. Levels of similarity between the 16S rRNA gene sequence of strain TAR-001(T) and those of the type strains of Brevundimonas species were 93.3-95.7%; highest sequence similarity was with the type strain of Brevundimonas diminuta. Levels of sequence similarity between those of the type strains of Caulobacter species were 94.9-96.0%; highest sequence similarity was with the type strain of Caulobacter mirabilis. The G+C content of strain TAR-001(T) was 67.6 mol%. Q-10 was the major respiratory isoprenoid quinone. The major fatty acids were C18:1ω7c and C16:0, and the presence of 1,2-di-O-acyl-3-O-[D-glucopyranosyl-(1→4)-α-D-glucopyranuronosyl]glycerol suggests strain TAR-001(T) is more closely to the genus Brevundimonas than to the genus Caulobacter. The mean DNA-DNA hybridization levels between strain TAR-001(T) and the type strains of two species of the genus Brevundimonas were higher than that of the genus Caulobacter. On the basis of polyphasic biological features and the 16S rRNA gene sequence comparison presented here, strain TAR-001(T) is considered to represent a novel species of the genus Brevundimonas, for which the name Brevundimonas abyssalis sp. nov. is proposed; the type strain is TAR-001(T) (=JCM 18150(T)=CECT 8073(T)).