The type and yield of lipopolysaccharide from symbiotically deficient rhizobium lipopolysaccharide mutants vary depending on the extraction method

Soluble CD14 produced by bovine mammary epithelial cells modulates their response to full length LPS

Bovine mastitis remains a major disease in cattle world-wide. In the mammary gland, mammary epithelial cells (MEC) are sentinels equipped with receptors allowing them to detect and respond to the invasion by bacterial pathogens, in particular Escherichia coli. Lipopolysaccharide (LPS) is the major E. coli motif recognized by MEC through its interaction with the TLR4 receptor and the CD14 co-receptor. Previous studies have highlighted the role of soluble CD14 (sCD14) in the efficient recognition of LPS molecules possessing a full-length O-antigen (LPSS). We demonstrate here that MEC are able to secrete CD14 and are likely to contribute to the presence of sCD14 in milk. We then investigated how sCD14 modulates and is required for the response of MEC to LPSS. This study highlights the key role of sCD14 for the full activation of the Myd88-independent pathway by LPSS. We also identified several lncRNA that are activated in MEC in response to LPS, including one lncRNA showing homologies with the mir-99a-let-7c gene (MIR99AHG). Altogether, our results show that a full response to LPS by mammary epithelial cells requires sCD14 and provide detailed information on how milk sCD14 can contribute to an efficient recognition of LPS from coliform pathogens.

Characterisation of a capsular polysaccharide from Moraxella nonliquefaciens CCUG 348T

Moraxella nonliquefaciens is a commensal of the human upper respiratory tract (URT) but on rare occasions is recovered in cases of ocular, septic and pulmonary infections. Hence there is interest in the pathogenic determinants of M. nonliquefaciens, of which outer membrane (OM) structures such as fimbriae and two capsular polysaccharide (CPS) structures, →3)-β-D-GalpNAc-(1→5)-β-Kdop-(2→ and →8)-α-NeuAc-(2→, have been reported in the literature. To further characterise its surface virulence factors, we isolated a novel CPS from M. nonliquefaciens type strain CCUG 348T. This structure was elucidated using NMR data obtained from CPS samples that were subjected to various degrees of mild acid hydrolysis. Together with GLC-MS data, the structure was resolved as a linear polymer composed of two GalfNAc residues consecutively added to Kdo, →3)-β-D-GalfNAc-(1→3)-α-D-GalfNAc-(1→5)-α-(8-OAc)Kdop-(2→. Supporting evidence for this material being CPS was drawn from the proposed CPS biosynthetic locus which encoded a potential GalfNAc transferase, a UDP-GalpNAc mutase for UDP-GalfNAc production and a putative CPS polymerase with predicted GalfNAc and Kdo transferase domains. This study describes a unique CPS composition reported in Moraxella spp. and offers genetic insights into the synthesis and expression of GalfNAc residues, which are rare in bacterial OM glycans.

O26 Polysaccharides as Key Players in Enteropathogenic E. coli Immune Evasion and Vaccine Development

Enteropathogenic Escherichia coli (EPEC) produce a capsule of polysaccharides identical to those composing the O-antigen polysaccharide of its LPS (lipopolysaccharide) molecules. In light of this, the impact of O26 polysaccharides on the immune evasion mechanisms of capsulated O26 EPEC compared to non-capsulated enterohemorrhagic Escherichia coli (EHEC) was investigated. Our findings reveal that there was no significant difference between the levels in EPEC and EHEC of rhamnose (2.8:2.5), a molecule considered to be a PAMP (Pathogen Associated Molecular Patterns). However, the levels of glucose (10:1.69), heptose (3.6:0.89) and N-acetylglucosamine (4.5:2.10), were significantly higher in EPEC than EHEC, respectively. It was also observed that the presence of a capsule in EPEC inhibited the deposition of C3b on the bacterial surface and protected the pathogen against lysis by the complement system. In addition, the presence of a capsule also protected EPEC against phagocytosis by macrophages. However, the immune evasion provided by the capsule was overcome in the presence of anti-O26 polysaccharide antibodies, and additionally, these antibodies were able to inhibit O26 EPEC adhesion to human epithelial cells. Finally, the results indicate that O26 polysaccharides can generate an effective humoral immune response, making them promising antigens for the development of a vaccine against capsulated O26 E. coli.

