Molecular cloning and characterization of a locus responsible for O acetylation of the O polysaccharide of Legionella pneumophila serogroup 1 lipopolysaccharide

A novel fold for acyltransferase-3 (AT3) proteins provides a framework for transmembrane acyl-group transfer

Acylation of diverse carbohydrates occurs across all domains of life and can be catalysed by proteins with a membrane bound acyltransferase-3 (AT3) domain (PF01757). In bacteria, these proteins are essential in processes including symbiosis, resistance to viruses and antimicrobials, and biosynthesis of antibiotics, yet their structure and mechanism are largely unknown. In this study, evolutionary co-variance analysis was used to build a computational model of the structure of a bacterial O-antigen modifying acetyltransferase, OafB. The resulting structure exhibited a novel fold for the AT3 domain, which molecular dynamics simulations demonstrated is stable in the membrane. The AT3 domain contains 10 transmembrane helices arranged to form a large cytoplasmic cavity lined by residues known to be essential for function. Further molecular dynamics simulations support a model where the acyl-coA donor spans the membrane through accessing a pore created by movement of an important loop capping the inner cavity, enabling OafB to present the acetyl group close to the likely catalytic resides on the extracytoplasmic surface. Limited but important interactions with the fused SGNH domain in OafB are identified, and modelling suggests this domain is mobile and can both accept acyl-groups from the AT3 and then reach beyond the membrane to reach acceptor substrates. Together this new general model of AT3 function provides a framework for the development of inhibitors that could abrogate critical functions of bacterial pathogens.

Diverse functions for acyltransferase-3 proteins in the modification of bacterial cell surfaces

The acylation of sugars, most commonly via acetylation, is a widely used mechanism in bacteria that uses a simple chemical modification to confer useful traits. For structures like lipopolysaccharide, capsule and peptidoglycan, that function outside of the cytoplasm, their acylation during export or post-synthesis requires transport of an activated acyl group across the membrane. In bacteria this function is most commonly linked to a family of integral membrane proteins – acyltransferase-3 (AT3). Numerous studies examining production of diverse extracytoplasmic sugar-containing structures have identified roles for these proteins in O-acylation. Many of the phenotypes conferred by the action of AT3 proteins influence host colonisation and environmental survival, as well as controlling the properties of biotechnologically important polysaccharides and the modification of antibiotics and antitumour drugs by Actinobacteria. Herein we present the first systematic review, to our knowledge, of the functions of bacterial AT3 proteins, revealing an important protein family involved in a plethora of systems of importance to bacterial function that is still relatively poorly understood at the mechanistic level. By defining and comparing this set of functions we draw out common themes in the structure and mechanism of this fascinating family of membrane-bound enzymes, which, due to their role in host colonisation in many pathogens, could offer novel targets for the development of antimicrobials.

The Role of Lipids in Legionella-Host Interaction

Legionella are Gram-stain-negative rods associated with water environments: either natural or man-made systems. The inhalation of aerosols containing Legionella bacteria leads to the development of a severe pneumonia termed Legionnaires' disease. To establish an infection, these bacteria adapt to growth in the hostile environment of the host through the unusual structures of macromolecules that build the cell surface. The outer membrane of the cell envelope is a lipid bilayer with an asymmetric composition mostly of phospholipids in the inner leaflet and lipopolysaccharides (LPS) in the outer leaflet. The major membrane-forming phospholipid of Legionella spp. is phosphatidylcholine (PC)-a typical eukaryotic glycerophospholipid. PC synthesis in Legionella cells occurs via two independent pathways: the N-methylation (Pmt) pathway and the Pcs pathway. The utilisation of exogenous choline by Legionella spp. leads to changes in the composition of lipids and proteins, which influences the physicochemical properties of the cell surface. This phenotypic plasticity of the Legionella cell envelope determines the mode of interaction with the macrophages, which results in a decrease in the production of proinflammatory cytokines and modulates the interaction with antimicrobial peptides and proteins. The surface-exposed O-chain of Legionella pneumophila sg1 LPS consisting of a homopolymer of 5-acetamidino-7-acetamido-8-O-acetyl-3,5,7,9-tetradeoxy-l-glycero-d-galacto-non-2-ulosonic acid is probably the first component in contact with the host cell that anchors the bacteria in the host membrane. Unusual in terms of the structure and function of individual LPS regions, it makes an important contribution to the antigenicity and pathogenicity of Legionella bacteria.

Acetylation of Surface Carbohydrates in Bacterial Pathogens Requires Coordinated Action of a Two-Domain Membrane-Bound Acyltransferase

Acyltransferase-3 (AT3) domain-containing membrane proteins are involved in O-acetylation of a diverse range of carbohydrates across all domains of life. In bacteria they are essential in processes including symbiosis, resistance to antimicrobials, and biosynthesis of antibiotics. Their mechanism of action, however, is poorly characterized. We analyzed two acetyltransferases as models for this important family of membrane proteins, which modify carbohydrates on the surface of the pathogen Salmonella enterica, affecting immunogenicity, virulence, and bacteriophage resistance. We show that when these AT3 domains are fused to a periplasmic partner domain, both domains are required for substrate acetylation. The data show conserved elements in the AT3 domain and unique structural features of the periplasmic domain. Our data provide a working model to probe the mechanism and function of the diverse and important members of the widespread AT3 protein family, which are required for biologically significant modifications of cell-surface carbohydrates.

