Escherichia coli K1 polysialic acid O-acetyltransferase gene, neuO, and the mechanism of capsule form variation involving a mobile contingency locus

Evolution of bacteriophages infecting encapsulated bacteria: lessons from Escherichia coli K1-specific phages

Bacterial capsules are not only important virulence factors, but also provide attachment sites for bacteriophages that possess capsule degrading enzymes as tailspike proteins. To gain insight into the evolution of these specialized viruses, we studied a panel of tailed phages specific for Escherichia coli K1, a neuroinvasive pathogen with a polysialic acid capsule. Genome sequencing of two lytic K1-phages and comparative analyses including a K1-prophage revealed that K1-phages did not evolve from a common ancestor. By contrast, each phage is related to a different progenitor type, namely T7-, SP6-, and P22-like phages, and gained new host specificity by horizontal uptake of an endosialidase gene. The new tailspikes emerged by combining endosialidase domains with the capsid binding module of the respective ancestor. For SP6-like phages, we identified a degenerated tailspike protein which now acts as versatile adaptor protein interconnecting tail and newly acquired tailspikes and demonstrate that this adapter utilizes an N-terminal undecapeptide interface to bind otherwise unrelated tailspikes. Combining biochemical and sequence analyses with available structural data, we provide new molecular insight into basic mechanisms that allow changes in host specificity while a conserved head and tail architecture is maintained. Thereby, the present study contributes not only to an improved understanding of phage evolution and host-range extension but may also facilitate the on purpose design of therapeutic phages based on well-characterized template phages.

Carbohydrate post-glycosylational modifications

Carbohydrate modification is a common phenomenon in nature. Many carbohydrate modifications such as some epimerization, O-acetylation, O-sulfation, O-methylation, N-deacetylation, and N-sulfation, take place after the formation of oligosaccharide or polysaccharide backbones. These modifications can be categorized as carbohydrate post-glycosylational modifications (PGMs). Carbohydrate PGMs further extend the complexity of the structures and the synthesis of carbohydrates and glycoconjugates. They also increase the capacity of the biological regulation that is achieved by finely tuning the structures of carbohydrates. Developing efficient methods to obtain structurally defined naturally occurring oligosaccharides, polysaccharides, and glycoconjugates with carbohydrate PGMs is essential for understanding the biological significance of carbohydrate PGMs. Combined with high-throughput screening methods, synthetic carbohydrates with PGMs are invaluable probes in structure-activity relationship studies. We illustrate here several classes of carbohydrates with PGMs and their applications. Recent progress in chemical, enzymatic, and chemoenzymatic syntheses of these carbohydrates and their derivatives are also presented.

Sialic acids: carbohydrate moieties that influence the biological and physical properties of biopharmaceutical proteins and living cells

Sialic acids are structurally diverse molecules that have important roles in the physiological reactions and characteristics of prokaryotes and eukaryotes. These include the ability to mask epitopes on underlying glycan chains and to repulse negatively charged moieties. Here, we describe the metabolism and immunological relevance of sialic acids and outline how their properties have been exploited by the pharmaceutical industry to enhance the therapeutic properties of proteins such as asparaginase and darbepoetin alpha.

The Escherichia coli argW-dsdCXA genetic island is highly variable, and E. coli K1 strains commonly possess two copies of dsdCXA

The genome sequences of Escherichia coli pathotypes reveal extensive genetic variability in the argW-dsdCXA island. Interestingly, the archetype E. coli K1 neonatal meningitis strain, strain RS218, has two copies of the dsdCXA genes for d-serine utilization at the argW and leuX islands. Because the human brain contains d-serine, an epidemiological study emphasizing K1 isolates surveyed the dsdCXA copy number and function. Forty of 41 (97.5%) independent E. coli K1 isolates could utilize d-serine. Southern blot hybridization revealed physical variability within the argW-dsdC region, even among 22 E. coli O18:K1:H7 isolates. In addition, 30 of 41 K1 strains, including 21 of 22 O18:K1:H7 isolates, had two dsdCXA loci. Mutational analysis indicated that each of the dsdA genes is functional in a rifampin-resistant mutant of RS218, mutant E44. The high percentage of K1 strains that can use d-serine is in striking contrast to our previous observation that only 4 of 74 (5%) isolates in the diarrheagenic E. coli (DEC) collection have this activity. The genome sequence of diarrheagenic E. coli isolates indicates that the csrRAKB genes for sucrose utilization are often substituted for dsdC and a portion of dsdX present at the argW-dsdCXA island of extraintestinal isolates. Among DEC isolates there is a reciprocal pattern of sucrose fermentation versus d-serine utilization. The ability to use d-serine is a trait strongly selected for among E. coli K1 strains, which have the ability to infect a wide range of extraintestinal sites. Conversely, diarrheagenic E. coli pathotypes appear to have substituted sucrose for d-serine as a potential nutrient.