A Journey from Structure to Function of Bacterial Lipopolysaccharides

Lipopolysaccharide (LPS) is a crucial constituent of the outer membrane of most Gram-negative bacteria, playing a fundamental role in the protection of bacteria from environmental stress factors, in drug resistance, in pathogenesis, and in symbiosis. During the last decades, LPS has been thoroughly dissected, and massive information on this fascinating biomolecule is now available. In this Review, we will give the reader a third millennium update of the current knowledge of LPS with key information on the inherent peculiar carbohydrate chemistry due to often puzzling sugar residues that are uniquely found on it. Then, we will drive the reader through the complex and multifarious immunological outcomes that any given LPS can raise, which is strictly dependent on its chemical structure. Further, we will argue about issues that still remain unresolved and that would represent the immediate future of LPS research. It is critical to address these points to complete our notions on LPS chemistry, functions, and roles, in turn leading to innovative ways to manipulate the processes involving such a still controversial and intriguing biomolecule.

Structural characterisation of the oligosaccharide from Moraxella bovoculi type strain 237 (ATCC BAA-1259) lipooligosaccharide

The Gram-negative bacterium Moraxella bovoculi is associated with infectious bovine keratoconjunctivitis (IBK), colloquially known as 'pink-eye'. IBK is an extremely contagious ocular disease of cattle. We report here the structure of the oligosaccharide derived from the lipooligosaccharide from M. bovoculi type strain 237 (also known as ATCC BAA-1259T). GLC-MS and correlation NMR analysis of the oligosaccharide revealed 5 sugar residues, with a notable central branched 3,4,6-α-D-Glcp. An additional α-D-Manp was present ~30% on the sub-terminal α-D-Manp of the 4-linked branch. This oligosaccharide structure was consistent with other members of the Moraxellaceae where no heptose was present and 5-linked Kdo was directly attached to the central 3,4,6-α-D-Glcp.

Purification and partial characterization of LdtP, a cell envelope modifying enzyme in Liberibacter asiaticus

Background

The aggressive spread of Liberibacter asiaticus, a bacterium closely associated with citrus greening, has given rise to an acute crisis in the citrus industry, making it imperative to expand the scientific knowledge base regarding L. asiaticus. Despite several endeavors to culture L. asiaticus, this bacterium has yet to be maintained in axenic culture, rendering identification and analysis of potential treatment targets challenging. Accordingly, a thorough understanding of biological mechanisms involved in the citrus host-microbe relationship is critical as a means of directing the search for future treatment targets. In this study, we evaluate the biochemical characteristics of CLIBASIA_01175, renamed LdtP (L,D-transpeptidase). Surrogate strains were used to evaluate its potential biological significance in gram-negative bacteria. A strain of E. coli carrying quintuple knock-outs of all genes encoding L,D-transpeptidases was utilized to demonstrate the activity of L. asiaticus LdtP.

Results

This complementation study demonstrated the periplasmic localization of mature LdtP and provided evidence for the biological role of LdtP in peptidoglycan modification. Further investigation highlighted the role of LdtP as a periplasmic esterase involved in modification of the lipid A moiety of the lipopolysaccharide. This work described, for the first time, an enzyme of the L,D-transpeptidase family with moonlighting enzyme activity directed to the modification of the bacterial cell wall and LPS.

Conclusions

Taken together, the data indicates that LdtP is a novel protein involved in an alternative pathway for modification of the bacterial cell, potentially affording L. asiaticus a means to survive within the host.

Electronic supplementary material

The online version of this article (10.1186/s12866-018-1348-8) contains supplementary material, which is available to authorized users.

