The branched CcsA/CckA-ChpT-CtrA phosphorelay of Sphingomonas melonis controls motility and biofilm formation

Recent advances in the development of bacterial response regulators inhibitors as antibacterial and/or antibiotic adjuvant agent: A new approach to combat bacterial resistance

The number of new antibacterial agents currently being discovered is insufficient to combat bacterial resistance. It is extremely challenging to find new antibiotics and to introduce them to the pharmaceutical market. Therefore, special attention must be given to find new strategies to combat bacterial resistance and prevent bacteria from developing resistance. Two-component system is a transduction system and the most prevalent mechanism employed by bacteria to respond to environmental changes. This signaling system consists of a membrane sensor histidine kinase that perceives environmental stimuli and a response regulator which acts as a transcription factor. The approach consisting of developing response regulators inhibitors with antibacterial activity or antibiotic adjuvant activity is a novel approach that has never been previously reviewed. In this review we report for the first time, the importance of targeting response regulators and summarizing all existing studies carried out from 2008 until now on response regulators inhibitors as antibacterial agents or / and antibiotic adjuvants. Moreover, we describe the antibacterial activity and/or antibiotic adjuvants activity against the studied bacterial strains and the mechanism of different response regulator inhibitors when it's possible.

Adaptation to Environmental Extremes Structures Functional Traits in Biological Soil Crust and Hypolithic Microbial Communities

Biological soil crusts (biocrusts) are widespread in drylands and deserts. At the microhabitat scale, they also host hypolithic communities that live under semitranslucent stones. Both environmental niches experience exposure to extreme conditions such as high UV radiation, desiccation, temperature fluctuations, and resource limitation. However, hypolithic communities are somewhat protected from extremes relative to biocrust communities. Conditions are otherwise similar, so comparing them can answer outstanding questions regarding adaptations to environmental extremes. Using metagenomic sequencing, we assessed the functional potential of dryland soil communities and identified the functional underpinnings of ecological niche differentiation in biocrusts versus hypoliths. We also determined the effect of the anchoring photoautotroph (moss or cyanobacteria). Genes and pathways differing in abundance between biocrusts and hypoliths indicate that biocrust communities adapt to the higher levels of UV radiation, desiccation, and temperature extremes through an increased ability to repair damaged DNA, sense and respond to environmental stimuli, and interact with other community members and the environment. Intracellular competition appears to be crucial to both communities, with biocrust communities using the Type VI Secretion System (T6SS) and hypoliths favoring a diversity of antibiotics. The dominant primary producer had a reduced effect on community functional potential compared with niche, but an abundance of genes related to monosaccharide, amino acid, and osmoprotectant uptake in moss-dominated communities indicates reliance on resources provided to heterotrophs by mosses. Our findings indicate that functional traits in dryland communities are driven by adaptations to extremes and we identify strategies that likely enable survival in dryland ecosystems. IMPORTANCE Biocrusts serve as a keystone element of desert and dryland ecosystems, stabilizing soils, retaining moisture, and serving as a carbon and nitrogen source in oligotrophic environments. Biocrusts cover approximately 12% of the Earth's terrestrial surface but are threatened by climate change and anthropogenic disturbance. Given their keystone role in ecosystem functioning, loss will have wide-spread consequences. Biocrust microbial constituents must withstand polyextreme environmental conditions including high UV exposure, desiccation, oligotrophic conditions, and temperature fluctuations over short time scales. By comparing biocrust communities with co-occurring hypolithic communities (which inhabit the ventral sides of semitranslucent stones and are buffered from environmental extremes), we identified traits that are likely key adaptations to extreme conditions. These include DNA damage repair, environmental sensing and response, and intracellular competition. Comparison of the two niches, which differ primarily in exposure levels to extreme conditions, makes this system ideal for understanding how functional traits are structured by the environment.