Membrane bound acyltransferase-3 (AT3) domain-containing proteins are implicated in a wide range of carbohydrate O-acyl modifications, but their mechanism of action is largely unknown. O-antigen acetylation by AT3 domain-containing acetyltransferases of Salmonella spp. can generate a specific immune response upon infection and can influence bacteriophage interactions. This study integrates in situ and in vitro functional analyses of two of these proteins, OafA and OafB (formerly F2GtrC), which display an “AT3-SGNH fused” domain architecture, where an integral membrane AT3 domain is fused to an extracytoplasmic SGNH domain. An in silico-inspired mutagenesis approach of the AT3 domain identified seven residues which are fundamental for the mechanism of action of OafA, with a particularly conserved motif in TMH1 indicating a potential acyl donor interaction site. Genetic and in vitro evidence demonstrate that the SGNH domain is both necessary and sufficient for lipopolysaccharide acetylation. The structure of the periplasmic SGNH domain of OafB identified features not previously reported for SGNH proteins. In particular, the periplasmic portion of the interdomain linking region is structured. Significantly, this region constrains acceptor substrate specificity, apparently by limiting access to the active site. Coevolution analysis of the two domains suggests possible interdomain interactions. Combining these data, we propose a refined model of the AT3-SGNH proteins, with structurally constrained orientations of the two domains. These findings enhance our understanding of how cells can transfer acyl groups from the cytoplasm to specific extracellular carbohydrates.

The Role of Legionella pneumophila Serogroup 1 Lipopolysaccharide in Host-Pathogen Interaction

The Legionella pneumophila TF3/1 mutant of the Corby strain, which possesses a point mutation in the active site of the O-acetyltransferase, synthesized the polysaccharide chain with a reduced degree of substitution with O-acetyl groups. The mutant did not produce a high-molecular-weight lipopolysaccharide (LPS) fraction above 12 kDa. The disturbances in LPS synthesis have an effect on the composition of other macromolecules (lipids and proteins), as indicated by differences in the infrared absorption spectra between the L. pneumophila Corby strain and its TF3/1 mutant. The wild-type strain contained less N⁺-CH₃ and C-N groups as well as more CH₃ groups than the mutant. The fatty acid composition showed that the wild type strain synthesized more branched acyl residues (a15:0, i16:0, and a17:0), a less unsaturated acid (16:1), and a straight-chain acid (18:0) than the mutant. The mutant synthesized approximately twice more a long-chain fatty acid (20:0) than the wild type. The main differences in the phospholipids between both strains were found in the classes of phosphatidylcholines and phosphatidylglycerols (PG). Substantial differences in the cell surface topography of these bacteria and their nanomechanical properties were shown by atomic force microscopy (AFM). The wild type strain had no undulated surface and produced numerous vesicles. In the case of the mutant type, the vesicles were not numerous, but there were grooves on the cell surface. The average roughness of the cell surface of the mutant was approximately twofold higher than in the wild-type strain. In turn, the wild-type strain exhibited much better adhesive properties than the mutant. The kinetic study of the interaction between the L. pneumophila strains and Acanthamoeba castellanii monitored by Förster resonance energy transfer revealed a pronounced difference, i.e., almost instantaneous and highly efficient binding of the L. pneumophila Corby strain to the amoeba surface, followed by penetration into the amoeba cells. This process was clearly not as efficient in the case of the mutant. The results point to LPS and, in particular, to the length of the polysaccharide fraction as an important L. pneumophila determinant involved in the process of adhesion to the host cell.

Distribution of lag-1 Alleles, ORF7, and ORF8 Genes of Lipopolysaccharide and Sequence-Based Types Among Legionella pneumophila Serogroup 1 Isolates in Japan and China

Approximately 85% of cases of Legionnaires' disease are caused by Legionella pneumophila serogroup 1. In this study, we analyzed the distribution of lag-1 alleles, ORF 7 and ORF 8 genes of lipopolysaccharide (LPS) and sequence-based types of 616 L. pneumophila serogroup 1 strains isolated in Japan (206 clinical, 225 environmental) and China (13 clinical and 172 environmental). The lag-1 gene was harbored by significantly more of the clinical isolates compared with the environmental isolates (90.3 vs. 19.1% and 61.6 vs. 3.0%, respectively; both P < 0.001). ORF 7 genes were detected in 51.0% of Japanese clinical and 36.0% of Japanese environmental (P = 0.001) isolates, as well as 15.3% of Chinese clinical and 9.9% of Chinese environmental isolates (P = 0.544). ORF 8 genes were detected in 12.1% of Japanese clinical and 5.8% of Japanese environmental (P = 0.017) isolates, as well as 7.7% of Chinese clinical and 3.4% of Chinese environmental isolates (P = 0.388). The Japanese and Chinese isolates were assigned to 203 and 36 different sequence-types (ST), respectively. ST1 was predominant. Most isolates with the same ST also had the same lag-1, ORF 7, and ORF 8 gene subgroups. In conclusion, the lag-1 was present in most of the clinical isolates, but was absent from most of the environmental isolates from both China and Japan, regardless of the water source and SBT type. PCR-based serotyping and subgrouping methods can be used to define a hierarchy of virulence genotypes that require stringent surveillance to prevent human disease.