Sensing of Escherichia coli and LPS by mammary epithelial cells is modulated by O-antigen chain and CD14

Escherichia coli is one of the major pathogens causing mastitis in dairy cattle. Yet, the factors which mediate the ability for E. coli to develop in the bovine mammary gland remain poorly elucidated. In a mouse model, infections induced by the reference mastitis E. coli P4 showed a strong colonisation of the mammary gland, while this strain had a low stimulating power on cells of the PS bovine mammary epithelial cell line. In order to understand if such a reduced response contributes to the severity of infection, a library of random mutants of P4 strain was screened to identify mutants inducing stronger response of PS cells. Among hyper-stimulating mutants, six were altered in genes involved in biosynthesis of lipopolysaccharide (LPS) and had lost their O-polysaccharide region, suggesting that the presence of O-antigen impairs the response of PS cells to LPS. Using purified smooth (S) and rough (R) fractions of LPS, we showed that the R-LPS fraction induced a stronger response from PS cells than the smooth LPS fraction. Biological activity of the S-LPS fraction could be restored by the addition of recombinant bovine CD14 (rbCD14), indicating a crucial role of CD14 in the recognition of S-LPS by Mammary Epithelial Cells (MEC). When S-LPS and R-LPS were injected in udder quarters of healthy lactating cows, an inflammation developed in all infused quarters, but the S-LPS induced a more intense pro-inflammatory response, possibly in relation to sizeable concentrations of CD14 in milk. Altogether, our results demonstrate that the O-antigen modulates the pro-inflammatory response of MEC to LPS, that S-LPS and R-LPS trigger different responses of MEC and that these responses depend on the presence of CD14.

Structure of the Lipopolysaccharide from the Bradyrhizobium sp. ORS285 rfaL Mutant Strain

The importance of the outer membrane and of its main constituent, lipopolysaccharide, in the symbiosis between rhizobia and leguminous host plants has been well studied. Here, the first complete structural characterization of the entire lipopolysaccharide from an O‐chain‐deficient Bradyrhizobium ORS285 rfaL mutant is achieved by a combination of chemical analysis, NMR spectroscopy, MALDI MS and MS/MS. The lipid A structure is shown to be consistent with previously reported Bradyrhizobium lipid A, that is, a heterogeneous blend of penta‐ to hepta‐acylated species carrying a nonstoichiometric hopanoid unit and possessing very‐long‐chain fatty acids ranging from 26:0(25‐OH) to 32:0(31‐OH). The structure of the core oligosaccharide region, fully characterized for the first time here, is revealed to be a nonphosphorylated linear chain with methylated sugar residues, with a heptose residue exclusively present in the outer core region, and with the presence of two singly substituted 3‐deoxy‐d‐manno‐oct‐2‐ulosonic acid (Kdo) residues, one of which is located in the outer core region. The lipid A moiety is linked to the core moiety through an uncommon 4‐substituted Kdo unit.

Structural analysis of lipopolysaccharides from Gram-negative bacteria

This review covers data on composition and structure of lipid A, core, and O-polysaccharide of the known lipopolysaccharides from Gram-negative bacteria. The relationship between the structure and biological activity of lipid A is discussed. The data on roles of core and O-polysaccharide in biological activities of lipopolysaccharides are presented. The structural homology of some oligosaccharide sequences of lipopolysaccharides to gangliosides of human cell membranes is considered.

Structures of the lipopolysaccharides from Rhizobium leguminosarum RBL5523 and its UDP-glucose dehydrogenase mutant (exo5)