The transcriptional regulator CtrA controls gene expression in Alphaproteobacteria phages: Evidence for a lytic deferment pathway

Pilitropic and flagellotropic phages adsorb to bacterial pili and flagella. These phages have long been used to investigate multiple aspects of bacterial physiology, such as the cell cycle control in the Caulobacterales. Targeting cellular appendages for adsorption effectively constrains the population of infectable hosts, suggesting that phages may have developed strategies to maximize their infective yield. Brevundimonas phage vB_BsubS-Delta is a recently characterized pilitropic phage infecting the Alphaproteobacterium Brevundimonas subvibrioides. Like other Caulobacterales, B. subvibrioides divides asymmetrically and its cell cycle is governed by multiple transcriptional regulators, including the master regulator CtrA. Genomic characterization of phage vB_BsubS-Delta identified the presence of a large intergenic region with an unusually high density of putative CtrA-binding sites. A systematic analysis of the positional distribution of predicted CtrA-binding sites in complete phage genomes reveals that the highly skewed distribution of CtrA-binding sites observed in vB_BsubS-Delta is an unequivocal genomic signature that extends to other pilli- and flagellotropic phages infecting the Alphaproteobacteria. Moreover, putative CtrA-binding sites in these phage genomes localize preferentially to promoter regions and have higher scores than those detected in other phage genomes. Phylogenetic and comparative genomics analyses show that this genomic signature has evolved independently in several phage lineages, suggesting that it provides an adaptive advantage to pili/flagellotropic phages infecting the Alphaproteobacteria. Experimental results demonstrate that CtrA binds to predicted CtrA-binding sites in promoter regions and that it regulates transcription of phage genes in unrelated Alphaproteobacteria-infecting phages. We propose that this focused distribution of CtrA-binding sites reflects a fundamental new aspect of phage infection, which we term lytic deferment. Under this novel paradigm, pili- and flagellotropic phages exploit the CtrA transduction pathway to monitor the host cell cycle state and synchronize lysis with the presence of infectable cells.

Hybrid Histidine Kinase WelA of Sphingomonas sp. WG Contributes to WL Gum Biosynthesis and Motility

Sphingomonas sp. WG produced WL gum with commercial utility potential in many industries. A hybrid sensor histidine kinase/response regulator WelA was identified to regulate the WL gum biosynthesis, and its function was evaluated by gene deletion strategy. The WL gum production and broth viscosity of mutant ΔwelA was only 44% and 0.6% of wild type strain at 72 h. The transcriptomic analysis of differentially expressed genes showed that WelA was mapped to CckA; ChpT, and CtrA in the CckA-ChpT-CtrA pathway was up-regulated. One phosphodiesterase was up-regulated by CtrA, and the intracellular c-di-GMP was decreased. Most genes involved in WL gum biosynthesis pathway was not significantly changed in ΔwelA except the up-regulated atrB and atrD and the down-regulated pmm. Furthermore, the up-regulated regulators ctrA, flaEY, flbD, and flaF may participate in the regulation of flagellar biogenesis and influenced motility. These results suggested that CckA-ChpT-CtrA pathway and c-di-GMP were involved in WL gum biosynthesis regulation. This work provides useful information on the understanding of molecular mechanisms underlying WL gum biosynthesis regulation.

Connection Between Chromosomal Location and Function of CtrA Phosphorelay Genes in Alphaproteobacteria

Most bacterial chromosomes are circular, with replication starting at one origin (ori) and proceeding on both replichores toward the terminus (ter). Several studies have shown that the location of genes relative to ori and ter can have profound effects on regulatory networks and physiological processes. The CtrA phosphorelay is a gene regulatory system conserved in most alphaproteobacteria. It was first discovered in Caulobacter crescentus where it controls replication and division into a stalked and a motile cell in coordination with other factors. The locations of the ctrA gene and targets of this response regulator on the chromosome affect their expression through replication-induced DNA hemi-methylation and specific positioning along a CtrA activity gradient in the dividing cell, respectively. Here we asked to what extent the location of CtrA regulatory network genes might be conserved in the alphaproteobacteria. We determined the locations of the CtrA phosphorelay and associated genes in closed genomes with unambiguously identifiable ori from members of five alphaproteobacterial orders. The location of the phosphorelay genes was the least conserved in the Rhodospirillales followed by the Sphingomonadales. In the Rhizobiales a trend toward certain chromosomal positions could be observed. Compared to the other orders, the CtrA phosphorelay genes were conserved closer to ori in the Caulobacterales. In contrast, the genes were highly conserved closer to ter in the Rhodobacterales. Our data suggest selection pressure results in differential positioning of CtrA phosphorelay and associated genes in alphaproteobacteria, particularly in the orders Rhodobacterales, Caulobacterales and Rhizobiales that is worth deeper investigation.

