A novel outer membrane protein, Wzi, is involved in surface assembly of the Escherichia coli K30 group 1 capsule

IS1-related large-scale deletion of chromosomal regions harbouring the oxygen-insensitive nitroreductase gene nfsB causes nitrofurantoin heteroresistance in Escherichia coli

Nitrofurantoin is a broad-spectrum first-line antimicrobial used for managing uncomplicated urinary tract infection (UTI). Loss-of-function mutations in chromosomal genes nfsA, nfsB and ribE of Escherichia coli are known to reduce nitrofurantoin susceptibility. Here, we report the discovery of nitrofurantoin heteroresistance in E. coli clinical isolates and a novel genetic mechanism associated with this phenomenon. Subpopulations with lower nitrofurantoin susceptibility than major populations (hereafter, nitrofurantoin-resistant subpopulations) in two E. coli blood isolates (previously whole-genome sequenced) were identified using population analysis profiling. Each isolate was known to have a loss-of-function mutation in nfsA. From each isolate, four nitrofurantoin-resistant isolates were derived at a nitrofurantoin concentration of 32 mg l-1, and a comparator isolate was obtained without any nitrofurantoin exposure. Genomes of derived isolates were sequenced on Illumina and Nanopore MinION systems. Genetic variation between isolates was determined based on genome assemblies and read mapping. Nitrofurantoin minimum inhibitory concentrations (MICs) of both blood isolates were 64 mg l-1, with MICs of major nitrofurantoin-susceptible populations varying from 4 to 8 mg l-1. Two to 99 c.f.u. per million demonstrated growth at the nitrofurantoin concentration of 32 mg l-1, which is distinct from that of a homogeneously susceptible or resistant isolate. Derived nitrofurantoin-resistant isolates had 11-66 kb deletions in chromosomal regions harbouring nfsB, and all deletions were immediately adjacent to IS1-family insertion sequences. Our findings demonstrate that the IS1-associated large-scale genetic deletion is a hitherto unrecognized mechanism of nitrofurantoin heteroresistance and could compromise UTI management. Further, frequencies of resistant subpopulations from nitrofurantoin-heteroresistant isolates may challenge conventional nitrofurantoin susceptibility testing in clinical settings.

The making of hypervirulent Klebsiella pneumoniae

Klebsiella pneumoniae is a notorious bacterium in clinical practice. Virulence, carbapenem-resistance and their convergence among K. pneumoniae are extensively discussed in this article. Hypervirulent K. pneumoniae (HvKP) has spread from the Asian Pacific Rim to the world, inducing various invasive infections, such as pyogenic liver abscess, endophthalmitis, and meningitis. Furthermore, HvKP has acquired more and more drug resistance. Among multidrug-resistant HvKP, hypervirulent carbapenem-resistant K. pneumoniae (Hv-CRKP), and carbapenem-resistant hypervirulent K. pneumoniae (CR-HvKP) are both devastating for their extreme drug resistance and virulence. The hypervirulence of HvKP is primarily attributed to hypercapsule, macromolecular exopolysaccharides, or excessive siderophores, although it has many other factors, for example, lipopolysaccharides, fimbriae, and porins. In contrast with classical determination of HvKP, that is, animal lethality test, molecular determination could be an optional and practical method after improvement. HvKP, including Hv-CRKP and CR-HvKP, has been progressing. R-M and CRISPR-Cas systems may play pivotal roles in such evolutions. Hv-CRKP and CR-HvKP, in particular the former, should be of severe concern due to their being more and more prevalent.

Evidence for a Widespread Third System for Bacterial Polysaccharide Export across the Outer Membrane Comprising a Composite OPX/β-Barrel Translocon

In Gram-negative bacteria, secreted polysaccharides have multiple critical functions. In Wzx/Wzy- and ABC transporter-dependent pathways, an outer membrane (OM) polysaccharide export (OPX) type translocon exports the polysaccharide across the OM. The paradigm OPX protein Wza of Escherichia coli is an octamer in which the eight C-terminal domains form an α-helical OM pore and the eight copies of the three N-terminal domains (D1 to D3) form a periplasmic cavity. In synthase-dependent pathways, the OM translocon is a 16- to 18-stranded β-barrel protein. In Myxococcus xanthus, the secreted polysaccharide EPS (exopolysaccharide) is synthesized in a Wzx/Wzy-dependent pathway. Here, using experiments, phylogenomics, and computational structural biology, we identify and characterize EpsX as an OM 18-stranded β-barrel protein important for EPS synthesis and identify AlgE, a β-barrel translocon of a synthase-dependent pathway, as its closest structural homolog. We also find that EpsY, the OPX protein of the EPS pathway, consists only of the periplasmic D1 and D2 domains and completely lacks the domain for spanning the OM (herein termed a D1D2OPX protein). In vivo, EpsX and EpsY mutually stabilize each other and interact in in vivo pulldown experiments supporting their direct interaction. Based on these observations, we propose that EpsY and EpsX make up and represent a third type of translocon for polysaccharide export across the OM. Specifically, in this composite translocon, EpsX functions as the OM-spanning β-barrel translocon together with the periplasmic D1D2OPX protein EpsY. Based on computational genomics, similar composite systems are widespread in Gram-negative bacteria. IMPORTANCE Bacteria secrete a wide variety of polysaccharides that have critical functions in, e.g., fitness, surface colonization, and biofilm formation and in beneficial and pathogenic human-, animal-, and plant-microbe interactions. In Gram-negative bacteria, export of these chemically diverse polysaccharides across the outer membrane depends on two known translocons, i.e., an outer membrane OPX protein in Wzx/Wzy- and ABC transporter-dependent pathways and an outer membrane 16- to 18-stranded β-barrel protein in synthase-dependent pathways. Here, using a combination of experiments in Myxococcus xanthus, phylogenomics, and computational structural biology, we provide evidence supporting that a third type of translocon can export polysaccharides across the outer membrane. Specifically, in this translocon, an outer membrane-spanning β-barrel protein functions together with an entirely periplasmic OPX protein that completely lacks the domain for spanning the OM. Computational genomics support that similar composite systems are widespread in Gram-negative bacteria.

HfaE Is a Component of the Holdfast Anchor Complex That Tethers the Holdfast Adhesin to the Cell Envelope

Bacteria use adhesins to colonize different surfaces and form biofilms. The species of the Caulobacterales order use a polar adhesin called holdfast, composed of polysaccharides, proteins, and DNA, to irreversibly adhere to surfaces. In Caulobacter crescentus, a freshwater Caulobacterales species, the holdfast is anchored at the cell pole via the holdfast anchor (Hfa) proteins HfaA, HfaB, and HfaD. HfaA and HfaD colocalize with holdfast and are thought to form amyloid-like fibers that anchor holdfast to the cell envelope. HfaB, a lipoprotein, is required for the translocation of HfaA and HfaD to the cell surface. Deletion of the anchor proteins leads to a severe defect in adherence resulting from holdfast not being properly attached to the cell and shed into the medium. This phenotype is greater in a ΔhfaB mutant than in a ΔhfaA ΔhfaD double mutant, suggesting that HfaB has other functions besides the translocation of HfaA and HfaD. Here, we identify an additional HfaB-dependent holdfast anchoring protein, HfaE, which is predicted to be a secreted protein. HfaE is highly conserved among Caulobacterales species, with no predicted function. In planktonic culture, hfaE mutants produce holdfasts and rosettes similar to those produced by the wild type. However, holdfasts from hfaE mutants bind to the surface but are unable to anchor cells, similarly to other anchor mutants. We showed that fluorescently tagged HfaE colocalizes with holdfast and that HfaE forms an SDS-resistant high-molecular-weight species consistent with amyloid fiber formation. We propose that HfaE is a novel holdfast anchor protein and that HfaE functions to link holdfast material to the cell envelope. IMPORTANCE For surface attachment and biofilm formation, bacteria produce adhesins that are composed of polysaccharides, proteins, and DNA. Species of the Caulobacterales produce a specialized polar adhesin, holdfast, which is required for permanent attachment to surfaces. In this study, we evaluate the role of a newly identified holdfast anchor protein, HfaE, in holdfast anchoring to the cell surface in two different members of the Caulobacterales with drastically different environments. We show that HfaE plays an important role in adhesion and biofilm formation in the Caulobacterales. Our results provide insights into bacterial adhesins and how they interact with the cell envelope and surfaces.

