The O-Antigen Capsule of Salmonella enterica Serovar Typhimurium Facilitates Serum Resistance and Surface Expression of FliC

Lipopolysaccharides and outer membrane proteins as main structures involved in complement evasion strategies of non-typhoidal Salmonella strains

Non-typhoidal Salmonella (NTS) infections pose a serious public health problem. In addition to the typical course of salmonellosis, an infection with Salmonella bacteria can often lead to parenteral infections and sepsis, which are particularly dangerous for children, the elderly and immunocompromised. Bacterial resistance to serum is a key virulence factor for the development of systemic infections. Salmonella, as an enterobacterial pathogen, has developed several mechanisms to escape and block the antibacterial effects of the complement system. In this review, we discuss the relevance of outer membrane polysaccharides to the complement evasion mechanisms of NTS strains. These include the influence of the overall length and density of the lipopolysaccharide molecules, modifications of the O-antigen lipopolysaccharide composition and the role of capsular polysaccharides in opsonization and protection of the outer membrane from the lytic action of complement. Additionally, we discuss specific outer membrane protein complement evasion mechanisms, such as recruitment of complement regulatory proteins, blocking assembly of late complement components to form the membrane attack complex and the proteolytic cleavage of complement proteins.

Molecular determinants of peaceful coexistence versus invasiveness of non-Typhoidal Salmonella: Implications in long-term side-effects

The genus Salmonella represents a wide range of strains including Typhoidal and Non-Typhoidal Salmonella (NTS) isolates that exhibit illnesses of varied pathophysiologies. The more frequent NTS ensues a self-limiting enterocolitis with rare occasions of bacteremia or systemic infections. These self-limiting Salmonella strains are capable of subverting and dampening the host immune system to achieve a more prolonged survival inside the host system thus leading to chronic manifestations. Notably, emergence of new invasive NTS isolates known as invasive Non-Typhoidal Salmonella (iNTS) have worsened the disease burden significantly in some parts of the world. NTS strains adapt to attain persister phenotype intracellularly and cause relapsing infections. These chronic infections, in susceptible hosts, are also capable of causing diseases like IBS, IBD, reactive arthritis, gallbladder cancer and colorectal cancer. The present understanding of molecular mechanism of how these chronic infections are manifested is quite limited. The current work is an effort to review the prevailing knowledge emanating from a large volume of research focusing on various forms of NTS infections including those that cause localized, systemic and persistent disease. The review will further dwell into the understanding of how this pathogen contributes to the associated long term sequelae.

Salmonella Biofilms Tolerate Hydrogen Peroxide by a Combination of Extracellular Polymeric Substance Barrier Function and Catalase Enzymes

The ability of Salmonella enterica subspecies enterica serovar Typhi (S. Typhi) to cause chronic gallbladder infections is dependent on biofilm growth on cholesterol gallstones. Non-typhoidal Salmonella (e.g. S. Typhimurium) also utilize the biofilm state to persist in the host and the environment. How the pathogen maintains recalcitrance to the host response, and oxidative stress in particular, during chronic infection is poorly understood. Previous experiments demonstrated that S. Typhi and S. Typhimurium biofilms are tolerant to hydrogen peroxide (H₂O₂), but that mutations in the biofilm extracellular polymeric substances (EPSs) O antigen capsule, colanic acid, or Vi antigen reduce tolerance. Here, biofilm-mediated tolerance to oxidative stress was investigated using a combination of EPS and catalase mutants, as catalases are important detoxifiers of H₂O₂. Using co-cultured biofilms of wild-type (WT) bacteria with EPS mutants, it was demonstrated that colanic acid in S. Typhimurium and Vi antigen in S. Typhi have a community function and protect all biofilm-resident bacteria rather than to only protect the individual cells producing the EPSs. However, the H₂O₂ tolerance deficiency of a O antigen capsule mutant was unable to be compensated for by co-culture with WT bacteria. For curli fimbriae, both WT and mutant strains are tolerant to H₂O₂ though unexpectedly, co-cultured WT/mutant biofilms challenged with H₂O₂ resulted in sensitization of both strains, suggesting a more nuanced oxidative resistance alteration in these co-cultures. Three catalase mutant (katE, katG and a putative catalase) biofilms were also examined, demonstrating significant reductions in biofilm H₂O₂ tolerance for the katE and katG mutants. Biofilm co-culture experiments demonstrated that catalases exhibit a community function. We further hypothesized that biofilms are tolerant to H₂O₂ because the physical barrier formed by EPSs slows penetration of H₂O₂ into the biofilm to a rate that can be mitigated by intra-biofilm catalases. Compared to WT, EPS-deficient biofilms have a heighted response even to low-dose (2.5 mM) H₂O₂ challenge, confirming that resident bacteria of EPS-deficient biofilms are under greater stress and have limited protection from H₂O₂. Thus, these data provide an explanation for how Salmonella achieves tolerance to H₂O₂ by a combination of an EPS-mediated barrier and enzymatic detoxification.