Rhizobial lipopolysaccharide (LPS) is required to establish an effective symbiosis with its host plant. An exo5 mutant of Rhizobium leguminosarum RBL5523, strain RBL5808, is defective in UDP-glucose (Glc) dehydrogenase that converts UDP-Glc to UDP-glucuronic acid (GlcA). This mutant is unable to synthesize either UDP-GlcA or UDP-galacturonic acid (GalA) and is unable to synthesize extracellular and capsular polysaccharides, lacks GalA in its LPS and is defective in symbiosis (Laus MC, Logman TJ, van Brussel AAN, Carlson RW, Azadi P, Gao MY, Kijne JW. 2004. Involvement of exo5 in production of surface polysaccharides in Rhizobium leguminosarum and its role in nodulation of Vicia sativa subsp. nigra. J Bacteriol. 186:6617-6625). Here, we determined and compared the structures of the RBL5523 parent and RBL5808 mutant LPSs. The parent LPS core oligosaccharide (OS), as with other R. leguminosarum and Rhizobium etli strains, is a Gal(1)Man(1)GalA(3)Kdo(3) octasaccharide in, which each of the GalA residues is terminally linked. The core OS from the mutant lacks all three GalA residues. Also, the parent lipid A consists of a fatty acylated GlcNGlcNonate or GlcNGlcN disaccharide that has a GalA residue at the 4'-position, typical of other R. leguminosarum and R. etli lipids A. The mutant lipid A lacks the 4'-GalA residue, and the proximal glycosyl residue was only present as GlcNonate. In spite of these alterations to the lipid A and core OSs, the mutant was still able to synthesize an LPS containing a normal O-chain polysaccharide (OPS), but at reduced levels. The structure of the OPS of the mutant LPS was identical to that of the parent and consists of an O-acetylated →4)-α-d-Glcp-(1→3)-α-d-QuipNAc-(1→ repeating unit.

Lipopolysaccharides in Rhizobium-legume symbioses

The establishment of nitrogen-fixing symbiosis between a legume plant and its rhizobial symbiont requires that the bacterium adapt to changing conditions that occur with the host plant that both promotes and allows infection of the host root nodule cell, regulates and resists the host defense response, permits the exchange of metabolites, and contributes to the overall health of the host. This adaptive process involves changes to the bacterial cell surface and, therefore, structural modifications to the lipopolysaccharide (LPS). In this chapter, we describe the structures of the LPSs from symbiont members of the Rhizobiales, the genetics and mechanism of their biosynthesis, the modifications that occur during symbiosis, and their possible functions.

Microbe-associated molecular patterns in innate immunity: Extraction and chemical analysis of gram-negative bacterial lipopolysaccharides

Bacterial lipopolysaccharides (LPSs) are the major component of the outer membrane of Gram-negative bacteria. They have a structural role since they contribute to the cellular rigidity by increasing the strength of cell wall and mediating contacts with the external environment that can induce structural changes to allow life in different conditions. Furthermore, the low permeability of the outer membrane acts as a barrier to protect bacteria from host-derived antimicrobial compounds. They also have a very important role in the elicitation of the animal and plant host innate immunity since they are microbe-associated molecular patterns, namely, they are glycoconjugates produced only by Gram-negative bacteria and are recognized as a molecular hallmark of invading microbes. LPSs are amphiphilic macromolecules generally comprising three defined regions distinguished by their genetics, structures, and function: the lipid A, the core oligosaccharide and a polysaccharide portion, the O-chain. In some Gram-negative bacteria, LPS can terminate with the core portion to form rough-type LPS (R-LPS, LOS). In this chapter, we will describe the isolation of both kinds of LPSs and their full chemical analysis, pivotal operations in the complete description of the primary structure of such important glycoconjugates.

The rkp-1 cluster is required for secretion of Kdo homopolymeric capsular polysaccharide in Sinorhizobium meliloti strain Rm1021

Under conditions of nitrogen stress, leguminous plants form symbioses with soil bacteria called rhizobia. This partnership results in the development of structures called root nodules, in which differentiated endosymbiotic bacteria reduce molecular dinitrogen for the host. The establishment of rhizobium-legume symbioses requires the bacterial synthesis of oligosaccharides, exopolysaccharides, and capsular polysaccharides. Previous studies suggested that the 3-deoxy-D-manno-oct-2-ulopyranosonic acid (Kdo) homopolymeric capsular polysaccharide produced by strain Sinorhizobium meliloti Rm1021 contributes to symbiosis with Medicago sativa under some conditions. However, a conclusive symbiotic role for this polysaccharide could not be determined due to a lack of mutants affecting its synthesis. In this study, we have further characterized the synthesis, secretion, and symbiotic function of the Kdo homopolymeric capsule. We showed that mutants lacking the enigmatic rkp-1 gene cluster fail to display the Kdo capsule on the cell surface but accumulate an intracellular polysaccharide of unusually high M(r). In addition, we have demonstrated that mutations in kdsB2, smb20804, and smb20805 affect the polymerization of the Kdo homopolymeric capsule. Our studies also suggest a role for the capsular polysaccharide in symbiosis. Previous reports have shown that the overexpression of rkpZ from strain Rm41 allows for the symbiosis of exoY mutants of Rm1021 that are unable to produce the exopolysaccharide succinoglycan. Our results demonstrate that mutations in the rkp-1 cluster prevent this phenotypic suppression of exoY mutants, although mutations in kdsB2, smb20804, and smb20805 have no effect.