LPS O Antigen Plays a Key Role in Klebsiella pneumoniae Capsule Retention

Despite the importance of encapsulation in bacterial pathogenesis, the biochemical mechanisms and forces that underpin retention of capsule by encapsulated bacteria are poorly understood. In Gram-negative bacteria, there may be interactions between lipopolysaccharide (LPS) core and capsule polymers, between capsule polymers with retained acyl carriers and the outer membrane, and in some bacteria, between the capsule polymers and Wzi, an outer membrane protein lectin. Our transposon studies in Klebsiella pneumoniae B5055 identified additional genes that, when insertionally inactivated, resulted in reduced encapsulation. Inactivation of the gene waaL, which encodes the ligase responsible for attaching the repeated O antigen of LPS to the LPS core, resulted in a significant reduction in capsule retention, measured by atomic force microscopy. This reduction in encapsulation was associated with increased sensitivity to human serum and decreased virulence in a murine model of respiratory infection and, paradoxically, with increased biofilm formation. The capsule in the WaaL mutant was physically smaller than that of the Wzi mutant of K. pneumoniae B5055. These results suggest that interactions between surface carbohydrate polymers may enhance encapsulation, a key phenotype in bacterial virulence, and provide another target for the development of antimicrobials that may avoid resistance issues associated with growth inhibition. IMPORTANCE Bacterial capsules, typically comprised of complex sugars, enable pathogens to avoid key host responses to infection, including phagocytosis. These capsules are synthesized within the bacteria, exported through the outer envelope, and then secured to the external surface of the organism by a force or forces that are incompletely described. This study shows that in the important hospital pathogen Klebsiella pneumoniae, the polysaccharide capsule is retained by interactions with other surface sugars, especially the repeated sugar molecule of the LPS molecule in Gram-negative bacteria known as "O antigen." This O antigen is joined to the LPS molecule by ligation, and loss of the enzyme responsible for ligation, a protein called WaaL, results in reduced encapsulation. Since capsules are essential to the virulence of many pathogens, WaaL might provide a target for new antimicrobial development, critical to the control of pathogens like K. pneumoniae that have become highly drug resistant.

Metagenomic Insights into Pathogenic Characterization of ST410 Acinetobacter nosocomialis Prevalent in China

Acinetobacter nosocomialis is a prevalent opportunistic pathogen that causes hospital-acquired infections. The increasing threats from A. nosocomialis infections have led to attention from the scientific and medical communities. Metagenomic next-generation sequencing (mNGS) was performed for an exudate specimen collected from an ICU patient with wound infection, followed by sepsis, in Tongji Hospital. Three assembly strategies were employed to recover the genome of A. nosocomialis in the metagenomic sample. Together with publicly available genomes of A. nosocomialis, the features of population genetics and molecular epidemiology were deeply analyzed. A draft genome was reconstructed for the metagenomic strain WHM01, derived from the ST410 A. nosocomialis dominating the microbial community, thereby prompting its highly pathogenic risk, which is associated with infection and persistence. The structure of the bacterial pangenome was characterized, including the 1862 core and 11,815 accessory genes present in the 157 strains. The genetic diversity of the genes coding for the 128 virulence factors assigned to 14 functional categories was uncovered in this nosocomial pathogen, such as the lipooligosaccharide, capsule, type IV pilus, and outer membrane proteins. Our work revealed genomic properties of ST410 A. nosocomialis, which is prevalent in China, and further highlighted that metagenomic surveillance may be a prospective application for evaluating the pathogenic characteristics of the nosocomial opportunistic pathogens.

Molecular Characterization of Klebsiella pneumoniae Isolated from Sputum in a Tertiary Hospital in Xinxiang, China

Background

In clinical practice, Klebsiella pneumoniae (K. pneumoniae) is a common opportunistic pathogen responsible for nosocomial infection. This study aimed to analyze the trend of antimicrobial susceptibility and virulent characteristics of K. pneumoniae isolated from sputum. In clinics, data of the current study will help in the clinical treatment of K. pneumoniae infection.

Results

The current research showed the resistance rates of the 20 K. pneumoniae isolates against 13 antibiotics ranged from 15.0% to 80.0%. The detection rate of extended spectrum β-lactamases (ESBLs) was up to 55%, while bla_SHV was the most prevalent ESBLs genes. Four strains (25.0%) of K. pneumoniae presented hypermucoviscous phenotype (HMV). Moreover, 18 strains (90.0%) showed the stronger biofilm-forming ability. wzi, wab_G, fim_H, mrk_D were the most prevalent virulence genes in current research. Ten strains were found capsule typing and the higher genetic diversity of colonizing K. pneumoniae in this region. K₁₉ exhibited a strong positive correlation with imipenem resistance, while K₁ showed strong correlations with mag_A . Furthermore, HMV phenotype showed significantly negative correlations with multidrug-resistant.

Conclusion

In the hospital, the antibiotic resistance of K. pneumoniae (isolated from sputum samples) has a serious concern. Additionally, strains of K. pneumoniae show the higher genetic diversity.

Identification of distinct capsule types associated with Serratia marcescens infection isolates

Serratia marcescens is a versatile opportunistic pathogen that can cause a variety of infections, including bacteremia. Our previous work established that the capsule polysaccharide (CPS) biosynthesis and translocation locus contributes to the survival of S. marcescens in a murine model of bacteremia and in human serum. In this study, we determined the degree of capsule genetic diversity among S. marcescens isolates. Capsule loci (KL) were extracted from >300 S. marcescens genome sequences and compared. A phylogenetic comparison of KL sequences demonstrated a substantial level of KL diversity within S. marcescens as a species and a strong delineation between KL sequences originating from infection isolates versus environmental isolates. Strains from five of the identified KL types were selected for further study and electrophoretic analysis of purified CPS indicated the production of distinct glycans. Polysaccharide composition analysis confirmed this observation and identified the constituent monosaccharides for each strain. Two predominant infection-associated clades, designated KL1 and KL2, emerged from the capsule phylogeny. Bacteremia strains from KL1 and KL2 were determined to produce ketodeoxynonulonic acid and N-acetylneuraminic acid, two sialic acids that were not found in strains from other clades. Further investigation of KL1 and KL2 sequences identified two genes, designated neuA and neuB, that were hypothesized to encode sialic acid biosynthesis functions. Disruption of neuB in a KL1 isolate resulted in the loss of sialic acid and CPS production. The absence of sialic acid and CPS production also led to increased susceptibility to internalization by a human monocytic cell line, demonstrating that S. marcescens phagocytosis resistance requires CPS. Together, these results establish the capsule genetic repertoire of S. marcescens and identify infection-associated clades with sialic acid CPS components.

Kaptive 2.0: updated capsule and lipopolysaccharide locus typing for the Klebsiella pneumoniae species complex

The outer polysaccharide capsule and lipopolysaccharide (LPS) antigens are key targets for novel control strategies targeting Klebsiella pneumoniae and related taxa from the K. pneumoniae species complex (KpSC), including vaccines, phage and monoclonal antibody therapies. Given the importance and growing interest in these highly diverse surface antigens, we had previously developed Kaptive, a tool for rapidly identifying and typing capsule (K) and outer LPS (O) loci from whole genome sequence data. Here, we report two significant updates, now freely available in Kaptive 2.0 (https://github.com/katholt/kaptive): (i) the addition of 16 novel K locus sequences to the K locus reference database following an extensive search of >17 000 KpSC genomes; and (ii) enhanced O locus typing to enable prediction of the clinically relevant O2 antigen (sub)types, for which the genetic determinants have been recently described. We applied Kaptive 2.0 to a curated dataset of >12 000 public KpSC genomes to explore for the first time, to the best of our knowledge, the distribution of predicted O (sub)types across species, sampling niches and clones, which highlighted key differences in the distributions that warrant further investigation. As the uptake of genomic surveillance approaches continues to expand globally, the application of Kaptive 2.0 will generate novel insights essential for the design of effective KpSC control strategies.

