The Sinorhizobium meliloti LpxXL and AcpXL Proteins Play Important Roles in Bacteroid Development within Alfalfa

Free-living Sinorhizobium meliloti lpxXL and acpXL mutants lack lipid A very-long-chain fatty acids (VLCFAs) and have reduced competitiveness in alfalfa. We demonstrate that LpxXL and AcpXL play important but distinct roles in bacteroid development and that LpxXL is essential for the modification of S. meliloti bacteroid lipid A with VLCFAs.Sinorhizobium meliloti and Brucella abortus form chronic intracellular infections within legumes and mammalian hosts, respectively (3, 20), and their BacA proteins play essential roles in these processes (8, 12). The precise function(s) of the BacA proteins has not been resolved, but free-living S. meliloti and B. abortus mutants lacking BacA have increased resistance to the glycopeptide bleomycin (9, 12) and there are ∼50% decreases in their lipid A very-long-chain fatty acid (VLCFA) contents (4, 7). It has also been determined that the increased resistance of an S. meliloti bacA null mutant to bleomycin and a truncated eukaryotic peptide, Bac7_1-16, is independent of its lipid A VLCFA alteration (6, 15). Together, these findings support a model in which BacA could have multiple nonoverlapping functions which lead to lipid A VLCFA modification and peptide uptake. The fact that two symbiotically defective S. meliloti BacA site-directed mutants (Q193G and R389G) (13) show defects in BacA-mediated lipid A VLCFA modification (4) but are still capable of peptide uptake (15) suggests that the S. meliloti lipid A VLCFA modification could play a key role in the symbiosis of this organism with alfalfa.Since the mechanism by which BacA leads to the lipid A VLCFA modification has not been resolved (4), S. meliloti mutants were constructed with mutations in the lpxXL and acpXL genes, which encode a lipid A VLCFA acyl transferase and a VLCFA acyl carrier protein directly involved in the biosynthesis of VLCFA-modified lipid A (5, 23). The S. meliloti lpxXL and acpXL mutants completely lack the lipid A VLCFA modification in their free-living states, but, unlike the S. meliloti bacA null mutant, these mutants can still form a successful symbiosis with alfalfa (5, 8, 23). However, the fact that the S. meliloti acpXL and lpxXL mutants are substantially less competitive in the alfalfa symbiosis than the parent strain (5, 23) indicates that the AcpXL and LpxXL proteins play important roles in at least one of the stages of the alfalfa symbiosis. Although the free-living S. meliloti acpXL and lpxXL mutants completely lack the lipid A VLCFA, they produce different species of lipid A (5). For example, in the absence of AcpXL, S. meliloti is able to modify lipid A with either C16:0 or C18:0 in the position normally modified with the VLCFA in the parent strain lipid A. This process is LpxXL dependent, as it does not occur in either an S. meliloti lpxXL single mutant or an S. meliloti acpXL lpxXL double mutant. In addition, since a Rhizobium leguminosarum acpXL mutant completely lacks the lipid A VLCFA modification in its free-living state but its lipid A is partially modified with the VLCFA to ∼58% of the amount in the parent strain lipid A during passage through peas (25), it is also possible that the S. meliloti acpXL mutant and possibly the S. meliloti lpxXL mutant undergo further lipid A changes during the interaction with alfalfa.In this study, we found that LpxXL and AcpXL play important but distinct roles in S. meliloti bacteroid development during alfalfa symbiosis. Additionally, we demonstrated that there is a minor host-induced AcpXL-independent mechanism by which S. meliloti bacteroid lipopolysaccharide (LPS) can be modified with the VLCFA. In contrast, we found that the LpxXL protein plays an essential role in the modification of S. meliloti bacteroids with VLCFAs.     

Identification of a Gene Cluster for the Biosynthesis of a Long,Galactose-Rich Exopolysaccharide in Lactobacillus rhamnosus GG and Functional Analysis of the Priming Glycosyltransferase