Separate pathways for O acetylation of polymeric and monomeric sialic acids and identification of sialyl O-acetyl esterase in Escherichia coli K1

O acetylation at carbon positions 7 or 9 of the sialic acid residues in the polysialic acid capsule of Escherichia coli K1 is catalyzed by a phase-variable contingency locus, neuO, carried by the K1-specific prophage, CUS-3. Here we describe a novel method for analyzing polymeric sialic acid O acetylation that involves the release of surface sialic acids by endo-N-acetylneuraminidase digestion, followed by fluorescent labeling and detection of quinoxalinone derivatives by chromatography. The results indicated that NeuO is responsible for the majority of capsule modification that takes place in vivo. However, a minor neuO-independent O acetylation pathway was detected that is dependent on the bifunctional polypeptide encoded by neuD. This pathway involves O acetylation of monomeric sialic acid and is regulated by another bifunctional enzyme, NeuA, which includes N-terminal synthetase and C-terminal sialyl O-esterase domains. A homologue of the NeuA C-terminal domain (Pm1710) in Pasteurella multocida was also shown to be an esterase, suggesting that it functions in the catabolism of acetylated environmental sialic acids. Our combined results indicate a previously unexpected complexity in the synthesis and catabolism of microbial sialic and polysialic acids. These findings are key to understanding the biological functions of modified sialic acids in E. coli K1 and other species and may provide new targets for drug or vaccine development.

Mobile contingency locus controlling Escherichia coli K1 polysialic acid capsule acetylation

Escherichia coli K1 is part of a reservoir of adherent, invasive facultative pathogens responsible for a wide range of human and animal disease including sepsis, meningitis, urinary tract infection and inflammatory bowel syndrome. A prominent virulence factor in these diseases is the polysialic acid capsular polysaccharide (K1 antigen), which is encoded by the kps/neu accretion domain inserted near pheV at 67 map units. Some E. coli K1 strains undergo form (phase) variation involving loss or gain of O-acetyl esters at carbon positions 7 or 9 of the individual sialic acid residues of the polysialic acid chains. Acetylation is catalysed by the receptor-modifying acetyl coenzyme-A-dependent O-acetyltransferase encoded by neuO, a phase variable locus mapping near the integrase gene of the K1-specific prophage, CUS-3, which is inserted in argW at 53.1 map units. As the first E. coli contingency locus shown to operate by a translational switch, further investigation of neuO should provide a better understanding of the invasive K1 pathotype. Minimal estimates of morbidity and economic costs associated with human infections caused by extraintestinal pathogenic E. coli strains such as K1 indicate at least 6.5 million cases with attendant medical costs exceeding 2.5 billion US dollars annually in the United States alone.

Biosynthesis and Assembly of Capsular Polysaccharides inEscherichia coli

Capsules are protective structures on the surfaces of many bacteria. The remarkable structural diversity in capsular polysaccharides is illustrated by almost 80 capsular serotypes in Escherichia coli. Despite this variation, the range of strategies used for capsule biosynthesis and assembly is limited, and E. coli isolates provide critical prototypes for other bacterial species. Related pathways are also used for synthesis and export of other bacterial glycoconjugates and some enzymes/processes have counterparts in eukaryotes. In gram-negative bacteria, it is proposed that biosynthesis and translocation of capsular polysaccharides to the cell surface are temporally and spatially coupled by multiprotein complexes that span the cell envelope. These systems have an impact on both a general understanding of membrane trafficking in bacteria and on bacterial pathogenesis.

Re-examining the role and random nature of phase variation

Phase variation in bacteria is often considered a random process that has evolved to facilitate immune evasion in a host. Here, alternative biological roles for this process are presented and discussed, incorporating recent studies on nonpathogenic and commensal bacterial species. Furthermore, the integration of phase variation into bacterial regulatory networks and the relevance of this for considering phase variation as a random process are reviewed. Novel approaches are needed to study phase variation and its biological roles, but the insights obtained can contribute significantly to our understanding of the dynamic behaviour of bacterial populations and their interactions with the environment.

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.

Identification of a sialate O-acetyltransferase from Campylobacter jejuni: demonstration of direct transfer to the C-9 position of terminalalpha-2, 8-linked sialic acid

We have identified a sialate O-acetyltransferase in the lipo-oligosaccharide biosynthesis locus of Campylobacter jejuni. Strains possessing this locus are known to produce sialylated outer core structures that mimic host gangliosides, and have been implicated in triggering the onset of Guillain-Barré syndrome. The acetyltransferase, which was cloned and expressed as a fusion construct in Escherichia coli, is soluble and homologous with members of the NodL-LacA-CysE family of O-acetyltransferases. This enzyme catalyzes the transfer of O-acetyl groups onto oligosaccharide-bound sialic acid, with a high specificity for terminal alpha2,8-linked residues. The modification is directed to C-9 and not C-7 as is believed to occur more commonly in other organisms. Despite their wide prevalence and importance in both eukaryotes and prokaryotes, this is the first report to describe the characterization of a purified sialate O-acetyltransferase.

The genome of bacteriophage K1F, a T7-like phage that has acquired the ability to replicate on K1 strains of Escherichia coli

Bacteriophage K1F specifically infects Escherichia coli strains that produce the K1 polysaccharide capsule. Like several other K1 capsule-specific phages, K1F encodes an endo-neuraminidase (endosialidase) that is part of the tail structure which allows the phage to recognize and degrade the polysaccharide capsule. The complete nucleotide sequence of the K1F genome reveals that it is closely related to bacteriophage T7 in both genome organization and sequence similarity. The most striking difference between the two phages is that K1F encodes the endosialidase in the analogous position to the T7 tail fiber gene. This is in contrast with bacteriophage K1-5, another K1-specific phage, which encodes a very similar endosialidase which is part of a tail gene "module" at the end of the phage genome. It appears that diverse phages have acquired endosialidase genes by horizontal gene transfer and that these genes or gene products have adapted to different genome and virion architectures.