Capsules and Extracellular Polysaccharides in Escherichia coli and Salmonella

Escherichia coli and Salmonella isolates produce a range of different polysaccharide structures that play important roles in their biology. E. coli isolates often possess capsular polysaccharides (K antigens), which form a surface structural layer. These possess a wide range of repeat-unit structures. In contrast, only one capsular polymer (Vi antigen) is found in Salmonella, and it is confined to typhoidal serovars. In both genera, capsules are vital virulence determinants and are associated with the avoidance of host immune defenses. Some isolates of these species also produce a largely secreted exopolysaccharide called colanic acid as part of their complex Rcs-regulated phenotypes, but the precise function of this polysaccharide in microbial cell biology is not fully understood. E. coli isolates produce two additional secreted polysaccharides, bacterial cellulose and poly-N-acetylglucosamine, which play important roles in biofilm formation. Cellulose is also produced by Salmonella isolates, but the genes for poly-N-acetylglucosamine synthesis appear to have been lost during its evolution toward enhanced virulence. Here, we discuss the structures, functions, relationships, and sophisticated assembly mechanisms for these important biopolymers.

Escherichia coli O127 group 4 capsule proteins assemble at the outer membrane

Enteropathogenic Escherichia coli O127 is encapsulated by a protective layer of polysaccharide made of the same strain specific O-antigen as the serotype lipopolysaccharide. Seven genes encoding capsule export functions comprise the group 4 capsule (gfc) operon. Genes gfcE, etk and etp encode homologs of the group 1 capsule secretion system but the upstream gfcABCD genes encode unknown functions specific to group 4 capsule export. We have developed an expression system for the large-scale production of the outer membrane protein GfcD. Contrary to annotations, we find that GfcD is a non-acylated integral membrane protein. Circular dichroism spectroscopy, light-scattering data, and the HHomp server suggested that GfcD is a monomeric β-barrel with 26 β-strands and an internal globular domain. We identified a set of novel protein-protein interactions between GfcB, GfcC, and GfcD, both in vivo and in vitro, and quantified the binding properties with isothermal calorimetry and biolayer interferometry. GfcC and GfcB form a high-affinity heterodimer with a K_D near 100 nM. This heterodimer binds to GfcD (K_D = 28 μM) significantly better than either GfcB or GfcC alone. These gfc proteins may form a complex at the outer membrane for group 4 capsule secretion or for a yet unknown function.

The Wzi outer membrane protein mediates assembly of a tight capsular polysaccharide layer on the Acinetobacter baumannii cell surface

Identification of novel therapeutic targets is required for developing alternate strategies to treat infections caused by the extensively drug-resistant bacterial pathogen, Acinetobacter baumannii. As capsular polysaccharide (CPS) is a prime virulence determinant required for evasion of host immune defenses, understanding the pathways for synthesis and assembly of this discrete cell-surface barrier is important. In this study, we assess cell-bound and cell-free CPS material from A. baumannii AB5075 wildtype and transposon library mutants and demonstrate that the Wzi outer membrane protein is required for the proper assembly of the CPS layer on the cell surface. Loss of Wzi resulted in an estimated 4.4-fold reduction in cell-associated CPS with a reciprocal increase in CPS material shed in the extracellular surrounds. Transmission electron microscopy revealed a disrupted CPS layer with sparse patches of CPS on the external face of the outer membrane when Wzi function was lost. However, this genotype did not have a significant effect on biofilm formation. Genetic analysis demonstrated that the wzi gene is ubiquitous in the species, though the nucleotide sequences were surprisingly diverse. Though divergence was not concomitant with variation at the CPS biosynthesis K locus, an association between wzi type and the first sugar of the CPS representing the base of the structure most likely to interact with Wzi was observed.

Human commensal gut Proteobacteria withstand type VI secretion attacks through immunity protein-independent mechanisms

While the major virulence factors for Vibrio cholerae, the cause of the devastating diarrheal disease cholera, have been extensively studied, the initial intestinal colonization of the bacterium is not well understood because non-human adult animals are refractory to its colonization. Recent studies suggest the involvement of an interbacterial killing device known as the type VI secretion system (T6SS). Here, we tested the T6SS-dependent interaction of V. cholerae with a selection of human gut commensal isolates. We show that the pathogen efficiently depleted representative genera of the Proteobacteria in vitro, while members of the Enterobacter cloacae complex and several Klebsiella species remained unaffected. We demonstrate that this resistance against T6SS assaults was mediated by the production of superior T6SS machinery or a barrier exerted by group I capsules. Collectively, our data provide new insights into immunity protein-independent T6SS resistance employed by the human microbiota and colonization resistance in general.

Virulence Factors in Hypervirulent Klebsiella pneumoniae

Hypervirulent Klebsiella pneumoniae (hvKP) has spread globally since first described in the Asian Pacific Rim. It is an invasive variant that differs from the classical K. pneumoniae (cKP), with hypermucoviscosity and hypervirulence, causing community-acquired infections, including pyogenic liver abscess, pneumonia, meningitis, and endophthalmitis. It utilizes a battery of virulence factors for survival and pathogenesis, such as capsule, siderophores, lipopolysaccharide, fimbriae, outer membrane proteins, and type 6 secretion system, of which the former two are dominant. This review summarizes these hvKP-associated virulence factors in order to understand its molecular pathogenesis and shed light on new strategies to improve the prevention, diagnosis, and treatment of hvKP-causing infection.

Molecular Epidemiology of Hypervirulent Carbapenemase-Producing Klebsiella pneumoniae

Objective

To investigate the overall distributions of key virulence genes in Klebsiella pneumoniae, especially the hypervirulent bla_KPC-positive K. pneumoniae (Hv-bla_KPC(+)-KP).

Methods

A total of 521 complete genomes of K. pneumoniae from GenBank were collected and analyzed. Multilocus sequence typing, molecular serotyping, antibiotic-resistance, virulence genes and plasmid replicon typing were investigated.

Results

Positive rates of virulence genes highly varied, ranging from 2.9 (c-rmpA/A2) to 99.6% (entB). Totally 207 strains presented positive fimH, mrkD, entB and wzi and 190 showed positive fimH, mrkD, entB, irp2 and wzi, which were the two primary modes. A total of 94, 165 and 29 strains were denoted as hypervirulent K. pneumoniae (HvKP), bla_KPC(+)-KP and Hv-bla_KPC(+)-KP. ST11 accounted for 17 among the 29 Hv-bla_KPC(+)-KP strains; Genes iucA, p-rmpA2 and p-rmpA were positive in 28, 26 and 18 Hv-bla_KPC(+)-KP strains respectively. Among the 29 Hv-bla_KPC(+)-KP strains exhibiting four super clusters from GenBank, IncHI1B plasmids carrying virulence genes and IncFII ones with bla_KPC were responsible for both 23 strains respectively.

Conclusions

Positive rates of virulence genes vary remarkably in K. pneumoniae. Genes iucA, p-rmpA2 and p-rmpA were primary ones inducing Hv-bla_KPC(+)-KP. IncHI1B plasmids carrying virulence genes and IncFII ones with bla_KPC constitute the primary combination responsible for Hv-bla_KPC(+)-KP. The making of Hv-bla_KPC(+)-KP is mostly via bla_KPC(+)-KP acquiring another plasmid harboring virulence genes.

A novel ItrA4 d-galactosyl 1-phosphate transferase is predicted to initiate synthesis of an amino sugar-lacking K92 capsular polysaccharide of Acinetobacter baumannii B8300

The K92 capsular polysaccharide (CPS) from Acinetobacter baumannii B8300 was studied by sugar analysis, Smith degradation, and one- and two-dimensional ¹H and ¹³C NMR spectroscopy. The elucidated CPS includes a branched pentasaccharide repeat unit containing one d-Galp and four l-Rhap residues; an atypical composition given that all A. baumannii CPS structures determined to date contain at least one amino sugar. Accordingly, biosynthesis of A. baumannii CPS types are initiated by initiating transferases (Itrs) that transfer 1-phosphate of either a 2-acetamido-2-deoxy-d-hexose, a 2-acetamido-2,6-dideoxy-d-hexose or a 2-acetamido-4-acylamino-2,4,6-trideoxy-d-hexose to an undecaprenyl phosphate (UndP) carrier. However, the KL92 capsule biosynthesis gene cluster in the B8300 genome sequence includes a gene for a novel Itr type, ItrA4, which is predicted to begin synthesis of the K92 CPS by transferring D-Galp 1-phosphate to the UndP lipid carrier. The itrA4 gene was found in a module transcribed in the opposite direction to the majority of the K locus. This module also includes an unknown open reading frame (orf_KL92), a gtr166 glycosyltransferase gene, and a wzi gene predicted to be involved in the attachment of CPS to the cell surface. Investigation into the origins of orf_KL92-gtr166-itrA4-wzi_KL92 revealed it might have originated from Acinetobacter junii.