Structure of compositionally simple lipopolysaccharide from marine synechococcus

Lipopolysaccharide (LPS) is the first defense against changing environmental factors for many bacteria. Here, we report the first structure of the LPS from cyanobacteria based on two strains of marine Synechococcus, WH8102 and CC9311. While enteric LPS contains some of the most complex carbohydrate residues in nature, the full-length versions of these cyanobacterial LPSs have neither heptose nor 3-deoxy-D-manno-octulosonic acid (Kdo) but instead 4-linked glucose as their main saccharide component, with low levels of glucosamine and galacturonic acid also present. Matrix-assisted laser desorption ionization mass spectrometry of the intact minimal core LPS reveals triacylated and tetraacylated structures having a heterogeneous mix of both hydroxylated and nonhydroxylated fatty acids connected to the diglucosamine backbone and a predominantly glucose outer core-like region for both strains. WH8102 incorporated rhamnose in this region as well, contributing to differences in sugar composition and possibly nutritional differences between the strains. In contrast to enteric lipid A, which can be liberated from LPS by mild acid hydrolysis, lipid A from these organisms could be produced by only two novel procedures: triethylamine-assisted periodate oxidation and acetolysis. The lipid A contains odd-chain hydroxylated fatty acids, lacks phosphate, and contains a single galacturonic acid. The LPS lacks any limulus amoebocyte lysate gelation activity. The highly simplified nature of LPSs from these organisms leads us to believe that they may represent either a primordial structure or an adaptation to the relatively higher salt and potentially growth-limiting phosphate levels in marine environments.

Comparison of three GC/MS methodologies for the analysis of fatty acids in Sinorhizobium meliloti: development of a micro-scale, one-vial method

Three protocols for fatty acid analysis in Sinorhizobium meliloti were improved by the addition of a number of standards/controls and a silylation step which allowed the determination of recoveries, extents of conversion of lipids to fatty acid methyl esters (FAMEs) and extents of side reactions. Basic hydrolysis followed by acid-catalyzed methylation and transmethylation with sodium methoxide, were the best for the analysis of 3-hydroxy- and cyclopropane fatty acids, respectively. A micro-scale, one-vial method that employed sodium methoxide/methanol was equally efficient and on a 1000-fold smaller scale than standard methods. Because this method avoids aqueous extractions, 3-hydroxybutanoic acid was detected as its trimethylsilyloxy methyl ester along with FAMEs.

The Sinorhizobium meliloti MsbA2 protein is essential for the legume symbiosis

Sinorhizobium meliloti is a beneficial legume symbiont, closely related to Brucella species, which are chronic mammalian pathogens. We discovered that the S. meliloti MsbA2 protein is essential to ensure the symbiotic interaction with the host plant, alfalfa. S. meliloti invades plant cells via plant-derived structures known as infection threads. However, in the absence of MsbA2, S. meliloti remains trapped within abnormally thickened infection threads and induces a heightened plant defence response, characterized by a substantial thickening of the nodule endodermis layer and the accumulation of polyphenolic compounds. The S. meliloti MsbA2 protein is homologous to the Escherichia coli lipopolysaccharide/phospholipid trafficking protein MsbA. However, MsbA2 was not essential for the membrane transport of either lipopolysaccharide or phospholipids in S. meliloti. We determined that the msbA2 gene is transcribed in free-living S. meliloti and that in the absence of MsbA2 the polysaccharide content of S. meliloti is altered. Consequently, we propose a model whereby the altered polysaccharide content of the S. meliloti msbA2 mutant could be responsible for its symbiotic defect by inducing an inappropriate host response.