Cell surface polysaccharides have an established role as virulence factors in human bacterial pathogens. Less documented are the biosynthesis and biological functions of surface polysaccharides in beneficial bacteria. We identified a gene cluster that encodes the enzymes and regulatory and transporter proteins for the different steps in the biosynthesis of extracellular polysaccharides (EPS) of the well-documented probiotic strain Lactobacillus rhamnosus GG. Subsequent mutation of the welE gene, encoding the priming glycosyltransferase within this cluster, and comparative phenotypic analyses of wild-type versus mutant strains confirmed the specific function of this gene cluster in the biosynthesis of high-molecular-weight, galactose-rich heteropolymeric EPS molecules. The phenotypic analyses included monomer composition determination, estimation of the polymer length of the isolated EPS molecules, and single-molecule force spectroscopy of the surface polysaccharides. Further characterization of the welE mutant also showed that deprivation of these long, galactose-rich EPS molecules results in an increased adherence and biofilm formation capacity of L. rhamnosus GG, possibly because of less shielding of adhesins such as fimbria-like structures.Bacterial surface polysaccharides are considered to be key macromolecules in determining microbe-host interactions, as they display a high degree of variety and diversity among bacterial species in terms of composition, monomer linkages, branching degree, polymer size, production level, etc. (24, 46). Since most bacteria contain more than one type of surface polysaccharides, such as lipopolysaccharides (O antigens), capsular polysaccharides (CPS), exopolysaccharides (EPS), and/or glycan chains as part of glycoproteins, the elucidation of their exact role is complex. Nevertheless, surface polysaccharides are now known to exert important functions at several stages during pathogenesis, including tissue adherence, biofilm formation, and evasion of host defenses such as phagocytosis (9, 24, 33). In addition to their role in pathogens, an important biological role for CPS and glycoproteins has also recently been shown in colonization of the gut by bacteria of the genus Bacteroides (10, 34).Conversely, the role of surface polysaccharides in probiotic-host interactions has not yet been studied in great detail. A probiotic bacterium is defined as “a live microorganism that, when administered or ingested in adequate amounts, confers a health benefit on the host” (18). Members of the genus Lactobacillus are commonly studied for their health-promoting capacities (26, 31, 37). As polysaccharides display a high diversity among lactobacilli, they are thought to be involved in determining strain-specific properties important for probiotic action, such as adhesion, stress resistance, and interactions with specific receptors and effectors of the host defense system (13, 56). Moreover, these EPS molecules are of interest in the dairy industry for conferring textural and rheological properties to fermented products such as yogurt and soft cheese (56). Nevertheless, detailed genetic and functional studies of EPS molecules of lactobacilli are currently limited (26, 56).Lactobacillus rhamnosus GG (ATCC 53103) is one of the probiotic strains with the largest number of proven health benefits (15). Several clinical trials have reported that L. rhamnosus GG can prevent and relieve certain types of diarrhea (22) and atopic disease (25) and reduce inflammation in some milder states of inflammatory bowel diseases (60). However, the cell surface factors or specific characteristics of L. rhamnosus GG that underlie these health benefits are largely unknown.We recently showed by single-molecule force spectroscopy (SMFS) with specific lectin tips that the cell surface of L. rhamnosus GG wild-type cells contains two major types of cell wall-associated polysaccharides (CW-PS) (21). The longest and most abundant polysaccharides are galactose-rich and seem to correspond with the EPS molecules of L. rhamnosus GG, which were previously structurally identified by Landersjö et al. (27) using nuclear magnetic resonance spectroscopy. Additionally, shorter, yet-uncharacterized glucose-rich polysaccharides are present on the L. rhamnosus GG surface (21). In the current study, we describe the identification and annotation of the L. rhamnosus GG gene cluster that encodes the enzymes and transporter and regulatory proteins involved in the biosynthesis of long, galactose-rich EPS molecules. This was experimentally confirmed by the construction of a knockout mutant of the corresponding priming glycosyltransferase and subsequent characterization of the surface polysaccharides of wild-type and mutant strains. We also studied the specific role of these EPS molecules in adherence to mucus and gut epithelial cells and in biofilm formation by L. rhamnosus GG.     