Genomic diversity and organization of complex polysaccharide biosynthesis clusters in the genus Dickeya

The pectinolytic genus Dickeya (formerly Erwinia chrysanthemi) comprises numerous pathogenic species which cause diseases in various crops and ornamental plants across the globe. Their pathogenicity is governed by complex multi-factorial processes of adaptive virulence gene regulation. Extracellular polysaccharides and lipopolysaccharides present on bacterial envelope surface play a significant role in the virulence of phytopathogenic bacteria. However, very little is known about the genomic location, diversity, and organization of the polysaccharide and lipopolysaccharide biosynthetic gene clusters in Dickeya. In the present study, we report the diversity and structural organization of the group 4 capsule (G4C)/O-antigen capsule, putative O-antigen lipopolysaccharide, enterobacterial common antigen, and core lipopolysaccharide biosynthesis clusters from 54 Dickeya strains. The presence of these clusters suggests that Dickeya has both capsule and lipopolysaccharide carrying O-antigen to their external surface. These gene clusters are key regulatory components in the composition and structure of the outer surface of Dickeya. The O-antigen capsule/group 4 capsule (G4C) coding region shows a variation in gene content and organization. Based on nucleotide sequence homology in these Dickeya strains, two distinct groups, G4C group I and G4C group II, exist. However, comparatively less variation is observed in the putative O-antigen lipopolysaccharide cluster in Dickeya spp. except for in Dickeya zeae. Also, enterobacterial common antigen and core lipopolysaccharide biosynthesis clusters are present mostly as conserved genomic regions. The variation in the O-antigen capsule and putative O-antigen lipopolysaccharide coding region in relation to their phylogeny suggests a role of multiple horizontal gene transfer (HGT) events. These multiple HGT processes might have been manifested into the current heterogeneity of O-antigen capsules and O-antigen lipopolysaccharides in Dickeya strains during its evolution.

Complete genome sequence of Shewanella marisflavi ECSMB14101, a red pigment synthesizing bacterium isolated from the East China Sea

Bacteria of the genus Shewanella have been studied for their versatile electron-accepting abilities, particularly for extracellular electron transfer via minerals. Shewanella marisflavi ECSMB14101 was isolated from naturally formed biofilms in the East China Sea. The genome of S. marisflavi ECSMB14101 encodes 3891 genes with a total size of 4,343,492 bp in one chromosome. Its GC content is 49.89%. S. marisflavi ECSMB14101 is able to synthesize a red pigment, which may be achieved through Cytochrome c3 and electron transfer to reduce Fe(III) oxide. The genomic data presented here could provide fundamental insights to better understand the physiological characteristics of S. marisflavi, the ecological significance of red pigment synthesis, and its inductive effects on the settlement of marine invertebrate larvae.

K-PAM: a unified platform to distinguish Klebsiella species K- and O-antigen types, model antigen structures and identify hypervirulent strains

A computational method has been developed to distinguish the Klebsiella species serotypes to aid in outbreak surveillance. A reliability score (estimated based on the accuracy of a specific K-type prediction against the dataset of 141 distinct K-types) average (ARS) that reflects the specificity between the Klebsiella species capsular polysaccharide biosynthesis and surface expression proteins, and their K-types has been established. ARS indicates the following order of potency in accurate serotyping: Wzx (ARS = 98.5%),Wzy (ARS = 97.5%),WbaP (ARS = 97.2%),Wzc (ARS = 96.4%),Wzb (ARS = 94.3%),WcaJ (ARS = 93.8%),Wza (ARS = 79.9%) and Wzi (ARS = 37.1%). Thus, Wzx, Wzy and WbaP can give more reliable K-typing compared with other proteins. A fragment-based approach has further increased the Wzi ARS from 37.1% to 80.8%. The efficacy of these 8 proteins in accurate K-typing has been confirmed by a rigorous testing and the method has been automated as K-PAM (www.iith.ac.in/K-PAM/). Testing also indicates that the use of multiple genes/proteins helps in reducing the K-type multiplicity, distinguishing the K-types that have identical K-locus (like KN3 and K35) and identifying the ancestral serotypes of Klebsiella spp. K-PAM has the facilities to O-type using Wzm (ARS = 85.7%) and Wzt (ARS = 85.7%) and identifies the hypervirulent Klebsiella species by the use of rmpA, rmpA2, iucA, iroB and peg-344 marker genes. Yet another highlight of the server is the repository of the modeled 11 O- and 79 K- antigen 3D structures.

Predicting antimicrobial resistance using conserved genes

A growing number of studies are using machine learning models to accurately predict antimicrobial resistance (AMR) phenotypes from bacterial sequence data. Although these studies are showing promise, the models are typically trained using features derived from comprehensive sets of AMR genes or whole genome sequences and may not be suitable for use when genomes are incomplete. In this study, we explore the possibility of predicting AMR phenotypes using incomplete genome sequence data. Models were built from small sets of randomly-selected core genes after removing the AMR genes. For Klebsiella pneumoniae, Mycobacterium tuberculosis, Salmonella enterica, and Staphylococcus aureus, we report that it is possible to classify susceptible and resistant phenotypes with average F1 scores ranging from 0.80-0.89 with as few as 100 conserved non-AMR genes, with very major error rates ranging from 0.11-0.23 and major error rates ranging from 0.10-0.20. Models built from core genes have predictive power in cases where the primary AMR mechanisms result from SNPs or horizontal gene transfer. By randomly sampling non-overlapping sets of core genes, we show that F1 scores and error rates are stable and have little variance between replicates. Although these small core gene models have lower accuracies and higher error rates than models built from the corresponding assembled genomes, the results suggest that sufficient variation exists in the core non-AMR genes of a species for predicting AMR phenotypes.

A Front Line on Klebsiella pneumoniae Capsular Polysaccharide Knowledge: Fourier Transform Infrared Spectroscopy as an Accurate and Fast Typing Tool

Klebsiella pneumoniae is nowadays recognized as one of the most defiant human pathogens, whose infections are increasingly more challenging to treat and control. Whole-genome sequencing (WGS) has been key for clarifying the population structure of K. pneumoniae, and it is still instrumental to provide insights into potential pathogenicity and evolutionary markers, such as the capsular locus. However, this information and WGS are still far from being accessible and translated into routine clinical microbiology laboratories as quick and cost-efficient strain diagnostic tools. Here, we propose a biochemical fingerprinting approach based on Fourier transform infrared spectroscopy (FT-IR) and multivariate data analysis tools for K. pneumoniae capsular typing that, because of its high resolution, speed, and low cost, can be an asset to provide enough information to support real-time epidemiology and infection control decisions. Besides, it provides a simple framework for phenotypic/biochemical validation of K. pneumoniae capsular diversity.

Genomics-based population analysis of multidrug-resistant (MDR) Klebsiella pneumoniae motivated a renewed interest on the capsule as an evolutionary and virulence marker of clinically relevant strains. Whole-genome sequencing (WGS)-based approaches have provided great insights into the genetic variability of the capsular locus, but genotypic-biochemical capsular (K)-type correlations are lacking, hindering the establishment of a reliable framework for K-type characterization and typing. To fill this gap, we combined molecular, comparative genomics, and multivariate data analysis tools with biochemical data on the capsular locus to support the usefulness of Fourier transform infrared (FT-IR) spectroscopy as a reliable K typing tool. To validate our approach, we used a representative collection of well-defined MDR K. pneumoniae lineages involved in local or nationwide epidemics in multiple countries. With this, we demonstrate a high accuracy and resolution of our FT-IR-based spectroscopy approach for K-type discrimination that is even higher than that provided by WGS. Moreover, the specific associations established between certain K types and specific K. pneumoniae lineages with high clinical relevance, together with the accuracy, simplicity, short time to result, and inexpensive features of the method, support the value of the developed FT-IR-based approach for an easy, fast, and cost-effective strain typing. This fulfills a still unmet need for tools to support real-time monitoring and control of K. pneumoniae infections. In addition, the genotypic-biochemical correlations established provide insights on sugar composition/structure of newly defined K. pneumoniae capsular types.