Genomic heterogeneity differentiates clinical and environmental subgroups of Legionella pneumophila sequence type 1

Legionella spp. are the cause of a severe bacterial pneumonia known as Legionnaires' disease (LD). In some cases, current genetic subtyping methods cannot resolve LD outbreaks caused by common, potentially endemic L. pneumophila (Lp) sequence types (ST), which complicates laboratory investigations and environmental source attribution. In the United States (US), ST1 is the most prevalent clinical and environmental Lp sequence type. In order to characterize the ST1 population, we sequenced 289 outbreak and non-outbreak associated clinical and environmental ST1 and ST1-variant Lp strains from the US and, together with international isolate sequences, explored their genetic and geographic diversity. The ST1 population was highly conserved at the nucleotide level; 98% of core nucleotide positions were invariant and environmental isolates unassociated with human disease (n = 99) contained ~65% more nucleotide diversity compared to clinical-sporadic (n = 139) or outbreak-associated (n = 28) ST1 subgroups. The accessory pangenome of environmental isolates was also ~30-60% larger than other subgroups and was enriched for transposition and conjugative transfer-associated elements. Up to ~10% of US ST1 genetic variation could be explained by geographic origin, but considerable genetic conservation existed among strains isolated from geographically distant states and from different decades. These findings provide new insight into the ST1 population structure and establish a foundation for interpreting genetic relationships among ST1 strains; these data may also inform future analyses for improved outbreak investigations.

Current and emerging Legionella diagnostics for laboratory and outbreak investigations

Legionnaires' disease (LD) is an often severe and potentially fatal form of bacterial pneumonia caused by an extensive list of Legionella species. These ubiquitous freshwater and soil inhabitants cause human respiratory disease when amplified in man-made water or cooling systems and their aerosols expose a susceptible population. Treatment of sporadic cases and rapid control of LD outbreaks benefit from swift diagnosis in concert with discriminatory bacterial typing for immediate epidemiological responses. Traditional culture and serology were instrumental in describing disease incidence early in its history; currently, diagnosis of LD relies almost solely on the urinary antigen test, which captures only the dominant species and serogroup, Legionella pneumophila serogroup 1 (Lp1). This has created a diagnostic "blind spot" for LD caused by non-Lp1 strains. This review focuses on historic, current, and emerging technologies that hold promise for increasing LD diagnostic efficiency and detection rates as part of a coherent testing regimen. The importance of cooperation between epidemiologists and laboratorians for a rapid outbreak response is also illustrated in field investigations conducted by the CDC with state and local authorities. Finally, challenges facing health care professionals, building managers, and the public health community in combating LD are highlighted, and potential solutions are discussed.

A structural comparison of lipopolysaccharide biosynthesis loci of Legionella pneumophila serogroup 1 strains

Background

The lipopolysaccharide (LPS) is the major immuno-dominant antigen of all Legionella species including L. pneumophila. Its diversity is the basis for the classification of L. pneumophila into serogroups and monoclonal subgroups and is thought to be involved in strain specific virulence. The understanding of the genetic basis of the LPS-antigen is incomplete. Thus, we analyzed the genetic locus involved in LPS-biosynthesis of L. pneumophila serogroup 1 (Sg1) strains with the focus on strain specific gene composition.

Results

The LPS-biosynthesis loci of 14 L. pneumophila Sg1 strains comprise two distinct regions: A 15 kb region containing LPS-biosynthesis genes that can be found in all L. pneumophila strains and a Sg1-specific 18 kb region. The 15 kb region is highly conserved among Sg1 strains as reflected by high homologies of single ORFs and by a consistent ORF arrangement. In contrast, the Sg1 specific 18 kb region is variable and partially disrupted by phage related genes. We propose that the region spanning from ORF 6 to ORF 11 of the Sg1-specific region is likely involved in late LPS-modification. Due to the high variability of this small region and various combinations of single ORFs within this region a strain specific LPS-structure could be synthesized including modifications of legionaminic acid derivates.

Conclusions

Our data clearly demonstrate that the gene structure of the LPS-biosynthesis locus of L. pneumophila Sg1 strains show significant interstrain variability. These data can be used for further functional analysis of the LPS synthesis to understand pathogenesis and reactivity with monoclonal antibodies. Moreover, variable but strain specific regions can serve as basis for the development of novel genotyping assays.