A GMMA-CPS-Based Vaccine for Non-Typhoidal Salmonella

Non-typhoidal Salmonella are a major cause of gastroenteritis worldwide, as well as causing bloodstream infections in sub-Saharan Africa with a high fatality rate. No vaccine is currently available for human use. Current vaccine development strategies are focused on capsular polysaccharides (CPS) present on the surface of non-typhoidal Salmonella. This study aimed to boost the amount of CPS purified from S. Typhimurium for immunization trials. Random mutagenesis with Tn10 transposon increased the production of CPS colanic acid, by 10-fold compared to wildtype. Immunization with colanic acid or colanic acid conjugated to truncated glycoprotein D or inactivated diphtheria toxin did not induce a protective immune response in mice. However, immunization with Generalized Modules for Membrane Antigens (GMMAs) isolated from colanic acid overproducing isolates reduced Salmonella colonization in mice. Our results support the development of a GMMA-CPS-based vaccine against non-typhoidal Salmonella.

Pathogenic signature of invasive non-typhoidal Salmonella in Africa: implications for vaccine development

Invasive non-typhoidal Salmonella (iNTS) infections are a leading cause of bacteremia in Sub-Saharan Africa (sSA), thereby representing a major public health threat. Salmonella Typhimurium clade ST313 and Salmonella Enteriditis lineages associated with Western and Central/Eastern Africa are among the iNTS serovars which are of the greatest concern due to their case-fatality rate, especially in children and in the immunocompromised population. Identification of pathogen-associated features and host susceptibility factors that increase the risk for invasive non-typhoidal salmonellosis would be instrumental for the design of targeted prevention strategies, which are urgently needed given the increasing spread of multidrug-resistant iNTS in Africa. This review summarizes current knowledge of bacterial traits and host immune responses associated with iNTS infections in sSA, then discusses how this knowledge can guide vaccine development while providing a summary of vaccine candidates in preclinical and early clinical development.

The response of yeast vacuolar proteins: A novel rapid tool for Salmonella sp. screening

Human health is recently affected by several factors in which food contamination is one of the most dangerous elements that damage directly on our bodies. In this study, we provided a novel approach for the rapid detection of Salmonella sp. at the molecular level using the response of Saccharomyces cerevisiae's vacuoles. First, an augmentation of vacuoles intensity was observed by confocal microscopy after treating Salmonella strains with yeast cells. Second, the vacuolar enzymes were isolated and then analyzed by two-dimensional electrophoresis for the screening of specific biomarkers. After that, various recombinant yeasts containing exclusive biomarkers were constructed by fusing these biomarkers with several fluorescent proteins. Finally, the recombinant strains showed the ability to detect Salmonella strains specifically by appropriate fluorescent signals from 20 CFU/mL after 15 Min of exposure.