IMPORTANCEKlebsiella pneumoniae is nowadays recognized as one of the most defiant human pathogens, whose infections are increasingly more challenging to treat and control. Whole-genome sequencing (WGS) has been key for clarifying the population structure of K. pneumoniae, and it is still instrumental to provide insights into potential pathogenicity and evolutionary markers, such as the capsular locus. However, this information and WGS are still far from being accessible and translated into routine clinical microbiology laboratories as quick and cost-efficient strain diagnostic tools. Here, we propose a biochemical fingerprinting approach based on Fourier transform infrared spectroscopy (FT-IR) and multivariate data analysis tools for K. pneumoniae capsular typing that, because of its high resolution, speed, and low cost, can be an asset to provide enough information to support real-time epidemiology and infection control decisions. Besides, it provides a simple framework for phenotypic/biochemical validation of K. pneumoniae capsular diversity.

Promising strategies for future treatment of Klebsiella pneumoniae biofilms

Klebsiella pneumoniae is a Gram-negative pathogenic bacterium that has the ability to aggregate as biofilm, representing one of the main agents in hospital infections, showing high rates of resistance to antibiotics. The K. pneumoniae biofilm aggregates are composed mainly of extracellular polysaccharides, eDNA and proteins. Besides, biofilms can attach to medical devices, such as endotracheal tubes and catheters, but are most dangerous on body surfaces. Here, we discuss the recent findings about the resistance mechanisms of K. pneumoniae biofilms, including genes and protein involved in 'classic', multidrug-resistant and hypervirulent strains, and also virulence factors. In addition, we also explore new strategies for possible treatment of these biofilms, and recently discovered molecules which may lead to future treatments.

Targeting the Sugary Armor of Klebsiella Species

The emergence of multidrug-resistant strains of Gram-negative Klebsiella species is an urgent global threat. The World Health Organization has listed Klebsiella pneumoniae as one of the global priority pathogens in critical need of next-generation antibiotics. Compared to other Gram-negative pathogens, K. pneumoniae accumulates a greater diversity of antimicrobial-resistant genes at a higher frequency. The evolution of a hypervirulent phenotype of K. pneumoniae is yet another concern. It has a broad ecological distribution affecting humans, agricultural animals, plants, and aquatic animals. Extracellular polysaccharides of Klebsiella, such as lipopolysaccharides, capsular polysaccharides, and exopolysaccharides, play crucial roles in conferring resistance against the host immune response, as well as in colonization, surface adhesion, and for protection against antibiotics and bacteriophages. These extracellular polysaccharides are major virulent determinants and are highly divergent with respect to their antigenic properties. Wzx/Wzy-, ABC-, and synthase-dependent proteinaceous nano-machineries are involved in the biosynthesis, transport, and cell surface expression of these sugar molecules. Although the proteins involved in the biosynthesis and surface expression of these sugar molecules represent potential drug targets, variation in the amino acid sequences of some of these proteins, in combination with diversity in their sugar composition, poses a major challenge to the design of a universal drug for Klebsiella infections. This review discusses the challenges in universal Klebsiella vaccine and drug development from the perspective of antigen sugar compositions and the proteins involved in extracellular antigen transport.

Evolution of Klebsiella pneumoniae with mucoid and non-mucoid type colonies within a single patient

We obtained nine Klebsiella pneumoniae isolates successively isolated from a single patient. Four pairs (M1-M4 and NM1-NM4) obtained simultaneously from the same site showed different colony types, mucoid and non-mucoid, while the final isolate (M5) was isolated alone from the blood and showed a mucoid phenotype. The whole genome of isolate M5 was sequenced de novo using the PacBio RSII system, while the others were sequenced with an Illumina Hiseq4000 and mapped to the genome sequences of M5. To identify insertions or deletions in the cps locus, we amplified and sequenced cps locus genes. We identified insertion sequence (IS) elements in several genes of the cps locus or one amino acid substitution in WcaJ in all non-mucoid isolates. Five additional amino acid alterations in RpsJ, LolE, Lon-2, PpsE, and a hypothetical protein were detected in some mucoid and non-mucoid isolates. Based on the genome data and cps locus sequences, the mucoid phenotype may have been lost or converted into the non-mucoid phenotype because of the insertion of IS elements or amino acid alterations at this locus. We inferred a within-host evolutionary scenario, in which non-mucoid variants emerged repeatedly from mucoid isolates, but may be short-lived because of their low fitness.

Transcriptional regulation of galF by RcsAB affects capsular polysaccharide formation in Klebsiella pneumoniae NTUH-K2044

RcsAB is an atypical two-component regulatory system that can regulate exopolysaccharide biosynthesis and is involved in the virulence of K. pneumoniae. The gene galF is well known as a gene involved in the biosynthesis of capsular polysaccharide (CPS). The specific DNA identification sequence for transcriptional regulation of RcsAB was found to be present in the promoter region of galF. This study aimed to detect the function of RcsAB in virulence and in biofilm and CPS formation. In addition, the transcriptional regulation of the galF gene in K. pneumoniae was studied. To determine the function of rcsAB gene, the wild-type K. pneumoniae strain NTUH-K2044 and the rcsAB knockout and complemented strains were used. The results showed decreased virulence, biofilm formation, and CPS levels in the rcsAB knockout strain. Complementation of the knockout by introducing an rcsAB fragment on an expression plasmid partially restored the virulence, biofilm, and CPS functions of the knockout strain. It indicated that the rcsAB genes might affect CPS formation and virulence of K. pneumonia. RT-qPCR, EMSA and DNase I footprinting assays were conducted to identify the transcriptional regulation of galF by RcsAB. RcsAB was seen to bind to the galF promoter-proximal region, and the binding site was further identified to be located from -177 bp to -152 bp upstream of the galF promoter. In conclusion, RcsAB could regulate the transcription of the galF gene positively by binding to the galF promoter DNA directly, and then affects the CPS formation of K. pneumonia.

Identification of Two Regulators of Virulence That Are Conserved in Klebsiella pneumoniae Classical and Hypervirulent Strains

Klebsiella pneumoniae is widely recognized as a pathogen with a propensity for acquiring antibiotic resistance. It is capable of causing a range of hospital-acquired infections (urinary tract infections [UTI], pneumonia, sepsis) and community-acquired invasive infections. The genetic heterogeneity of K. pneumoniae isolates complicates our ability to understand the virulence of K. pneumoniae. Characterization of virulence factors conserved between strains as well as strain-specific factors will improve our understanding of this important pathogen. The MarR family of regulatory proteins is widely distributed in bacteria and regulates cellular processes such as antibiotic resistance and the expression of virulence factors. Klebsiella encodes numerous MarR-like proteins, and they likely contribute to the ability of K. pneumoniae to respond to and survive under a wide variety of environmental conditions, including those present in the human body. We tested loss-of-function mutations in all the marR homologues in a murine pneumonia model and found that two (kvrA and kvrB) significantly impacted the virulence of K1 and K2 capsule type hypervirulent (hv) strains and that kvrA affected the virulence of a sequence type 258 (ST258) classical strain. In the hv strains, kvrA and kvrB mutants displayed phenotypes associated with reduced capsule production, mucoviscosity, and transcription from galF and manC promoters that drive expression of capsule synthesis genes. In contrast, kvrA and kvrB mutants in the ST258 strain had no effect on capsule gene expression or capsule-related phenotypes. Thus, KvrA and KvrB affect virulence in classical and hv strains but the effect on virulence may not be exclusively due to effects on capsule production.

In addition to having a reputation as the causative agent for hospital-acquired infections as well as community-acquired invasive infections, Klebsiella pneumoniae has gained widespread attention as a pathogen with a propensity for acquiring antibiotic resistance. Due to the rapid emergence of carbapenem resistance among K. pneumoniae strains, a better understanding of virulence mechanisms and identification of new potential drug targets are needed. This study identified two novel regulators (KvrA and KvrB) of virulence in K. pneumoniae and demonstrated that their effect on virulence in invasive strains is likely due in part to effects on capsule production (a major virulence determinant) and hypermucoviscosity. KvrA also impacts the virulence of classical strains but does not appear to affect capsule gene expression in this strain. KvrA and KvrB are conserved among K. pneumoniae strains and thus could regulate capsule expression and virulence in diverse strains regardless of capsule type.