A mutation in the Xanthomonas oryzae pv. oryzae wxoD gene affects xanthan production and chemotaxis

Xanthomonas oryzae pv. oryzae causes bacterial blight in rice (Oryza sativa L.). The effect of a mutation in the wxoD gene, that encodes a putative O-antigen acetylase, on xanthan production as well as bacterial chemotaxis was investigated. The mutation increased xanthan production by 52 %. The mutant strain was non-motile on semi-solid agar swarm plates. In addition, several genes involved in chemotaxis, including the cheW, cheV, cheR, and cheD genes, were down-regulated by a mutation in the wxoD gene. Thus, the mutation in the wxoD gene affects xanthan production as well as bacterial chemotaxis. However, the wxoD gene is not essential for the virulence of X. oryzae.

Characterization of legionella lipopolysaccharide

The lipopolysaccharide(LPS) of Legionella spp. is an immuno-dominant antigen and the basis for Legionella pneumophila serogroup classification. The LPS shows a peculiar structure composed of a very hydrophobic lipid A acylated by long chain fatty acids and an O-antigen-specific chain consisting of homopolymeric legionaminic acid. In this chapter we describe a method for the isolation of LPS from L. pneumophila. In the first part we describe the chemical purification, in the second part we outline the application of monoclonal antibody (mAb) in Western blot and immuno-localization by indirect immunofluorescence. This report does not describe physico-chemical methods that analyze the structure of lipopolysaccharide entities.

Lipopolysaccharide of Legionella pneumophila shed in a liquid culture as a nonvesicular fraction arrests phagosome maturation in amoeba and monocytic host cells

After uptake by susceptible host cells, Legionella pneumophila displays the ability to arrest phagolysosome fusion. To elucidate the role of lipopolysaccharide (LPS) in this mechanism, we investigated its influence on Acanthamoeba castellanii, A/J mouse macrophages and human monocytes. For this, legionellae were cultured in broth to the replicative, noninfectious phase or to the infectious phase expressing virulence traits. Shed LPS-enriched outer membrane vesicles (OMV) and LPS species <300 kDa were obtained from L. pneumophila Corby strains possessing the virulence-associated LPS epitope recognized by monoclonal antibody (MAb) 3/1 and its mutant TF 3/1, which has lost this epitope due to a mutation in the lag-1 gene. The shed LPS components were attached by specific antibodies to latex beads and added to the host cells for phagocytosis. We demonstrated for the first time that evasion of lysosomal degradation of phagosomes for up to 5 h can also be set off by LPS that is not tied up in OMV. Moreover, our cell culture models showed that the influence of MAb 3/1-positive and -negative LPS was identical. Our data clearly substantiate that LPS is an independent factor for evading lysosomal degradation, which is independent of the bacterial expression of known virulence traits.

Impact of the virulence-associated MAb3/1 epitope on the physicochemical surface properties of Legionella pneumophila sg1: An issue to explain infection potential?

The relationship between the presence/absence of the virulence-associated MAb3/1 epitope of sixteen Legionella pneumophila serogroup 1 strains and their respective surface physicochemical properties is evidenced from electrokinetic measurements (microelectrophoresis) performed as a function of KNO(3) electrolyte concentration (range 1-100mM, pH∼6.5). Among the bacteria selected, nine original strains constitute the Dresden reference panel and differ according to the presence/absence of the virulence-associated monoclonal antibody MAb3/1 of the O-specific chain of the lipopolysaccharides (LPS). Five isogenic Lens strains, also investigated in the current study, present the epitope MAb3/1 of their LPS and were involved to some extent in the outbreak that stroke the Nord Pas-de-Calais region (France) in 2004. All bacteria exhibit the typical electrokinetic features of soft (permeable) particles. On the basis of Ohshima's model, analysis of the electrophoretic mobility data allows evaluating the intraparticular flow penetration length 1/λ(0) and the (negative) volume charge density ρ(0) that both reflect the structure and chemical composition of the soft bacterial component. Our results show that the virulent MAb3/1 positive strains are characterized on average by 1/λ(0) and ǀρ(0)ǀ values that are about 1.5 times larger and 5 times lower, respectively, than those derived for lesser virulent (MAb3/1 negative) strains. In other words, on average the soft surface layer of MAb3/1 positive strains is significantly less charged and more permeable than those of MAb3/1 negative strains. The intimate correlation between virulence-associated MAb3/1 epitope and charge density carried by the bacterial envelop was further confirmed by lower 1/λ(0) and greater ǀρ(0)ǀ values for lag-1 mutant CS332 strain, lacking the MAb3/1 epitope, compared to the parental strain AM511. A closer inspection of the dispersion in 1/λ(0) and ǀρ(0)ǀ data over the ensemble of analysed bacteria together with the reported number of Legionnaires' disease cases they are responsible for, points out the charge density ǀρ(0)ǀ as the parameter that is most suitable for discriminating highly virulent (MAb3/1 positive) from less virulent (MAb3/1 negative) strains. Although short-range interaction determines infection process, our results suggest that the infection potential of Legionella pneumophila serogroup 1 may be also controlled significantly by non-specific long-range electrostatic repulsion the bacteria undergo when approaching negatively charged host cells to be infected.