Identification of two fnr genes and characterisation of their role in the anaerobic switch in Sphingopyxis granuli strain TFA

Sphingopyxis granuli strain TFA is able to grow on the organic solvent tetralin as the only carbon and energy source. The aerobic catabolic pathway for tetralin, the genes involved and their regulation have been fully characterised. Unlike most of the bacteria belonging to the sphingomonads group, this strain is able to grow in anoxic conditions by respiring nitrate, though not nitrite, as the alternative electron acceptor. In this work, two fnr-like genes, fnrN and fixK, have been identified in strain TFA. Both genes are functional in E. coli and Sphingopyxis granuli although fixK, whose expression is apparently activated by FnrN, seems to be much less effective than fnrN in supporting anaerobic growth. Global transcriptomic analysis of a ΔfnrN ΔfixK double mutant and identification of Fnr boxes have defined a minimal Fnr regulon in this bacterium. However, expression of a substantial number of anaerobically regulated genes was not affected in the double mutant. Additional regulators such regBA, whose expression is also activated by Fnr, might also be involved in the anaerobic response. Anaerobically induced stress response genes were not regulated by Fnr but apparently induced by stress conditions inherent to anaerobic growth, probably due to accumulation of nitrite and nitric oxide.

Generating asymmetry in a changing environment: cell cycle regulation in dimorphic alphaproteobacteria

While many bacteria divide by symmetric binary fission, some alphaproteobacteria have strikingly asymmetric cell cycles, producing offspring that differs significantly in their morphology and reproductive state. To establish this asymmetry, these species employ a complex cell cycle regulatory pathway based on two-component signaling cascades. At the center of this network is the essential DNA-binding response regulator CtrA, which acts as a transcription factor controlling numerous genes with cell cycle-relevant functions as well as a regulator of chromosome replication. The DNA-binding activity of CtrA is controlled at the level of both protein phosphorylation and stability, dependent on an intricate network of regulatory proteins, whose function is tightly coordinated in time and space. CtrA is differentially activated in the two (developing) offspring, thereby establishing distinct transcriptional programs that ultimately determine their distinct cell fates. Phase-separated polar microdomains of changing composition sequester proteins involved in the (in-)activation and degradation of CtrA specifically at each pole. In this review, we summarize the current knowledge of the CtrA pathway and discuss how it has evolved to regulate the cell cycle of morphologically distinct alphaproteobacteria.

Genetic and physiological analysis of biofilm formation on different plastic surfaces by Sphingomonas sp. strain S2M10 reveals an essential function of sphingan biosynthesis

Alphaproteobacteria belonging to the group of the sphingomonads are frequently found in biofilms colonizing pure-water systems, where they cause technical and hygienic problems. In this study, physiological properties of sphingomonads for biofilm formation on plastic surfaces were analysed. Sphingomonas sp. strain S2M10 was isolated from a used water-filtration membrane and submitted to transposon mutagenesis for isolating mutants with altered biofilm formation. Mutants showing strongly decreased biofilm formation carried transposon insertions in genes for the biosynthesis of the polysaccharide sphingan and for flagellar motility. Flagella-mediated attachment was apparently important for biofilm formation on plastic materials of intermediate hydrophobicity, while a mutant with defect in spnB, encoding the first enzyme in sphingan biosynthesis, showed no biofilm formation on all tested materials. Sphingan-dependent biofilm formation was induced in the presence of specific carbon sources while it was not induced in complex medium with yeast extract and tryptone. The regulation of sphingan-based biofilm formation was investigated by interfering with the CckA/ChpT/CtrA phosphorelay, a central signal-transduction pathway in most Alphaproteobacteria. Construction and ectopic expression of a kinase-deficient histidine kinase CckA caused cell elongation and massive sphingan-dependent cell aggregation. In addition, it caused increased activity of the promotor of spnB. In conclusion, these results indicate that sphingan-based biofilm formation by sphingomonads might be triggered by specific carbon sources under prototrophic conditions resembling a milieu that often prevails in pure-water systems.