A New Borrelia Species Defined by Multilocus Sequence Analysis of Housekeeping Genes

Analysis of Lyme borreliosis (LB) spirochetes, using a novel multilocus sequence analysis scheme, revealed that OspA serotype 4 strains (a rodent-associated ecotype) of Borrelia garinii were sufficiently genetically distinct from bird-associated B. garinii strains to deserve species status. We suggest that OspA serotype 4 strains be raised to species status and named Borrelia bavariensis sp. nov. The rooted phylogenetic trees provide novel insights into the evolutionary history of LB spirochetes.Multilocus sequence typing (MLST) and multilocus sequence analysis (MLSA) have been shown to be powerful and pragmatic molecular methods for typing large numbers of microbial strains for population genetics studies, delineation of species, and assignment of strains to defined bacterial species (4, 13, 27, 40, 44). To date, MLST/MLSA schemes have been applied only to a few vector-borne microbial populations (1, 6, 30, 37, 40, 41, 47).Lyme borreliosis (LB) spirochetes comprise a diverse group of zoonotic bacteria which are transmitted among vertebrate hosts by ixodid (hard) ticks. The most common agents of human LB are Borrelia burgdorferi (sensu stricto), Borrelia afzelii, Borrelia garinii, Borrelia lusitaniae, and Borrelia spielmanii (7, 8, 12, 35). To date, 15 species have been named within the group of LB spirochetes (6, 31, 32, 37, 38, 41). While several of these LB species have been delineated using whole DNA-DNA hybridization (3, 20, 33), most ecological or epidemiological studies have been using single loci (5, 9-11, 29, 34, 36, 38, 42, 51, 53). Although some of these loci have been convenient for species assignment of strains or to address particular epidemiological questions, they may be unsuitable to resolve evolutionary relationships among LB species, because it is not possible to define any outgroup. For example, both the 5S-23S intergenic spacer (5S-23S IGS) and the gene encoding the outer surface protein A (ospA) are present only in LB spirochete genomes (36, 43). The advantage of using appropriate housekeeping genes of LB group spirochetes is that phylogenetic trees can be rooted with sequences of relapsing fever spirochetes. This renders the data amenable to detailed evolutionary studies of LB spirochetes.LB group spirochetes differ remarkably in their patterns and levels of host association, which are likely to affect their population structures (22, 24, 46, 48). Of the three main Eurasian Borrelia species, B. afzelii is adapted to rodents, whereas B. valaisiana and most strains of B. garinii are maintained by birds (12, 15, 16, 23, 26, 45). However, B. garinii OspA serotype 4 strains in Europe have been shown to be transmitted by rodents (17, 18) and, therefore, constitute a distinct ecotype within B. garinii. These strains have also been associated with high pathogenicity in humans, and their finer-scale geographical distribution seems highly focal (10, 34, 52, 53).In this study, we analyzed the intra- and interspecific phylogenetic relationships of B. burgdorferi, B. afzelii, B. garinii, B. valaisiana, B. lusitaniae, B. bissettii, and B. spielmanii by means of a novel MLSA scheme based on chromosomal housekeeping genes (30, 48).     

The Twin Arginine Transport System Appears To Be Essential for Viability in Sinorhizobium meliloti