Distribution of lag-1 alleles and sequence-based types among Legionella pneumophila serogroup 1 clinical and environmental isolates in the United States

Approximately 84% of legionellosis cases are due to Legionella pneumophila serogroup 1. Moreover, a majority of L. pneumophila serogroup 1 clinical isolates react positively with monoclonal antibody 2 (MAb2) of the international standard panel. Over 94% of the legionellosis outbreaks investigated by the Centers for Disease Control and Prevention are due to this subset of L. pneumophila serogroup 1. To date, there is no complete explanation for the enhanced ability of these strains to cause disease. To better characterize these organisms, we subtyped 100 clinical L. pneumophila serogroup 1 isolates and 50 environmental L. pneumophila serogroup 1 isolates from the United States by (i) reactivity with MAb2, (ii) presence of a lag-1 gene required for the MAb2 epitope, and (iii) sequence-based typing analysis. Our results showed that the MAb2 epitope and lag-1 gene are overrepresented in clinical L. pneumophila serogroup 1 isolates. MAb2 recognized 75% of clinical isolates but only 6% of environmental isolates. Similarly, 75% of clinical isolates but only 8% of environmental isolates harbored lag-1. We identified three distinct lag-1 alleles, referred to as Philadelphia, Arizona, and Lens alleles, among 79 isolates carrying this gene. The Arizona allele is described for the first time in this study. We identified 59 different sequence types (STs), and 34 STs (58%) were unique to the United States. Our results support the hypothesis that a select group of STs may have an enhanced ability to cause legionellosis. Combining sequence typing and lag-1 analysis shows that STs tend to associate with a single lag-1 allele type, suggesting a hierarchy of virulence genotypes. Further analysis of ST and lag-1 profiles may identify genotypes of L. pneumophila serogroup 1 that warrant immediate intervention.

PCR-based 'serotyping' of Legionella pneumophila

Currently, several PCR assays based on 16S rRNA and virulence-associated genes are available for detection of Legionella pneumophila. So far, no genotyping method has been published that can discriminate between serogroups and monoclonal subgroups of the most common L. pneumophila serogroup 1. Our first approach was to analyse LPS-associated genes of seven L. pneumophila serogroup 1 strains, and we developed two PCR-based methods specific for serogroup 1. Specific DNA fragments could be amplified from all the serogroup 1 strains (n=43) including the strains from the American Type Culture Collection. In contrast, none of the strains from serogroups 2-15 (n=41) contained these specific gene regions. In a second approach, primers specific for the lag-1 gene, encoding an O-acetyltransferase, which is responsible for the presence of the LPS epitope recognized by mAb 3/1, were designed and tested for their ability to differentiate between mAb 3/1-positive and -negative strains. All mAb 3/1-positive strains (n=30) contained the lag-1 gene, but in turn 4 of 13 tested mAb 3/1-negative strains were also positive in the PCR. Thus, the discrimination between mAb 3/1-positive and mAb 3/1-negative subgroups could not be achieved for all strains. In a third approach, two intergenic regions expected to be specific for monoclonal subgroup Knoxville and closely related subgroups Benidorm/Bellingham were identified and used for selective genotyping. These intergenic regions could not only be amplified in every tested strain belonging to the subgroups Knoxville, Benidorm and Bellingham, but also in some strains of other unrelated subgroups. The two PCR approaches with primers specific for serogroup 1 genes definitely represent a valuable tool in outbreak investigations and for risk assessment. They also might be used for culture-independent diagnosis of legionellosis caused by L. pneumophila serogroup 1.

Random mutagenesis of Legionella pneumophila reveals genes associated with lipopolysaccharide synthesis and recognition by typing monoclonal antibodies

AIMS

To use random mutagenesis for the characterization of Legionella pneumophila lipopolysaccharide (LPS) components and serotypes.

METHODS AND RESULTS

Five strains belonging to different serogroups and/or monoclonal subgroups were mutagenized using a mini-Tn10 transposon. Exactly 11 819 mutants were checked for alterations in LPS using at least 11 monoclonal antibodies (mAbs) that define L. pneumophila serotypes. Among the mutants, five different mini-Tn10 insertions were identified. Four mutants originating from serogroup-1 did not lose their serogroup-specific epitope, but did sustain subtler changes that resulted in switches to different mAb subgroups. In contrast, a mutant from serogroup-6 lost its serogroup-specific epitope, while retaining a serogroup-cross-reacting epitope.

CONCLUSIONS

Random mutagenesis is a valuable tool for LPS epitope mapping. While some characteristics of L. pneumophila LPS can be altered, others appear resistant to mutagenesis. This underscores both the flexibility and rigidity of LPS architecture in L. pneumophila.

SIGNIFICANCE AND IMPACT OF THE STUDY

Losses of L. pneumophila LPS epitopes can result in new serotypes, changes that might escape detection by current DNA-based typing schemes. But, as the frequency of these changes is rare, based upon our observations, serotyping should remain an important tool for identifying L. pneumophila in water systems that are implicated in human infection.