Rhizobium etli CE3 bacteroid lipopolysaccharides are structurally similar but not identical to those produced by cultured CE3 bacteria

Rhizobium etli CE3 bacteroids were isolated from Phaseolus vulgaris root nodules. The lipopolysaccharide (LPS) from the bacteroids was purified and compared with the LPS from laboratory-cultured R. etli CE3 and from cultures grown in the presence of anthocyanin. Comparisons were made of the O-chain polysaccharide, the core oligosaccharide, and the lipid A. Although LPS from CE3 bacteria and bacteroids are structurally similar, it was found that bacteroid LPS had specific modifications to both the O-chain polysaccharide and lipid A portions of their LPS. Cultures grown with anthocyanin contained modifications only to the O-chain polysaccharide. The changes to the O-chain polysaccharide consisted of the addition of a single methyl group to the 2-position of a fucosyl residue in one of the five O-chain trisaccharide repeat units. This same change occurred for bacteria grown in the presence of anthocyanin. This methylation change correlated with the inability of bacteroid LPS and LPS from anthocyanin-containing cultures to bind the monoclonal antibody JIM28. The core oligosaccharide region of bacteroid LPS and from anthocyanin-grown cultures was identical to that of LPS from normal laboratory-cultured CE3. The lipid A from bacteroids consisted exclusively of a tetraacylated species compared with the presence of both tetra- and pentaacylated lipid A from laboratory cultures. Growth in the presence of anthocyanin did not affect the lipid A structure. Purified bacteroids that could resume growth were also found to be more sensitive to the cationic peptides, poly-l-lysine, polymyxin-B, and melittin.

Involvement of exo5 in production of surface polysaccharides in Rhizobium leguminosarum and its role in nodulation of Vicia sativa subsp. nigra

Analysis of two exopolysaccharide-deficient mutants of Rhizobium leguminosarum, RBL5808 and RBL5812, revealed independent Tn5 transposon integrations in a single gene, designated exo5. As judged from structural and functional homology, this gene encodes a UDP-glucose dehydrogenase responsible for the oxidation of UDP-glucose to UDP-glucuronic acid. A mutation in exo5 affects all glucuronic acid-containing polysaccharides and, consequently, all galacturonic acid-containing polysaccharides. Exo5-deficient rhizobia do not produce extracellular polysaccharide (EPS) or capsular polysaccharide (CPS), both of which contain glucuronic acid. Carbohydrate composition analysis and nuclear magnetic resonance studies demonstrated that EPS and CPS from the parent strain have very similar structures. Lipopolysaccharide (LPS) molecules produced by the mutant strains are deficient in galacturonic acid, which is normally present in the core and lipid A portions of the LPS. The sensitivity of exo5 mutant rhizobia to hydrophobic compounds shows the involvement of the galacturonic acid residues in the outer membrane structure. Nodulation studies with Vicia sativa subsp. nigra showed that exo5 mutant rhizobia are impaired in successful infection thread colonization. This is caused by strong agglutination of EPS-deficient bacteria in the root hair curl. Root infection could be restored by simultaneous inoculation with a Nod factor-defective strain which retained the ability to produce EPS and CPS. However, in this case colonization of the nodule tissue was impaired.

Disruption of dTDP-rhamnose biosynthesis modifies lipopolysaccharide core, exopolysaccharide production, and root colonization in Azospirillum brasilense

The interaction between Azospirillum brasilense and plants is not fully understood, although several bacterial surface components like exopolysaccharides (EPS), flagella, and capsular polysaccharides are required for attachment and colonization. While in other plant-bacteria associations (Rhizobium-legume, Pseudomonas-potato), lipopolysaccharides (LPS) play a key role in the establishment of an effective association, their role in the root colonization by Azospirillum had not been determined. In this study, we isolated a Tn5 mutant of A. brasilense Cd (EJ1) with an apparently modified LPS core structure, non-mucoid colony morphology, increased EPS production, and affected in maize root colonization. A 3790-bp region revealed the presence of three complete open reading frames designated rmlC, rmlB and rmlD. The beginning of a fourth open reading frame was found and designated rmlA. These genes are organized in a cluster which shows homology to the cluster involved in the synthesis of dTDP-rhamnose in other bacteria. Additionally, the analysis of the monosaccharide composition of LPSs showed a diminution of rhamnose compared to the wild-type strain.