Immunomodulatory Roles of Polysaccharide Capsules in the Intestine

The interplay between the immune system and the microbiota in the human intestine dictates states of health vs. disease. Polysaccharide capsules are critical elements of bacteria that protect bacteria against environmental and host factors, including the host immune system. This review summarizes the mechanisms by which polysaccharide capsules from commensal and pathogenic bacteria in the gut microbiota modulate the innate and adaptive immune systems in the intestine. A deeper understanding of the roles of polysaccharide capsules in microbiota-immune interactions will provide a basis to harness their therapeutic potential to advance human health.

Eculizumab for Severe Thrombotic Microangiopathy Secondary to Surgical Invasive Stress and Bleeding

Atypical hemolytic uremic syndrome (aHUS) is an extremely rare condition caused by an excessive activation of the complement pathway based on genetic or acquired dysfunctions in complement regulation, leading to thrombotic microangiopathy (TMA). A complement-amplifying condition (CAC) can trigger aHUS occurrence along with complement abnormality. We herein report a case of severe TMA after laparoscopic myomectomy in a healthy woman. This case was eventually diagnosed as complement-mediated TMA secondary to surgical invasive stress as a CAC, with no definitive diagnosis of aHUS despite a genetic test. The patient fully recovered after several eculizumab administrations.

Structure of the capsule and lipopolysaccharide O-antigen from the channel catfish pathogen, Aeromonas hydrophila

A hypervirulent A. hydrophila (vAh) pathotype has been identified as the etiologic agent responsible for disease outbreaks in farmed carp species and channel catfish (Ictalurus punctatus) in China and the Southeastern United States, respectively. The possible route of infection has previously been unknown; however, virulence is believed to be multifactorial, involving the production/secretion of several virulence factors, including a high molecular weight group 4 capsular polysaccharide. Here we present chemical structural evidence of a novel capsule- and LPS-associated O-antigen found present in vAh isolated during these disease outbreaks. In this study, the chemical structure of the vAh O-antigen was determined by chemical analysis, Smith degradation, mass spectrometry, and 2D proton and carbon nuclear magnetic resonance (NMR) spectroscopy and found to be unique among described bacterial O-antigens. The O-antigen consists of hexasaccharide repeating units featuring a 4)-α-l-Fucp-(1-3)-β-d-GlcpNAc-(1-4)-α-l-Fucp-(1-4)-β-d-Glcp-(1- backbone, substituted with single residue side chains of α-d-Glcp and α-d-Quip3NAc linked to O-3 of the two fucose residues. The polysaccharide is partially O-acetylated on O-6 of the 4-substituted β-Glcp residue.

Applications of different solvents and conditions for differential extraction of lipopolysaccharide in Gram-negative bacteria

Lipopolysaccharide (LPS) is one of the major components in the outer membrane of Gram-negative bacteria. However, its heterogeneity and variability in different bacteria and differentiation conditions make it difficult to extract all of the structural variants. We designed a solution to improve quality and biological activity of LPS extracted from various bacteria with different types of LPS, as compared to conventional methods. We introduced a quality index as a simple measure of LPS purity in terms of a degree of polysaccharide content detected by absorbance at 204 nm. Further experiments using gel electrophoresis, endotoxin test, and macrophage activation test were performed to evaluate the performance and reliability of a proposed 'T-sol' method and the biological effectiveness and character of the LPS products. We presented that the T-sol method had differential effects on extraction of a RAW 264.7 cell-activating LPS, which was effective in the macrophage activation with similar effects in stimulating the production of TNF-alpha. In conclusion, the T-sol method provides a simple way to improve quality and biological activity of LPS with high yield.