The twin arginine transport (Tat) system is responsible for transporting prefolded proteins to the periplasmic space. The Tat pathway has been implicated in many bacterial cellular functions, including motility, biofilm formation, and pathogenesis and symbiosis. Since the annotation of Sinorhizobium meliloti Rm1021 genome suggests that there may be up to 94 putative Tat substrates, we hypothesized that characterizing the twin arginine transport system in this organism might yield unique data that could help in the understanding of twin arginine transport. To initiate this work we attempted a targeted mutagenesis of the tat locus. Despite repeated attempts using a number of different types of media, the attempts at mutation construction were unsuccessful unless the experiment was carried out in a strain that was merodiploid for tatABC. In addition, it was shown that a plasmid carrying tatABC was stable in the absence of antibiotic selection in a tat deletion background. Finally, fluorescence microscopy and live/dead assays of these cultures show a high proportion of dead and irregularly shaped cells, suggesting that the loss of tatABC is inversely correlated with viability. Taken together, the results of this work provide evidence that the twin arginine transport system of S. meliloti appears to be essential for viability under all the conditions that we had tested.Sinorhizobium meliloti is a Gram-negative alphaproteobacterium capable of entering into a symbiotic relationship with leguminous plants such as alfalfa. Within the rhizosphere, rhizobia are capable of sensing flavones or isoflavones secreted by the host plant (4, 46, 57). In response, a cascade of events ensues that leads to the eventual attachment of the bacteria to the plant root, infection thread development, and finally release of the bacteria within the differentiated plant cells of the developing nodule structure (34, 45). It is within this tightly regulated environment that the rhizobia express the genes that encode the proteins required for nitrogen fixation and that result in the reduction of atmospheric N₂ to NH_4. In exchange for the production of nitrogen, the plant provides nutrients for the bacteria to grow and to establish the symbiotic relationship (33, 50).Protein targeting and translocation are important processes for correct cellular function within all living organisms. It is predicted in Escherichia coli that more than 450 proteins are transported across the cytoplasmic membrane (43). The vast majority of these proteins are transported through the general secretory (Sec) system, with a minority being transported by the more recently discovered twin arginine transport (Tat) pathway (43). Proteins that are targeted to the cytoplasmic membrane in Gram-negative bacteria via the Sec system rely on a core set of proteins that include SecA, a protein that has ATPase function, SecYEG, which define the minimum membrane transport apparatus, and in some cases a chaperone protein, SecB (18, 54). The translated protein is carried toward the membrane with help from the chaperone SecB and relayed to the SecYEG apparatus that threads the proteins through the membrane in a linear fashion, with the energy for transport being derived from the hydrolysis of ATP, which is provided by SecA (18).In contrast, the Tat system is believed to transport proteins that have already undergone folding and, in many cases, cofactor insertion (41, 60). In brief, following protein translation, a chaperone may be involved to help transfer the substrate to the TatBC complex, where the TatC component recognizes the twin arginine signal motif, (S/T)RRXFLK (1, 42). The TatBC complex subsequently recruits TatA oligomers that coordinately make up the membrane pore required for transport (8, 29, 31). Using the pH gradient (ΔpH), the Tat substrate protein is transported through the TatA pore in its folded state and integrated into the membrane or transported further to the periplasmic space (3, 39).Approximately 30 proteins are predicted to be transported through the Tat system in E. coli (43). The majority of these appear to be expressed or function anaerobically (43). Interestingly, bioinformatic analysis of S. meliloti and Rhizobium leguminosarum suggests that a much larger number of proteins may use the Tat system in these organisms (36). In addition, these organisms are classified as obligate aerobic organisms (12, 28, 55).Since tat mutations have been shown to affect many bacteria-host interactions (17, 25, 36, 49, 62), we set out to construct a tat mutation in S. meliloti to elucidate the role that tat may have in determining the bacteria''s ability to interact with its host plant and affect nodule development. Moreover, we reasoned that a tat mutation in S. meliloti might help characterize putative Tat substrates in a different model organism. Surprisingly, we were able to construct a tat mutation only in a merodiploid strain that contained the tatABC genes on a plasmid in trans. Using plasmid stability, transduction experiments, and live/dead assays, we show that the tat region in S. meliloti appears to be required for viability and is an essential region of the chromosome. This is the first work to show that Tat is required for viability in a bacterial species.     

Role of Extracellular Polymeric Substances in the Surface Chemical Reactivity of Hymenobacter aerophilus,a Psychrotolerant Bacterium