Properties and partial purification of sialate-O-acetyltransferase from bovine submandibular glands

The O-acetylation of sialic acids in various positions is a frequent modification of these residues in glycoproteins and glycolipids of higher animals and some bacteria. Sialic acid O-acetylation is involved in the regulation of many cell biological and pathophysiological events. Since the properties and the structural and molecular genetic aspects of the eukaryotic sialate O-acetyltransferases are not yet known, we attempted to isolate the enzyme from bovine submandibular glands. O-Acetyltransferase was solubilised from its microsomal location with a zwitterionic detergent and enriched by approximately 50-fold in three steps, including affinity chromatography on coenzyme A. It exhibits a molecular mass of 150-160 kDa. Evidence was obtained for the putative existence of a low-molecular-mass, dialysable enzyme activator. The enzyme showed best activity with CMP-N-acetylneuraminic acid (CMP-Neu5Ac), followed by N-acetylneuraminic acid (Neu5Ac). These compounds, as well as AcCoA, have high affinity for both the microsome-bound and the partially purified O-acetyltransferase. CoA is a strong inhibitor. N-Acetyl-9-O-acetylneuraminic acid was found to be the main reaction product. No evidence was obtained for the involvement of an isomerase that might be responsible for the migration of O-acetyl groups within the sialic acid side chain.

Addition of neuA, the gene encoding N-acylneuraminate cytidylyl transferase, increases the discriminatory ability of the consensus sequence-based scheme for typing Legionella pneumophila serogroup 1 strains

The standard sequence-based method for the typing of Legionella pneumophila serogroup 1 strains was extended by using the gspA and neuA alleles. The use of neuA as a seventh allele for typing significantly increased the index of discrimination calculated for a panel of unrelated strains (from 0.932 to 0.963) and subdivided some known large common complexes (e.g., 1,4,3,1,1,1). This modification to the standard method is proposed as the method of choice in the epidemiological investigation of L. pneumophila infections.

The group B streptococcal sialic acid O-acetyltransferase is encoded by neuD, a conserved component of bacterial sialic acid biosynthetic gene clusters

Nearly two dozen microbial pathogens have surface polysaccharides or lipo-oligosaccharides that contain sialic acid (Sia), and several Sia-dependent virulence mechanisms are known to enhance bacterial survival or result in host tissue injury. Some pathogens are also known to O-acetylate their Sias, although the role of this modification in pathogenesis remains unclear. We report that neuD, a gene located within the Group B Streptococcus (GBS) Sia biosynthetic gene cluster, encodes a Sia O-acetyltransferase that is itself required for capsular polysaccharide (CPS) sialylation. Homology modeling and site-directed mutagenesis identified Lys-123 as a critical residue for Sia O-acetyltransferase activity. Moreover, a single nucleotide polymorphism in neuD can determine whether GBS displays a "high" or "low" Sia O-acetylation phenotype. Complementation analysis revealed that Escherichia coli K1 NeuD also functions as a Sia O-acetyltransferase in GBS. In fact, NeuD homologs are commonly found within Sia biosynthetic gene clusters. A bioinformatic approach identified 18 bacterial species with a Sia biosynthetic gene cluster that included neuD. Included in this list are the sialylated human pathogens Legionella pneumophila, Vibrio parahemeolyticus, Pseudomonas aeruginosa, and Campylobacter jejuni, as well as an additional 12 bacterial species never before analyzed for Sia expression. Phylogenetic analysis shows that NeuD homologs of sialylated pathogens share a common evolutionary lineage distinct from the poly-Sia O-acetyltransferase of E. coli K1. These studies define a molecular genetic approach for the selective elimination of GBS Sia O-acetylation without concurrent loss of sialylation, a key to further studies addressing the role(s) of this modification in bacterial virulence.

Genetics of capsule O-acetylation in serogroup C, W-135 and Y meningococci

Capsular polysaccharides of serogroup C, W-135 and Y meningococci were previously reported to be O-acetylated at the sialic acid residues. There is evidence that O-acetylation affects the immunogenicity of polysaccharide vaccines. We identified genes indispensable for O-acetylation of serogroup C, W-135 and Y meningococci downstream of the capsule synthesis genes siaA-D. The genes were co-transcribed with the sia operon as shown by reverse transcription polymerase chain reaction analysis. The putative capsular polysaccharide O-acetyltransferases were designated OatC and OatWY. The protein OatWY of serogroups W-135 and Y showed sequence homologies to members of the NodL-LacA-CysE family of bacterial acetyltransferases, whereas no sequence homology with any known protein in the different databases was found for the serogroup C protein OatC. In serogroup W-135 and Y meningococci, several clonal lineages either lacked OatWY or OatWY was inactivated by insertion of IS1301. For serogroup C meningococci, we observed in vitro phase variation of O-acetylation, which resulted from slipped-strand mispairing in homopolymeric tracts. This finding explains the observation of naturally occurring de-O-acetylated serogroup C meningococci. Our report is the first description of sequences of sialic acid O-acetyltransferase genes that have not been cloned from either other bacterial or mammalian organisms.