Integrative and quantitative view of the CtrA regulatory network in a stalked budding bacterium

The Alphaproteobacteria show a remarkable diversity of cell cycle-dependent developmental patterns, which are governed by the conserved CtrA pathway. Its central component CtrA is a DNA-binding response regulator that is controlled by a complex two-component signaling network, mediating distinct transcriptional programs in the two offspring. The CtrA pathway has been studied intensively and was shown to consist of an upstream part that reads out the developmental state of the cell and a downstream part that integrates the upstream signals and mediates CtrA phosphorylation. However, the role of this circuitry in bacterial diversification remains incompletely understood. We have therefore investigated CtrA regulation in the morphologically complex stalked budding alphaproteobacterium Hyphomonas neptunium. Compared to relatives dividing by binary fission, H. neptunium shows distinct changes in the role and regulation of various pathway components. Most notably, the response regulator DivK, which normally links the upstream and downstream parts of the CtrA pathway, is dispensable, while downstream components such as the pseudokinase DivL, the histidine kinase CckA, the phosphotransferase ChpT and CtrA are essential. Moreover, CckA is compartmentalized to the nascent bud without forming distinct polar complexes and CtrA is not regulated at the level of protein abundance. We show that the downstream pathway controls critical functions such as replication initiation, cell division and motility. Quantification of the signal flow through different nodes of the regulatory cascade revealed that the CtrA pathway is a leaky pipeline and must involve thus-far unidentified factors. Collectively, the quantitative system-level analysis of CtrA regulation in H. neptunium points to a considerable evolutionary plasticity of cell cycle regulation in alphaproteobacteria and leads to hypotheses that may also hold in well-established model organisms such as Caulobacter crescentus.

Bacteria show a variety of morphologies and life cycles. This is especially true for members of the Alphaproteobacteria, a bacterial class of considerable ecological, medical, and biotechnological importance. The alphaproteobacterial cell cycle is regulated by a conserved regulatory pathway mediated by CtrA, a DNA-binding response regulator that acts as a transcriptional regulator and repressor of replication initiation. CtrA controls the expression of many genes with critical roles in cell growth, division, and differentiation. The contribution of changes in the CtrA regulatory network to the diversification of alphaproteobacterial species is still incompletely understood. Therefore, we comprehensively studied CtrA regulation in the stalked budding bacterium Hyphomonas neptunium, a morphologically complex species that multiplies by forming buds at the end of a stalk-like cellular extension. Our results show that this distinct mode of growth is accompanied by marked differences in the importance and subcellular localization of several CtrA pathway components. Moreover, quantitative analysis of the signal flow through the pathway indicates that its different nodes are less tightly connected than previously thought, suggesting the existence of so-far unidentified factors. Our results indicate a considerable plasticity of the CtrA regulatory network and reveal novel features that may also apply to other alphaproteobacterial species.

Interactions among Redox Regulators and the CtrA Phosphorelay in Dinoroseobacter shibae and Rhodobacter capsulatus

Bacteria employ regulatory networks to detect environmental signals and respond appropriately, often by adjusting gene expression. Some regulatory networks influence many genes, and many genes are affected by multiple regulatory networks. Here, we investigate the extent to which regulatory systems controlling aerobic–anaerobic energetics overlap with the CtrA phosphorelay, an important system that controls a variety of behavioral processes, in two metabolically versatile alphaproteobacteria, Dinoroseobacter shibae and Rhodobacter capsulatus. We analyzed ten available transcriptomic datasets from relevant regulator deletion strains and environmental changes. We found that in D. shibae, the CtrA phosphorelay represses three of the four aerobic–anaerobic Crp/Fnr superfamily regulator-encoding genes (fnrL, dnrD, and especially dnrF). At the same time, all four Crp/Fnr regulators repress all three phosphorelay genes. Loss of dnrD or dnrF resulted in activation of the entire examined CtrA regulon, regardless of oxygen tension. In R. capsulatus FnrL, in silico and ChIP-seq data also suggested regulation of the CtrA regulon, but it was only with loss of the redox regulator RegA where an actual transcriptional effect on the CtrA regulon was observed. For the first time, we show that there are complex interactions between redox regulators and the CtrA phosphorelays in these bacteria and we present several models for how these interactions might occur.