Bacterial surface layers, such as extracellular polymeric substances (EPS), are known to play an important role in metal sorption and biomineralization; however, there have been very few studies investigating how environmentally induced changes in EPS production affect the cell''s surface chemistry and reactivity. Acid-base titrations, cadmium adsorption assays, and Fourier transform infrared spectroscopy (FT-IR) were used to characterize the surface reactivities of Hymenobacter aerophilus cells with intact EPS (WC) or stripped of EPS (SC) and purified EPS alone. Linear programming modeling of titration data showed SC to possess functional groups corresponding to phosphoryl (pK_a ∼6.5), phosphoryl/amine (pK_a ∼7.9), and amine/hydroxyl (pK_a ∼9.9). EPS and WC both possess carboxyl groups (pK_a ∼5.1 to 5.8) in addition to phosphoryl and amine groups. FT-IR confirmed the presence of polysaccharides and protein in purified EPS that can account for the additional carboxyl groups. An increased ligand density was observed for WC relative to that for SC, leading to an increase in the amount of Cd adsorbed (0.53 to 1.73 mmol/liter per g [dry weight] and 0.53 to 0.59 mmol/liter per g [dry weight], respectively). Overall, the presence of EPS corresponds to an increase in the number and type of functional groups on the surface of H. aerophilus that is reflected by increased metal adsorption relative to that for EPS-free cells.Acid-base titrations are frequently used to characterize microbial cell surface reactivity, in particular, the ability of the cell to adsorb and desorb protons (19, 21, 47). This ability is conferred by the presence of proton-reactive surface functional groups that are also responsible for the surface adsorption of other cations, including dissolved metals. Thus, a microbe''s ability to immobilize metals and influence metal transport is largely dependent on the nature of the reactive sites found at the cell-water interface, namely, their concentrations and chemical affinities (in terms of equilibrium surface stability constants) for cations such as protons and metals.Both Gram-negative and Gram-positive bacteria have been characterized extensively using acid-base titration to determine their reactivity with respect to geochemical processes (18, 22, 33). To date, most work has focused on mesophilic and strictly heterotrophic model organisms; however, some work has also been done with cyanobacteria (29, 44) and thermophiles (19, 47). While proton sorption assays provide information on surface site densities and acidity constants, a more direct assessment of a microbe''s ability to interact with aqueous metals is the metal adsorption assay, where a cell''s ability to adsorb metal ions from solution is measured over a range of pH values. Metal adsorption assays have been used to characterize microbes from a wide variety of environments to determine their potential for bioremediation of heavy metal contamination (21, 26), their influence on geochemical cycling (5, 16), and their ability to serve as nucleation sites for mineral authigenesis (3, 43). Although more than 80% of the Earth''s biosphere is cold (37), to our knowledge there have been no published studies of acid-base surface chemistry for psychrotolerant bacteria, although recent studies examining metal adsorption have been published (53, 54).Extracellular polymeric substances (EPS) are produced by both prokaryotes and eukaryotes in a wide variety of environments (15). Although the relative quantities of EPS components are highly variable, polysaccharides are usually dominant, with proteins and, to a lesser extent, nucleic acids and lipids also present (15). The production of EPS can be important in mediation of environmental interactions, such as adhesion to surfaces and aggregation (35, 49); mineral weathering (28, 51); microbial tolerance of toxic metals through sequestration of metal ions outside the cell (1, 11); and biomineralization (27). Indeed, the stability of metal-surface complexes is great enough to affect metal mobility in many aqueous systems (14), which can, in turn, affect the distribution of metals in the environment (32).The physical and chemical characteristics of EPS have usually been studied using cells with intact EPS or on purified EPS (7, 8, 17, 34, 41, 51). Interestingly, few studies have compared cells with and without surface layers such as EPS (44, 47), despite the fact that EPS and other external layers alter the cell surface presented to the environment, potentially changing both the type and the quantity of functional groups available for environmental interactions. Accordingly, the purpose of this study was to determine the changes in cell surface reactivity resulting from the production of EPS by Hymenobacter aerophilus, a psychrotolerant bacterium. Acid-base titrations and cadmium adsorption assays were used to compare the numbers and types of functional groups on the surfaces of bacterial cells presenting intact EPS and those from which EPS had been removed mechanically.     

Cultivation of Fastidious Bacteria by Viability Staining and Micromanipulation in a Soil Substrate Membrane System

Soil substrate membrane systems allow for microcultivation of fastidious soil bacteria as mixed microbial communities. We isolated established microcolonies from these membranes by using fluorescence viability staining and micromanipulation. This approach facilitated the recovery of diverse, novel isolates, including the recalcitrant bacterium Leifsonia xyli, a plant pathogen that has never been isolated outside the host.The majority of bacterial species have never been recovered in the laboratory (1, 14, 19, 24). In the last decade, novel cultivation approaches have successfully been used to recover “unculturables” from a diverse range of divisions (23, 25, 29). Most strategies have targeted marine environments (4, 23, 25, 32), but soil offers the potential for the investigation of vast numbers of undescribed species (20, 29). Rapid advances have been made toward culturing soil bacteria by reformulating and diluting traditional media, extending incubation times, and using alternative gelling agents (8, 21, 29).The soil substrate membrane system (SSMS) is a diffusion chamber approach that uses extracts from the soil of interest as the growth substrate, thereby mimicking the environment under investigation (12). The SSMS enriches for slow-growing oligophiles, a proportion of which are subsequently capable of growing on complex media (23, 25, 27, 30, 32). However, the SSMS results in mixed microbial communities, with the consequent difficulty in isolation of individual microcolonies for further characterization (10).Micromanipulation has been widely used for the isolation of specific cell morphotypes for downstream applications in molecular diagnostics or proteomics (5, 15). This simple technology offers the opportunity to select established microcolonies of a specific morphotype from the SSMS when combined with fluorescence visualization (3, 11). Here, we have combined the SSMS, fluorescence viability staining, and advanced micromanipulation for targeted isolation of viable, microcolony-forming soil bacteria.