Small alarmones (p)ppGpp regulate virulence associated traits and pathogenesis of Salmonella enterica serovar Typhi

How Salmonella enterica serovar Typhi (S. Typhi), an important human pathogen, survives the stressful microenvironments inside the gastrointestinal tract and within macrophages remains poorly understood. We report here that S. Typhi has a bonafide stringent response (SR) system, which is mediated by (p)ppGpp and regulates multiple virulence-associated traits and the pathogenicity of the S. Typhi Ty2 strain. In an iron overload mouse model of S. Typhi infection, the (p)ppGpp⁰ (Ty2ΔRelAΔSpoT) strain showed minimal systemic spread and no mortality, as opposed to 100% death of the mice challenged with the isogenic wild-type strain. Ty2ΔRelAΔSpoT had markedly elongated morphology with incomplete septa formation and demonstrated severely attenuated motility and chemotaxis due to the loss of flagella. Absence of the Vi-polysaccharide capsule rendered the mutant strain highly susceptible to complement-mediated lysis. The phenotypes of Ty2ΔRelAΔSpoT was contributed by transcriptional repression of several genes, including fliC, tviA, and ftsZ, as found by reverse transcriptase quantitative polymerase chain reaction and gene complementation studies. Finally, Ty2ΔRelAΔSpoT had markedly reduced invasion into intestinal epithelial cells and significantly attenuated survival within macrophages. To the best of our knowledge, this was the first study that addressed SR in S. Typhi and showed that (p)ppGpp was essential for optimal pathogenic fitness of the organism.

Comparative stress response to food preservation conditions of ST19 and ST213 genotypes of Salmonella enterica serotype Typhimurium

The replacement of the most prevalent Salmonella enterica genotypes has been documented worldwide. Here we tested the hypothesis that the current prevalent sequence type ST213 of serotype Typhimurium in Mexico has a higher resistance to stressful food preservation conditions than the displaced sequence ST19. ST19 showed higher cell viability percentages than ST213 in osmotic (685 mM NaCl) and acidic (pH 3.5) stress conditions and in combination with refrigeration (4 °C) and ambient (≈22 °C) temperatures. Both genotypes showed the same poststress recovery growth. ST213 formed biofilm and filamentous cells (FCs) under stress, whereas ST19 did not. ST213 cells also showed higher motility. The capacity of ST213 to form FCs may explain its lower viability percentages when compared with ST19, i.e., ST213 cells divided less under stress conditions, but FCs had the same recovery capacity of ST19 cells. ST213 presented a higher unsaturated/saturated fatty acids ratio (0.5-0.6) than ST19 (0.2-0.5), which indicates higher membrane fluidity. The transcript levels of the rpoS gene were similar between genotypes under the experimental conditions employed. Biofilm formation, the generation of FCs, cell motility and membrane modification seem to make ST213 more resistant than ST19 to food preservation environments.

Screening of c-di-GMP-Regulated Exopolysaccharides in Host Interacting Bacteria

Bacterial exopolysaccharides (EPS) often confer a survival advantage by protecting the cell against abiotic and biotic stresses, including host defensive factors. They are also main components of the extracellular matrix involved in cell-cell recognition, surface adhesion and biofilm formation. Biosynthesis of a growing number of EPS has been reported to be regulated by the ubiquitous second messenger c-di-GMP, which promotes the transition to a biofilm mode of growth in an intimate association with the eukaryotic host. Here we describe a strategy based on the combination of an approach to artificially increase the intracellular level of c-di-GMP in virtually any gram-negative bacteria with a high throughput screening (HTS) for the identification of monosaccharide composition and carbohydrate fingerprinting of novel EPS, or modified variants, that can be involved in host-bacteria interactions.

Edwardsiella tarda Sip2: A Serum-Induced Protein That Is Essential to Serum Survival, Acid Resistance, Intracellular Replication, and Host Infection