The CtrA Regulon of Rhodobacter sphaeroides Favors Adaptation to a Particular Lifestyle

Activation of the two-component system formed by CckA, ChpT, and CtrA (kinase, phosphotransferase, and response regulator, respectively) in Rhodobacter sphaeroides does not occur under the growth conditions commonly used in the laboratory. However, it is possible to isolate a gain-of-function mutant in CckA that turns the system on. Using massive parallel transcriptome sequencing (RNA-seq), we identified 321 genes that are differentially regulated by CtrA. From these genes, 239 were positively controlled and 82 were negatively regulated. Genes encoding the Fla2 polar flagella and gas vesicle proteins are strongly activated by CtrA. Genes involved in stress responses as well as several transcriptional factors are also positively controlled, whereas the photosynthetic and CO₂ fixation genes are repressed. Potential CtrA-binding sites were bioinformatically identified, leading to the proposal that at least 81 genes comprise the direct regulon. Based on our results, we ponder that the transcriptional response orchestrated by CtrA enables a lifestyle in which R. sphaeroides will effectively populate the surface layer of a water body enabled by gas vesicles and will remain responsive to chemotactic stimuli using the chemosensoring system that controls the Fla2 flagellum. Simultaneously, fine-tuning of photosynthesis and stress responses will reduce the damage caused by heat and high light intensity in this water stratum. In summary, in this bacterium CtrA has evolved to control physiological responses that allow its adaptation to a particular lifestyle instead of controlling the cell cycle as occurs in other species.IMPORTANCE Cell motility in Alphaproteobacteria is frequently controlled by the CckA, ChpT, and CtrA two-component system. Under the growth conditions commonly used in the laboratory, ctrA is transcriptionally inactive in Rhodobacter sphaeroides, and motility depends on the Fla1 flagellar system that was acquired by a horizontal transfer event. Likely, the incorporation of this flagellar system released CtrA from the strong selective pressure of being the main motility regulator, allowing this two-component system to specialize and respond to some specific conditions. Identifying the genes that are directly regulated by CtrA could help us understand the conditions in which the products of this regulon are required. Massive parallel transcriptome sequencing (RNA-seq) revealed that CtrA orchestrates an adaptive response that contributes to the colonization of a particular environmental niche.

Precise timing of transcription by c-di-GMP coordinates cell cycle and morphogenesis in Caulobacter

Bacteria adapt their growth rate to their metabolic status and environmental conditions by modulating the length of their G1 period. Here we demonstrate that a gradual increase in the concentration of the second messenger c-di-GMP determines precise gene expression during G1/S transition in Caulobacter crescentus. We show that c-di-GMP stimulates the kinase ShkA by binding to its central pseudo-receiver domain, activates the TacA transcription factor, and initiates a G1/S-specific transcription program leading to cell morphogenesis and S-phase entry. Activation of the ShkA-dependent genetic program causes c-di-GMP to reach peak levels, which triggers S-phase entry and promotes proteolysis of ShkA and TacA. Thus, a gradual increase of c-di-GMP results in precise control of ShkA-TacA activity, enabling G1/S-specific gene expression that coordinates cell cycle and morphogenesis.

Bacteria adapt their growth rate to their metabolic status and environmental conditions by modulating the length of their G1 period. Here the authors show that an increase in c-di-GMP concentration modulates the activity of kinase ShkA and transcription factor TacA, thus enabling G1/S transition in Caulobacter.

The response of Sphingopyxis granuli strain TFA to the hostile anoxic condition

Sphingomonads comprises a group of interesting aerobic bacteria because of their ubiquity and metabolic capability of degrading many recalcitrant contaminants. The tetralin-degrader Sphingopyxis granuli strain TFA has been recently reported as able to anaerobically grow using nitrate as the alternative electron acceptor and so far is the only bacterium with this ability within the sphingomonads group. To understand how strain TFA thrives under anoxic conditions, a differential transcriptomic analysis while growing under aerobic or anoxic conditions was performed. This analysis has been validated and complemented with transcription kinetics of representative genes of different functional categories. Results show an extensive change of the expression pattern of this strain in the different conditions. Consistently, the most induced operon in anoxia codes for proteases, presumably required for extensive changes in the protein profile. Besides genes that respond to lack of oxygen in other bacteria, there are a number of genes that respond to stress or to damage of macromolecules, including genes of the SOS DNA-damage response, which suggest that anoxic conditions represent a hostile environment for this bacterium. Interestingly, growth under anoxic conditions also resulted in repression of all flagellar and type IV pilin genes, which suggested that this strain shaves its appendages off while growing in anaerobiosis.