Extraction of cyanobacterial endotoxin

To simplify our efforts in acquiring toxicological information on endotoxins produced by cyanobacteria, a method development study was undertaken to identify relatively hazard-free and efficient procedures for their extraction. One article sourced and two novel methods were evaluated for their ability to extract lipopolysaccharides (LPSs) or endotoxins from cyanobacteria. The Limulus polyphemus amoebocyte lysate (LAL) assay was employed to compare the performance of a novel method utilizing a 1-butanol-water (HBW) solvent system to that of Westphal's (1965) phenol-water system (HPW) for the extraction of endotoxin from various cyanobacteria. The traditional HPW method extracted from 3- to 12-fold more endotoxin from six different cyanobacterial blooms and culture materials than did the novel HBW method. In direct contrast, the novel HBW method extracted ninefold more endotoxin from a non-microcystin producing Microcystis aeruginosa culture as compared to the HPW method. A solvent system utilizing N,N'-dimethylformamide-water (HDW) was compared to both the HPW and HBW methods for the extraction of endotoxin from natural samples of Anabaena circinalis, Microcystis flos-aquae, and a 1:1 mixture of Microcystis aeruginosa/Microcystisflos-aquae. The LAL activities of these extracts showed that the novel HDW method extracted two- and threefold more endotoxin from the Anabaena sample that did the HBW and HPW methods, respectively. The HDW method also extracted approximately 1.5-fold more endotoxin from the Microcystis flos-aquae sample as compared to both the HBW and HPW methods. On the other hand, the HBW method extracted 2- and 14-fold more endotoxin from the Microcystis flos-aquae/Microcystis aeruginosa mixture than did the HPW and HDW methods, respectively. Results of this study demonstrate that significant disparities exist between the physicochemical properties of the cell wall constituents not only of different cyanobacterial species but also of different strains of the same cyanobacterial species, as showing by the varying effectiveness of the solvent systems investigated. Therefore, a sole method cannot be regarded as universal and superior for the extraction of endotoxins from cyanobacteria. Nevertheless, the ability of the novel HBW and HDW methods to utilize easily handled organic solvents that are less hazardous than phenol render them attractive alternatives to the standard HPW method.

Sinorhizobium meliloti acpXL mutant lacks the C28 hydroxylated fatty acid moiety of lipid A and does not express a slow migrating form of lipopolysaccharide

Lipid A is the hydrophobic anchor of lipopolysaccharide (LPS) in the outer membrane of Gram-negative bacteria. Lipid A of all Rhizobiaceae is acylated with a long fatty acid chain, 27-hydroxyoctacosanoic acid. Biosynthesis of this long acyl substitution requires a special acyl carrier protein, AcpXL, which serves as a donor of C28 (omega-1)-hydroxylated fatty acid for acylation of rhizobial lipid A (Brozek, K.A., Carlson, R.W., and Raetz, C. R. (1996) J. Biol. Chem. 271, 32126-32136). To determine the biological function of the C28 acylation of lipid A, we constructed an acpXL mutant of Sinorhizobium meliloti strain 1021. Gas-liquid chromatography and mass spectrometry analysis of the fatty acid composition showed that the acpXL mutation indeed blocked C28 acylation of lipid A. SDS-PAGE analysis of acpXL mutant LPS revealed only a fast migrating band, rough LPS, whereas the parental strain 1021 manifested both rough and smooth LPS. Regardless of this, the LPS of parental and mutant strains had a similar sugar composition and exposed the same antigenic epitopes, implying that different electrophoretic profiles might account for different aggregation properties of LPS molecules with and without a long acyl chain. The acpXL mutant of strain 1021 displayed sensitivity to deoxycholate, delayed nodulation of Medicago sativa, and a reduced competitive ability. However, nodules elicited by this mutant on roots of M. sativa and Medicago truncatula had a normal morphology and fixed nitrogen. Thus, the C28 fatty acid moiety of lipid A is not crucial, but it is beneficial for establishing an effective symbiosis with host plants. acpXL lies upstream from a cluster of five genes, including msbB (lpxXL), which might be also involved in biosynthesis and transfer of the C28 fatty acid to the lipid A precursor.