A hospital-associated outbreak of Legionnaires' disease caused by Legionella pneumophila serogroup 1 is characterized by stable genetic fingerprinting but variable monoclonal antibody patterns

An outbreak of 18 pneumonia cases caused by Legionella pneumophila serogroup 1 occurred at a Swedish university hospital 1996 to 1999. Eight clinical isolates obtained by culture from the respiratory tract were compared to 20 environmental isolates from the hospital and to 21 epidemiologically unrelated isolates in Sweden, mostly from patients, by using pulsed-field gel electrophoresis (PFGE), amplified fragment length polymorphism analysis (AFLP), and monoclonal antibody (MAb) typing. All patients and most environmental isolates from the outbreak hospital belonged to the same genotypic cluster in both PFGE and AFLP. This genotype was distinctly different from other strains, including a cluster from a second hospital in a different part of the country. The MAb subtype of the outbreak clone was Knoxville except for three isolates that were Oxford. A variation in the MAb reactivity pattern was also found in a second genotypic cluster. These changes in the MAb reactivity pattern were due to the absence or presence of the lag-1 gene coding for an O-acetyltransferase that is responsible for expression of the lipopolysaccharide epitope recognized by MAb 3/1 of the Dresden Panel. In all MAb 3/1-positive strains, the lag-1 gene was present on a genetic element that was bordered by a direct repeat that showed a high degree of sequence homology. Due to this homology, the lag-1 gene region seemed to be an unstable element in the chromosome. MAb patterns are thus a valuable adjunct to genotyping methods in defining subgroups inside a genotypic cluster of L. pneumophila sg 1.

Lipopolysaccharides from Legionella and Rhizobium stimulate mouse bone marrow granulocytes via Toll-like receptor 2

Lipopolysaccharide (LPS) derived from enterobacteria elicit in several cell types cellular responses that are restricted in the use of Toll-like receptor 4 (TLR4) as the principal signal-transducing molecule. A tendency to consider enterobacterial LPS as a prototypic LPS led some authors to present this mechanism as a paradigm accounting for all LPSs in all cell types. However, the structural diversity of LPS does not allow such a general statement. By using LPSs from bacteria that do not belong to the Enterobacteriaceae, we show that in bone marrow cells (BMCs) the LPS of Rhizobium species Sin-1 and of three strains of Legionella pneumophila require TLR2 rather than TLR4 to elicit the expression of CD14. In addition, exposure of BMCs from TLR4-deficient (C3H/HeJ) mice to the lipid A fragment of the Bordetella pertussis LPS inhibits their activation by the Legionella lipid A. The data show selective action of different LPSs via different TLRs, and suggest that TLR2 can interact with many lipid A structures, leading to either agonistic or specific antagonistic effects.

Epitope mapping of the O-chain polysaccharide of Legionella pneumophila serogroup 1 lipopolysaccharide by saturation-transfer-difference NMR spectroscopy

Two modifications of 5-acetimidoylamino-7-acetamido-3,5,7,9-tetradeoxy-D-glycero-D-galacto-non-2-ulosonic acid (5-N-acetimidoyl-7-N-acetyllegionaminic acid) in the O-chain polysaccharide (OPS) of the Legionella pneumophila serogroup 1 lipopolysaccharide (LPS) concern N-methylation of the 5-N-acetimidoyl group in legionaminic acid. Both N-methylated substituents, the (N,N-dimethylacetimidoyl) amino and acetimidoyl(N-methyl)amino group, could be allocated to one single legionaminic acid residue in the long- and middle-chain OPS, respectively. Using mutants devoid of N-methylated legionaminic acid derivatives, it could be shown that N-methylation of legionaminic acid correlated with the expression of the mAb 2625 epitope. In the present study we investigated the binding of the LPS-specific monoclonal antibody mAb 2625 to isolated OPS with surface-plasmon-resonance biomolecular interaction analysis and saturation-transfer-difference (STD) NMR spectroscopy in order to map the mAb 2625 epitope on a molecular level. It could be demonstrated that the binding affinity of the N-methylated legionaminic acid derivatives was independent from the size of the isolated OPS molecular species. In addition, STD NMR spectroscopic studies with polysaccharide ligands with an average molecular mass of up to 14 kDa revealed that binding was mainly mediated via the N-methylated acetimidoylamino group and via the closely located 7-N-acetyl group of the respective legionaminic acid residue, thus indicating these derivatives to represent the major epitope of mAb 2625.