Establishment of a Molecular Serotyping Scheme and a Multiplexed Luminex-Based Array for Enterobacter aerogenes

Serotyping based on surface polysaccharide antigens is important for the clinical detection and epidemiological surveillance of pathogens. Polysaccharide gene clusters (PSgcs) are typically responsible for the diversity of bacterial surface polysaccharides. Through whole-genome sequencing and analysis, eight putative PSgc types were identified in 23 Enterobacter aerogenes strains from several geographic areas, allowing us to present the first molecular serotyping system for E. aerogenes. A conventional antigenic scheme was also established and correlated well with the molecular serotyping system that was based on PSgc genetic variation, indicating that PSgc-based molecular typing and immunological serology provide equally valid results. Further, a multiplex Luminex-based array was developed, and a double-blind test was conducted with 97 clinical specimens from Shanghai, China, to validate our array. The results of these analyses indicated that strains containing PSgc4 and PSgc7 comprised the predominant groups. We then examined 86 publicly available E. aerogenes strain genomes and identified an additional seven novel PSgc types, with PSgc10 being the most abundant type. In total, our study identified 15 PSgc types in E. aerogenes, providing the basis for a molecular serotyping scheme. From these results, differing epidemic patterns were identified between strains that were predominant in different regions. Our study highlights the feasibility and reliability of a serotyping system based on PSgc diversity, and for the first time, presents a molecular serotyping system, as well as an antigenic scheme for E. aerogenes, providing the basis for molecular diagnostics and epidemiological surveillance of this important emerging pathogen.

Capsule Production and Glucose Metabolism Dictate Fitness during Serratia marcescens Bacteremia

Serratia marcescens is an opportunistic pathogen that causes a range of human infections, including bacteremia, keratitis, wound infections, and urinary tract infections. Compared to other members of the Enterobacteriaceae family, the genetic factors that facilitate Serratia proliferation within the mammalian host are less well defined. An in vivo screen of transposon insertion mutants identified 212 S. marcescens fitness genes that contribute to bacterial survival in a murine model of bloodstream infection. Among those identified, 11 genes were located within an 18-gene cluster encoding predicted extracellular polysaccharide biosynthesis proteins. A mutation in the wzx gene contained within this locus conferred a loss of fitness in competition infections with the wild-type strain and a reduction in extracellular uronic acids correlating with capsule loss. A second gene, pgm, encoding a phosphoglucomutase exhibited similar capsule-deficient phenotypes, linking central glucose metabolism with capsule production and fitness of Serratia during mammalian infection. Further evidence of the importance of central metabolism was obtained with a pfkA glycolytic mutant that demonstrated reduced replication in human serum and during murine infection. An MgtB magnesium transporter homolog was also among the fitness factors identified, and an S. marcescens mgtB mutant exhibited decreased growth in defined medium containing low concentrations of magnesium and was outcompeted ~10-fold by wild-type bacteria in mice. Together, these newly identified genes provide a more complete understanding of the specific requirements for S. marcescens survival in the mammalian host and provide a framework for further investigation of the means by which S. marcescens causes opportunistic infections.

Serratia marcescens is a remarkably prolific organism that replicates in diverse environments, including as an opportunistic pathogen in human bacteremia. The genetic requirements for S. marcescens survival in the mammalian bloodstream were defined in this work by transposon insertion sequencing. In total, 212 genes that contribute to bacterial fitness were identified. When sorted via biological function, two of the major fitness categories identified herein were genes encoding capsule polysaccharide biogenesis functions and genes involved in glucose utilization. Further investigation determined that certain glucose metabolism fitness genes are also important for the generation of extracellular polysaccharides. Together, these results identify critical biological processes that allow S. marcescens to colonize the mammalian bloodstream.

E. coli Group 1 Capsular Polysaccharide Exportation Nanomachinary as a Plausible Antivirulence Target in the Perspective of Emerging Antimicrobial Resistance

Bacteria evolving resistance against the action of multiple drugs and its ability to disseminate the multidrug resistance trait(s) across various strains of the same bacteria or different bacterial species impose serious threat to public health. Evolution of such multidrug resistance is due to the fact that, most of the antibiotics target bacterial survival mechanisms which exert selective pressure on the bacteria and aids them to escape from the action of antibiotics. Nonetheless, targeting bacterial virulence strategies such as bacterial surface associated polysaccharides biosynthesis and their surface accumulation mechanisms may be an attractive strategy, as they impose less selective pressure on the bacteria. Capsular polysaccharide (CPS) or K-antigen that is located on the bacterial surface armors bacteria from host immune response. Thus, unencapsulating bacteria would be a good strategy for drug design, besides CPS itself being a good vaccine target, by interfering with CPS biosynthesis and surface assembly pathway. Gram-negative Escherichia coli uses Wzy-polymerase dependent (Groups 1 and 4) and ATP dependent (Groups 1 and 3) pathways for CPS production. Considering E. coli as a case in point, this review explains the structure and functional roles of proteins involved in Group 1 Wzy dependent CPS biosynthesis, surface expression and anchorage in relevance to drug and vaccine developments.

Key diffusion mechanisms involved in regulating bidirectional water permeation across E. coli outer membrane lectin

Capsular polysaccharides (CPSs) are major bacterial virulent determinants that facilitate host immune evasion. E. coli group1 K30_CPS is noncovalently attached to bacterial surface by Wzi, a lectin. Intriguingly, structure based phylogenetic analysis indicates that Wzi falls into porin superfamily. Molecular dynamics (MD) simulations further shed light on dual role of Wzi as it also functions as a bidirectional passive water specific porin. Such a functional role of Wzi was not realized earlier, due to the occluded pore. While five water specific entry points distributed across extracellular & periplasmic faces regulate the water diffusion involving different mechanisms, a luminal hydrophobic plug governs water permeation across the channel. Coincidently, MD observed open state structure of “YQF” triad is seen in sugar-binding site of sodium-galactose cotransporters, implicating its involvement in K30_CPS surface anchorage. Importance of Loop 5 (L5) in membrane insertion is yet another highlight. Change in water diffusion pattern of periplasmic substitution mutants suggests Wzi’s role in osmoregulation by aiding in K30_CPS hydration, corroborating earlier functional studies. Water molecules located inside β-barrel of Wzi crystal structure further strengthens the role of Wzi in osmoregulation. Thus, interrupting water diffusion or L5 insertion may reduce bacterial virulence.

Klebsiella pneumoniae: Going on the Offense with a Strong Defense

Klebsiella pneumoniae causes a wide range of infections, including pneumonias, urinary tract infections, bacteremias, and liver abscesses. Historically, K. pneumoniae has caused serious infection primarily in immunocompromised individuals, but the recent emergence and spread of hypervirulent strains have broadened the number of people susceptible to infections to include those who are healthy and immunosufficient. Furthermore, K. pneumoniae strains have become increasingly resistant to antibiotics, rendering infection by these strains very challenging to treat. The emergence of hypervirulent and antibiotic-resistant strains has driven a number of recent studies. Work has described the worldwide spread of one drug-resistant strain and a host defense axis, interleukin-17 (IL-17), that is important for controlling infection. Four factors, capsule, lipopolysaccharide, fimbriae, and siderophores, have been well studied and are important for virulence in at least one infection model. Several other factors have been less well characterized but are also important in at least one infection model. However, there is a significant amount of heterogeneity in K. pneumoniae strains, and not every factor plays the same critical role in all virulent Klebsiella strains. Recent studies have identified additional K. pneumoniae virulence factors and led to more insights about factors important for the growth of this pathogen at a variety of tissue sites. Many of these genes encode proteins that function in metabolism and the regulation of transcription. However, much work is left to be done in characterizing these newly discovered factors, understanding how infections differ between healthy and immunocompromised patients, and identifying attractive bacterial or host targets for treating these infections.

Adhesins Involved in Attachment to Abiotic Surfaces by Gram-Negative Bacteria

During the first step of biofilm formation, initial attachment is dictated by physicochemical and electrostatic interactions between the surface and the bacterial envelope. Depending on the nature of these interactions, attachment can be transient or permanent. To achieve irreversible attachment, bacterial cells have developed a series of surface adhesins promoting specific or nonspecific adhesion under various environmental conditions. This article reviews the recent advances in our understanding of the secretion, assembly, and regulation of the bacterial adhesins during biofilm formation, with a particular emphasis on the fimbrial, nonfimbrial, and discrete polysaccharide adhesins in Gram-negative bacteria.