Learning from the master: targets and functions of the CtrA response regulator in Brucella abortus and other alpha-proteobacteria

The α-proteobacteria are a fascinating group of free-living, symbiotic and pathogenic organisms, including the Brucella genus, which is responsible for a worldwide zoonosis. One common feature of α-proteobacteria is the presence of a conserved response regulator called CtrA, first described in the model bacterium Caulobacter crescentus, where it controls gene expression at different stages of the cell cycle. Here, we focus on Brucella abortus and other intracellular α-proteobacteria in order to better assess the potential role of CtrA in the infectious context. Comparative genomic analyses of the CtrA control pathway revealed the conservation of specific modules, as well as the acquisition of new factors during evolution. The comparison of CtrA regulons also suggests that specific clades of α-proteobacteria acquired distinct functions under its control, depending on the essentiality of the transcription factor. Other CtrA-controlled functions, for instance motility and DNA repair, are proposed to be more ancestral. Altogether, these analyses provide an interesting example of the plasticity of a regulation network, subject to the constraints of inherent imperatives such as cell division and the adaptations to diversified environmental niches.

Architecture of divergent flagellar promoters controlled by CtrA in Rhodobacter sphaeroides

BACKGROUND

Rhodobacter sphaeroides has two sets of flagellar genes, fla1 and fla2, that are responsible for the synthesis of two different flagellar structures. The expression of the fla2 genes is under control of CtrA. In several α-proteobacteria CtrA is also required for the expression of the flagellar genes, but the architecture of CtrA-dependent promoters has only been studied in detail in Caulobacter crescentus. In many cases the expression of fla genes originates from divergent promoters located a few base pairs apart, suggesting a particular arrangement of the cis-acting sites.

RESULTS

Here we characterized several control regions of the R. sphaeroides fla2 genes and analyzed in detail two regions containing the divergent promoters flgB2p-fliI2p, and fliL2p-fliF2p. Binding sites for CtrA of these promoters were identified in silico and tested by site directed mutagenesis. We conclude that each one of these promoter regions has a particular arrangement, either a single CtrA binding site for activation of fliL2p and fliF2p, or two independent sites for activation of flgB2p and fliI2p. ChIP experiments confirmed that CtrA binds to the control region containing the flgB2 and fliI2 promoters, supporting the notion that CtrA directly controls the expression of the fla2 genes. The flgB and fliI genes are syntenic and show a short intercistronic region in closely related bacterial species. We analyzed these regions and found that the arrangement of the CtrA binding sites varies considerably.

CONCLUSIONS

The results in this work reveal the arrangement of the fla2 divergent promoters showing that CtrA promotes transcriptional activation using more than a single architecture.

Transcriptome Analysis of Novosphingobium pentaromativorans US6-1 Reveals the Rsh Regulon and Potential Molecular Mechanisms of N-acyl-l-homoserine Lactone Accumulation

In most bacteria, a bifunctional Rsh responsible for (p)ppGpp metabolism is the key player in stringent response. To date, no transcriptome-wide study has been conducted to investigate the Rsh regulon, and the molecular mechanism of how Rsh affects the accumulation of N-acyl-l-homoserine lactone (AHL) remains unknown in sphingomonads. In this study, we identified an rsh_US6–1 gene by sequence analysis in Novosphingobium pentaromativorans US6-1, a member of the sphingomonads. RNA-seq was used to determine transcription profiles of the wild type and the ppGpp-deficient rsh_US6–1 deletion mutant (∆rsh). There were 1540 genes in the Rsh_US6–1 regulon, including those involved in common traits of sphingomonads such as exopolysaccharide biosynthesis. Furthermore, both RNA-seq and quantitative real-time polymerase chain reaction (qRT-PCR) showed essential genes for AHL production (novI and novR) were positively regulated by Rsh_US6–1 during the exponential growth phase. A degradation experiment indicated the reason for the AHL absence in ∆rsh was unrelated to the AHL degradation. According to RNA-seq, we proposed σ^E, DksA, Lon protease and RNA degradation enzymes might be involved in the Rsh_US6–1-dependent expression of novI and novR. Here, we report the first transcriptome-wide analysis of the Rsh regulon in sphingomonads and investigate the potential mechanisms regulating AHL accumulation, which is an important step towards understanding the regulatory system of stringent response in sphingomonads.