A Rhizobium leguminosarum AcpXL mutant produces lipopolysaccharide lacking 27-hydroxyoctacosanoic acid

The structure of the lipid A from Rhizobium etli and Rhizobium leguminosarum lipopolysaccharides (LPSs) lacks phosphate and contains a galacturonosyl residue at its 4' position, an acylated 2-aminogluconate in place of the proximal glucosamine, and a very long chain omega-1 hydroxy fatty acid, 27-hydroxyoctacosanoic acid (27OHC28:0). The 27OHC28:0 moiety is common in lipid A's among members of the Rhizobiaceae and also among a number of the facultative intracellular pathogens that form chronic infections, e.g., Brucella abortus, Bartonella henselae, and Legionella pneumophila. In this paper, a mutant of R. leguminosarum was created by placing a kanamycin resistance cassette within acpXL, the gene which encodes the acyl carrier protein for 27OHC28:0. The result was an LPS containing a tetraacylated lipid A lacking 27OHC28:0. A small amount of the mutant lipid A may contain an added palmitic acid residue. The mutant is sensitive to changes in osmolarity and an increase in acidity, growth conditions that likely occur in the nodule microenvironment. In spite of the probably hostile microenvironment of the nodule, the acpXL mutant is still able to form nitrogen-fixing root nodules even though the appearance and development of nodules are delayed. Therefore, it is possible that the acpXL mutant has a host-inducible mechanism which enables it to adapt to these physiological changes.

Deficiency of a Sinorhizobium meliloti BacA mutant in alfalfa symbiosis correlates with alteration of the cell envelope

The BacA protein is essential for the long-term survival of Sinorhizobium meliloti and Brucella abortus within acidic compartments in plant and animal cells, respectively. Since both the S. meliloti and B. abortus bacA mutants have an increased resistance to bleomycin, it was hypothesized that BacA was a transporter of bleomycin and bleomycin-like compounds into the bacterial cell. However, our finding that the S. meliloti bacA mutant also has an increased sensitivity to detergents, a hydrophobic dye, ethanol, and acid pH supported a model in which BacA function affects the bacterial cell envelope. In addition, an S. meliloti lpsB mutant that is defective at a stage in infection of the host similar to that found for a bacA mutant is also sensitive to the same agents, and the carbohydrate content of its lipopolysaccharide (LPS) is altered. However, analysis of crude preparations of the bacA mutant LPS suggested that, unlike that for LpsB, BacA function did not affect the carbohydrate composition of the LPS. Rather, we found that at least one function of BacA is to affect the distribution of LPS fatty acids, including a very-long-chain fatty acid thought to be unique to the alpha-proteobacteria, including B. abortus.

Identification of an ATP-binding cassette transporter for export of the O-antigen across the inner membrane in Rhizobium etli based on the genetic, functional, and structural analysis of an lps mutant deficient in O-antigen

For O-antigen lipopolysaccharide (LPS) synthesis in bacteria, transmembrane migration of undecaprenyl pyrophosphate-bound O-antigen oligosaccharide subunits or polysaccharide occurs before ligation to the core region of the LPS molecule. In this study, we identified by mutagenesis an ATP-binding cassette transporter in Rhizobium etli CE3 that is likely responsible for the translocation of the O-antigen across the inner plasma membrane. Mutant FAJ1200 LPS lacks largely the O-antigen, as shown by SDS-polyacrylamide gel electrophoresis and confirmed by immunoblot analysis. Furthermore, LPS isolated from FAJ1200 is totally devoid of any O-chain glycosyl residues and contains only those glycosyl residues that can be expected for the inner core region. The membrane component and the cytoplasmic ATP-binding component of the ATP-binding cassette transporter are encoded by wzm and wzt, respectively. The Tn5 transposon in mutant FAJ1200 is inserted in the wzm gene. This mutation resulted in an Inf- phenotype in bean plants.