Masquerading microbial pathogens: capsular polysaccharides mimic host-tissue molecules

The increasing prevalence of antibiotic-resistant bacteria portends an impending postantibiotic age, characterized by diminishing efficacy of common antibiotics and routine application of multifaceted, complementary therapeutic approaches to treat bacterial infections, particularly multidrug-resistant organisms. The first line of defense for most bacterial pathogens consists of a physical and immunologic barrier known as the capsule, commonly composed of a viscous layer of carbohydrates that are covalently bound to the cell wall in Gram-positive bacteria or often to lipids of the outer membrane in many Gram-negative bacteria. Bacterial capsular polysaccharides are a diverse class of high molecular weight polysaccharides contributing to virulence of many human pathogens in the gut, respiratory tree, urinary tract, and other host tissues, by hiding cell surface components that might otherwise elicit host immune response. This review highlights capsular polysaccharides that are structurally identical or similar to polysaccharides found in mammalian tissues, including polysialic acid and glycosaminoglycan capsules hyaluronan, heparosan, and chondroitin. Such nonimmunogenic coatings render pathogens insensitive to certain immune responses, effectively increasing residence time in host tissues and enabling pathologically relevant population densities to be reached. Biosynthetic pathways and capsular involvement in immune system evasion are described, providing a basis for potential therapies aimed at supplementing or replacing antibiotic treatment.

O-antigens of bacteria of the genus providencia: structure, serology, genetics, and biosynthesis

The genus Providencia consists of eight species of opportunistic pathogenic enterobacteria that can cause enteric diseases and urinary tract infections. The existing combined serological classification scheme of three species, P. alcalifaciens, P. stuartii, and P. rustigianii, is based on the specificity of O-antigens (O-polysaccharides) and comprises 63 O-serogroups. Differences between serogroups are related to polymorphism at a specific genome locus, the O-antigen gene cluster, responsible for O-antigen biosynthesis. This review presents data on structures of 36 O-antigens of Providencia, many of which contain unusual monosaccharides and non-carbohydrate components. The structural data correlate with the immunospecificity of the O-antigens and enable substantiation on a molecular level of serological relationships within the genus Providencia and between strains of Providencia and bacteria of the genera Proteus, Escherichia, and Salmonella. Peculiar features of the O-antigen gene cluster organization in 10 Providencia serogroups and biosynthetic pathways of nucleotide precursors of specific monosaccharide components of the O-antigens also are discussed.

wzi Gene sequencing, a rapid method for determination of capsular type for Klebsiella strains

Pathogens of the genus Klebsiella have been classified into distinct capsular (K) types for nearly a century. K typing of Klebsiella species still has important applications in epidemiology and clinical microbiology, but the serological method has strong practical limitations. Our objective was to evaluate the sequencing of wzi, a gene conserved in all capsular types of Klebsiella pneumoniae that codes for an outer membrane protein involved in capsule attachment to the cell surface, as a simple and rapid method for the prediction of K type. The sequencing of a 447-nucleotide region of wzi distinguished the K-type reference strains with only nine exceptions. A reference wzi sequence database was created by the inclusion of multiple strains representing K types associated with high virulence and multidrug resistance. A collection of 119 prospective clinical isolates of K. pneumoniae were then analyzed in parallel by wzi sequencing and classical K typing. Whereas K typing achieved typeability for 81% and discrimination for 94.4% of the isolates, these figures were 98.1% and 98.3%, respectively, for wzi sequencing. The prediction of K type once the wzi allele was known was 94%. wzi sequencing is a rapid and simple method for the determination of the K types of most K. pneumoniae clinical isolates.

Wzi is an outer membrane lectin that underpins group 1 capsule assembly in Escherichia coli

Many pathogenic bacteria encase themselves in a polysaccharide capsule that provides a barrier to the physical and immunological challenges of the host. The mechanism by which the capsule assembles around the bacterial cell is unknown. Wzi, an integral outer-membrane protein from Escherichia coli, has been implicated in the formation of group 1 capsules. The 2.6 Å resolution structure of Wzi reveals an 18-stranded β-barrel fold with a novel arrangement of long extracellular loops that blocks the extracellular entrance and a helical bundle that plugs the periplasmic end. Mutagenesis shows that specific extracellular loops are required for in vivo capsule assembly. The data show that Wzi binds the K30 carbohydrate polymer and, crucially, that mutants functionally deficient in vivo show no binding to K30 polymer in vitro. We conclude that Wzi is a novel outer-membrane lectin that assists in the formation of the bacterial capsule via direct interaction with capsular polysaccharides.

Variation in the complex carbohydrate biosynthesis loci of Acinetobacter baumannii genomes

Extracellular polysaccharides are major immunogenic components of the bacterial cell envelope. However, little is known about their biosynthesis in the genus Acinetobacter, which includes A. baumannii, an important nosocomial pathogen. Whether Acinetobacter sp. produce a capsule or a lipopolysaccharide carrying an O antigen or both is not resolved. To explore these issues, genes involved in the synthesis of complex polysaccharides were located in 10 complete A. baumannii genome sequences, and the function of each of their products was predicted via comparison to enzymes with a known function. The absence of a gene encoding a WaaL ligase, required to link the carbohydrate polymer to the lipid A-core oligosaccharide (lipooligosaccharide) forming lipopolysaccharide, suggests that only a capsule is produced. Nine distinct arrangements of a large capsule biosynthesis locus, designated KL1 to KL9, were found in the genomes. Three forms of a second, smaller variable locus, likely to be required for synthesis of the outer core of the lipid A-core moiety, were designated OCL1 to OCL3 and also annotated. Each K locus includes genes for capsule export as well as genes for synthesis of activated sugar precursors, and for glycosyltransfer, glycan modification and oligosaccharide repeat-unit processing. The K loci all include the export genes at one end and genes for synthesis of common sugar precursors at the other, with a highly variable region that includes the remaining genes in between. Five different capsule loci, KL2, KL6, KL7, KL8 and KL9 were detected in multiply antibiotic resistant isolates belonging to global clone 2, and two other loci, KL1 and KL4, in global clone 1. This indicates that this region is being substituted repeatedly in multiply antibiotic resistant isolates from these clones.

Polysaccharide export outer membrane proteins in Gram-negative bacteria

Polysaccharide export outer membrane proteins of Gram-negative bacteria are involved in the export of polysaccharides across the outer membrane. The mechanisms of polysaccharide export across the outer membrane in Gram-negative bacteria are not yet completely clear. However, the mechanisms of polysaccharide assembly in Escherichia coli have been intensively investigated. Here, we mainly review the current understanding of the assembly mechanisms of group 1 capsular polysaccharide, group 2 capsular polysaccharide and lipopolysaccharide of E. coli, and the current structures and interactions of some polysaccharide export outer membrane proteins with other proteins involved in polysaccharide export in Gram-negative bacteria. In addition, LptD may be targeted by peptidomimetic antibiotics in Gram-negative bacteria. We also give insights into the directions of future research regarding the mechanisms of polysaccharide export.

Pyrosequencing-based analysis reveals a novel capsular gene cluster in a KPC-producing Klebsiella pneumoniae clinical isolate identified in Brazil

BACKGROUND

An important virulence factor of Klebsiella pneumoniae is the production of capsular polysaccharide (CPS), a thick mucus layer that allows for evasion of the host's defense and creates a barrier against antibacterial peptides. CPS production is driven mostly by the expression of genes located in a locus called cps, and the resulting structure is used to distinguish between different serotypes (K types). In this study, we report the unique genetic organization of the cps cluster from K. pneumoniae Kp13, a clinical isolate recovered during a large outbreak of nosocomial infections that occurred in a Brazilian teaching hospital.

RESULTS

A pyrosequencing-based approach showed that the cps region of Kp13 (cpsKp13) is 26.4 kbp in length and contains genes common, although not universal, to other strains, such as the rmlBADC operon that codes for L-rhamnose synthesis. cpsKp13 also presents some unique features, like the inversion of the wzy gene and a unique repertoire of glycosyltransferases. In silico comparison of cpsKp13 RFLP pattern with 102 previously published cps PCR-RFLP patterns showed that cpsKp13 is distinct from the C patterns of all other K serotypes. Furthermore, in vitro serotyping showed only a weak reaction with capsular types K9 and K34. We confirm that K9 cps shares common genes with cpsKp13 such as the rmlBADC operon, but lacks features like uge and Kp13-specific glycosyltransferases, while K34 capsules contain three of the five sugars that potentially form the Kp13 CPS.