N-Methylation in polylegionaminic acid is associated with the phase-variable epitope of Legionella pneumophila serogroup 1 lipopolysaccharide. Identification of 5-(N,N-dimethylacetimidoyl)amino and 5-acetimidoyl(N-methyl)amino-7-acetamido-3,5,7,9-tetradeoxynon-2-ulosonic acid in the O-chain polysaccharide

Previously, a phase-variable epitope was detected in the virulent wild-type strain RC1 of Legionella pneumophila serogroup 1 subgroup OLDA using a lipopolysaccharide-specific monoclonal antibody, mAb 2625 [Lüneberg, E., Zähringer, U., Knirel, Y. A., Steinmann, D., Hartmann, M., Steinmetz, I., Rohde, M., Kohl, J. & Frosch, M. (1998) J.Exp. Med. 188, 49-60]. In the present study, an isogenic mutant strain, termed 5215, was constructed by deletion of genes involved in the biosynthesis of the mAb 2625 epitope. Mutant 5215 was as virulent as the parental wild-type RC1 but did not bind mAb 2625. The two strains showed no difference in the core oligosaccharide and lipid A but in the O-chain polysaccharide structure, which is a homopolymer of 5-acetimidoylamino-7-acetamido-3,5,7,9-tetradeoxy-d-glycero-d-galacto-non-2-ulosonic acid (a derivative of legionaminic acid). NMR spectroscopic studies revealed a hitherto unknown modification of bacterial polysaccharides in the wild-type strain, namely N-methylation of the 5-acetimidoylamino group on a single legionaminic acid residue that is located, most likely, proximal to the core oligosaccharide. Two major N-methylated substituents, the (N,N-dimethylacetimidoyl)amino and acetimidoyl(N-methyl) amino groups, could be allocated to the long- and middle-chain O-polysaccharide species, respectively. N-Methylation of legionaminic acid that was absent from the isogenic mutant 5215 and from the spontaneous phase variant 811, correlated with the presence of the mAb 2625 epitope.

Complex O-acetylation in Legionella pneumophila serogroup 1 lipopolysaccharide. Evidence for two genes involved in 8-O-acetylation of legionaminic acid

A putative gene encoding an O-acetyl transferase, lag-1, is involved in biosynthesis of the O-polysaccharide (polylegionaminic acid) in some Legionella pneumophila serogroup 1 strains. To study the effect of the presence and absence of the gene on the O-polysaccharide O-acetylation, lag-1 from strain Philadelphia 1 was expressed in trans in the naturally lag-1-negative OLDA strain RC1, and immunoblot analysis revealed that the lag-1-encoded O-acetyl transferase is active. O-Polysaccharides of different size were prepared from the lipopolysaccharides of wild-type and transformant strains by mild acid degradation followed by gel-permeation chromatography. Using NMR spectroscopy and MALDI-TOF mass spectrometry, it was found that O-acetylation of the first three legionaminic acid residues next to the core occurs in the short-chain O-polysaccharide (<10 sugars) from both strains. Hence, there is another O-acetyl transferase encoded by a gene different from lag-1. In the longer-chain O-polysaccharide, a legionaminic acid residue proximal to the core is N-methylated and could be further 8-O-acetylated in the lag-1-dependent manner. Only strains expressing a functional lag-1 gene were recognized in Western blot analysis by monoclonal antibody 3/1 requiring 8-O-acetylated polylegionaminic acid for binding. The highly O-acetylated outer core region of the lipopolysaccharide is involved in the epitope of another serogroup 1-specific monoclonal antibody termed LPS-1. The O-acetylation pattern of the L. pneumophila serogroup 1 core oligosaccharide was revised using MALDI-TOF mass spectrometry. lag-1-independent O-acetylation of the core and short-chain O-polysaccharide was found to be a common feature of L. pneumophila serogroup 1 strains. The biological importance of conserved lag-1-independent and variable lag-1-dependent O-acetylation is discussed.

Cloning and functional characterization of a 30 kb gene locus required for lipopolysaccharide biosynthesis in Legionella pneumophila

The spontaneous Legionella pneumophila lipopolysaccharide (LPS) mutant 137, which did not bind the LPS-specific mAb 2625, was complemented with a genomic library from the parental wild-type strain. Transformants were screened for reconstitution of the wild-type LPS phenotype, able to bind mAb 2625. By this strategy, a 32,661 bp region comprising 30 open reading frames (Orfs) was identified. Orfs with significant homologies to genes encoding enzymes required for LPS or capsule biosynthesis of Gram-negative bacteria were located on the gene locus. The mutation of strain 137 could be assigned to a deletion of a cytosine residue in Orf 8. The protein encoded by Orf 8 exhibited homology to bacterial methyl-transferases. The L. pneumophila LPS gene locus included genes with deduced products likely to be involved in LPS core oligosaccharide biosynthesis (rmlA-D, rhamnosyl-transferases, acetyl-transferase) as well as LPS O-chain biosynthesis and translocation (mnaA, neuB, neuA, wecA, wzt, wzm). The neuA (Orf 25) and neuB (Orf 24) gene products were functionally characterized by complementation of the capsule negative E. coli K1 mutants EV5 and EV24, respectively. By introduction of the L. pneumophila neuA gene into E. coli EV5 and the neuB gene into EV24, expression of the K1 polysialic acid capsule could be restored. We, therefore, conclude that the biosynthesis pathway of legionaminic acid, the structural unit of the L. pneumophila Sg1 O-antigen, might be similar to the biosynthesis of sialic acid. Southern blot analysis indicated the entire gene locus to be present in L. pneumophila serogroup (Sg)1 strains, whereas only parts of the DNA stretch hybridized to DNA from Sg2 to Sg14 strains.