CONCLUSIONS

We report the first description of a cps cluster from a Brazilian clinical isolate of a KPC-producing K. pneumoniae. The gathered data including K-serotyping support that Kp13's K-antigen belongs to a novel capsular serotype. The CPS of Kp13 probably includes L-rhamnose and D-galacturonate in its structure, among other residues. Because genes involved in L-rhamnose biosynthesis are absent in humans, this pathway may represent potential targets for the development of antimicrobial agents. Studying the capsular serotypes of clinical isolates is of great importance for further development of vaccines and/or novel therapeutic agents. The distribution of K-types among multidrug-resistant isolates is unknown, but our findings may encourage scientists to perform K-antigen typing of KPC-producing strains worldwide.

Localization and molecular characterization of putative O antigen gene clusters of Providencia species

Enterobacteria of the genus Providencia are opportunistic human pathogens associated with urinary tract and wound infections, as well as enteric diseases. The lipopolysaccharide (LPS) O antigen confers major antigenic variability upon the cell surface and is used for serotyping of Gram-negative bacteria. Recently, Providencia O antigen structures have been extensively studied, but no data on the location and organization of the O antigen gene cluster have been reported. In this study, the four Providencia genome sequences available were analysed, and the putative O antigen gene cluster was identified in the polymorphic locus between the cpxA and yibK genes. This finding provided the necessary information for designing primers, and cloning and sequencing the O antigen gene clusters from five more Providencia alcalifaciens strains. The gene functions predicted in silico were in agreement with the known O antigen structures; furthermore, annotation of the genes involved in the three-step synthesis of GDP-colitose (gmd, colD and colC) was supported by cloning and biochemical characterization of the corresponding enzymes. In one strain (P. alcalifaciens O39), no polysaccharide product of the gene cluster in the cpxA-yibK locus was found, and hence genes for synthesis of the existing O antigen are located elsewhere in the genome. In addition to the putative O antigen synthesis genes, homologues of wza, wzb, wzc and (in three strains) wzi, required for the surface expression of capsular polysaccharides, were found upstream of yibK in all species except Providencia rustigianii, suggesting that the LPS of these species may be attributed to the so-called K LPS (K(LPS)). The data obtained open a way for development of a PCR-based typing method for identification of Providencia isolates.

Legionella pneumophila LbtU acts as a novel, TonB-independent receptor for the legiobactin siderophore

Gram-negative Legionella pneumophila produces a siderophore (legiobactin) that promotes lung infection. We previously determined that lbtA and lbtB are required for the synthesis and secretion of legiobactin. DNA sequence and reverse transcription-PCR (RT-PCR) analyses now reveal the presence of an iron-repressed gene (lbtU) directly upstream of the lbtAB-containing operon. In silico analysis predicted that LbtU is an outer membrane protein consisting of a 16-stranded transmembrane β-barrel, multiple extracellular domains, and short periplasmic tails. Immunoblot analysis of cell fractions confirmed an outer membrane location for LbtU. Although replicating normally in standard media, lbtU mutants, like lbtA mutants, were impaired for growth on iron-depleted agar media. While producing typical levels of legiobactin, lbtU mutants were unable to use supplied legiobactin to stimulate growth on iron-depleted media and displayed an inability to take up iron. Complemented lbtU mutants behaved as the wild type did. The lbtU mutants were also impaired for infection in a legiobactin-dependent manner. Together, these data indicate that LbtU is involved in the uptake of legiobactin and, based upon its location, is most likely the Legionella siderophore receptor. The sequence and predicted two-dimensional (2D) and 3D structures of LbtU were distinct from those of all known siderophore receptors, which generally contain a 22-stranded β-barrel and an extended N terminus that binds TonB in order to transduce energy from the inner membrane. This observation coupled with the fact that L. pneumophila does not encode TonB suggests that LbtU is a new type of receptor that participates in a form of iron uptake that is mechanistically distinct from the existing paradigm.

Crystallization and preliminary diffraction analysis of Wzi, a member of the capsule export and assembly pathway in Escherichia coli

External polysaccharide capsules provide a physical barrier that is employed by many species of bacteria for the purposes of host evasion and persistence. Wzi is a 53 kDa outer membrane β-barrel protein that is thought to play a role in the attachment of group 1 capsular polysaccharides to the cell surface. The purification and crystallization of an Escherichia coli homologue of Wzi is reported and diffraction data from native and selenomethionine-incorporated protein crystals are presented. Crystals of C-terminally His6-tagged Wzi diffracted to 2.8 Å resolution. Data processing showed that the crystals belonged to the orthorhombic space group C222, with unit-cell parameters a=128.8, b=152.8, c=94.4 Å, α=β=γ=90°. A His-tagged selenomethionine-containing variant of Wzi has also been crystallized in the same space group and diffraction data have been recorded to 3.8 Å resolution. Data processing shows that the variant crystal has similar unit-cell parameters to the native crystal.

A localized multimeric anchor attaches the Caulobacter holdfast to the cell pole

Caulobacter crescentus attachment is mediated by the holdfast, a complex of polysaccharide anchored to the cell by HfaA, HfaB and HfaD. We show that all three proteins are surface exposed outer membrane (OM) proteins. HfaA is similar to fimbrial proteins and assembles into a high molecular weight (HMW) form requiring HfaD, but not holdfast polysaccharide. The HfaD HMW form is dependent on HfaA but not on holdfast polysaccharide. We show that HfaA and HfaD form homomultimers and that they require HfaB for stability and OM translocation. All three proteins localize to the late pre-divisional flagellar pole, remain at this pole in swarmer cells, and localize at the stalk tip after the stalk is synthesized at the same pole. Hfa protein localization requires the holdfast polysaccharide secretion proteins and the polar localization factor PodJ. An hfaB mutant is much more severely deficient in adherence and holdfast attachment than hfaA and hfaD mutants. An hfaA, hfaD double mutant phenocopies either single mutant, suggesting that HfaB is involved in holdfast attachment beyond secretion of HfaA and HfaD. We hypothesize that HfaB secretes HfaA and HfaD across the outer membrane, and the three proteins form a complex anchoring the holdfast to the stalk.

Role of NtrC-regulated exopolysaccharides in the biofilm formation and pathogenic interaction of Vibrio vulnificus

Vibrio vulnificus has been shown to require a global transcription factor, NtrC for mature biofilm development via controlling the biosyntheses of lipopolysaccharide and exopolysaccharide (EPS). Biofilm formation and EPS production were dramatically increased in a medium including a tricarboxylic acid cycle-intermediate as a carbon source. These phenotypes required functional NtrC and were abolished by the addition of ammonium chloride. During the initial stage of biofilm formation, both expression of the ntrC gene and the cellular content of NtrC protein increased. Thus, the regulatory roles of NtrC in EPS biosynthesis were studied with three gene clusters for EPS biosyntheses. Transcriptions of the three clusters were positively controlled by NtrC and showed maximal expression at the early stage of biofilm development. Mutants deficient in one of the genes (VV1_2661, VV2_1579 and VV1_2305) in each cluster showed decreased production of EPS, attenuated ability to form biofilm and lowered cytoadherence to human epithelial cells. However, mutations in VV2_1579 and VV1_2305 resulted in lower cytotoxicity to human cells and mortality to mice than the mutation in VV1_2661. These results demonstrate that NtrC-regulated EPS are crucial in biofilm formation of V. vulnificus, and some EPS components play important roles in interacting with hosts.

Structure and Assembly of Escherichia coli Capsules

The capsule is a cell surface structure composed of long-chain polysaccharides that envelops many isolates of Escherichia coli. It protects the cell against host defenses or physical environmental stresses, such as desiccation. The component capsular polysaccharides (CPSs) are major surface antigens in E. coli. They are named K antigens (after the German word Kapsel). Due to variations in CPS structures, more than 80 serologically unique K antigens exist in E. coli. Despite the hypervariability in CPS structures, only two capsule-assembly strategies exist in E. coli. These have led to the assignment of group 1 and group 2 capsules, and many of the key elements of the corresponding assembly pathways have been resolved. Structural features, as well as genetic and regulatory variations, give rise to additional groups 3 and 4. These employ the same biosynthesis processes described in groups 2 and 1, respectively. Each isolate possesses a distinctive set of cytosolic and inner-membrane enzymes, which generate a precise CPS structure, defining a given K serotype. Once synthesized, a multiprotein complex is needed to translocate the nascent CPS across the Gram-negative cell envelope to the outer surface of the outer membrane, where the capsule structure is assembled. While the translocation machineries for group 1 and group 2 CPSs are fundamentally different from one another, they possess no specificity for a given CPS structure. Each is conserved in all isolates producing capsules belonging to a particular group.