Edwardsiella tarda is a broad-host pathogen that can infect mammals, reptiles, and fish. E. tarda exhibits a remarkable ability to survive in host serum and replicate in host phagocytes, but the underlining mechanism is unclear. In this study, in order to identify E. tarda proteins involved in serum resistance, iTRAQ proteomic analysis was performed to examine the whole-cell protein profiles of TX01, a pathogenic E. tarda isolate, in response to serum treatment. Of the differentially expressed proteins identified, one (named Sip2) possesses a conserved hydrogenase domain and is homologous to the putative hydrogenase accessory protein HypB. When Sip2 was expressed in Escherichia coli, it significantly enhanced the survival of the host cells in serum. Compared to TX01, the sip2 knockout, TX01Δsip2, was dramatically reduced in the ability of hydrogenase activity, serum resistance, intracellular replication, dissemination in fish tissues, and causing mortality in infected fish. The lost virulence capacities of TX01Δsip2 were restored by complementation with the sip2 gene. Furthermore, TX01Δsip2 was significantly reduced in the capacity to grow under low pHs and iron-depleted conditions, and was unable to maintain its internal pH in acidic environment. Taken together, these results indicate that Sip2 is a novel serum-induced protein that is essential to serum resistance, cellular and tissue infection, and coping with acidic stress via its ability to modulate intracellular pH.

Periplasmic depolymerase provides insight into ABC transporter-dependent secretion of bacterial capsular polysaccharides

Capsules are critical virulence determinants for bacterial pathogens. They are composed of capsular polysaccharides (CPSs) with diverse structures, whose assembly on the cell surface is often powered by a conserved ABC transporter. Current capsule-assembly models include a contiguous trans-envelope channel directing nascent CPSs from the transporter to the cell surface. This conserved apparatus is an attractive target for antivirulence antimicrobial development. This work describes a CPS depolymerizing lyase enzyme found in the Burkholderiales and unique structural features that define its mechanism, CPS specificity, and evolution to function in the periplasm in a noncatabolic role. The activity of this enzyme provides evidence that CPS assembled in an ABC transporter-dependent system is exposed to periplasm during translocation to the cell surface.

Capsules are surface layers of hydrated capsular polysaccharides (CPSs) produced by many bacteria. The human pathogen Salmonella enterica serovar Typhi produces “Vi antigen” CPS, which contributes to virulence. In a conserved strategy used by bacteria with diverse CPS structures, translocation of Vi antigen to the cell surface is driven by an ATP-binding cassette (ABC) transporter. These transporters are engaged in heterooligomeric complexes proposed to form an enclosed translocation conduit to the cell surface, allowing the transporter to power the entire process. We identified Vi antigen biosynthesis genetic loci in genera of the Burkholderiales, which are paradoxically distinguished from S. Typhi by encoding VexL, a predicted pectate lyase homolog. Biochemical analyses demonstrated that VexL is an unusual metal-independent endolyase with an acidic pH optimum that is specific for O-acetylated Vi antigen. A 1.22-Å crystal structure of the VexL-Vi antigen complex revealed features which distinguish common secreted catabolic pectate lyases from periplasmic VexL, which participates in cell-surface assembly. VexL possesses a right-handed parallel β-superhelix, of which one face forms an electropositive glycan-binding groove with an extensive hydrogen bonding network that includes Vi antigen acetyl groups and confers substrate specificity. VexL provided a probe to interrogate conserved features of the ABC transporter-dependent export model. When introduced into S. Typhi, VexL localized to the periplasm and degraded Vi antigen. In contrast, a cytosolic derivative had no effect unless export was disrupted. These data provide evidence that CPS assembled in ABC transporter-dependent systems is actually exposed to the periplasm during envelope translocation.

Salmonella O48 Serum Resistance is Connected with the Elongation of the Lipopolysaccharide O-Antigen Containing Sialic Acid

Complement is one of the most important parts of the innate immune system. Some bacteria can gain resistance against the bactericidal action of complement by decorating their outer cell surface with lipopolysaccharides (LPSs) containing a very long O-antigen or with specific outer membrane proteins. Additionally, the presence of sialic acid in the LPS molecules can provide a level of protection for bacteria, likening them to human cells, a phenomenon known as molecular mimicry. Salmonella O48, which contains sialic acid in the O-antigen, is the major cause of reptile-associated salmonellosis, a worldwide public health problem. In this study, we tested the effect of prolonged exposure to human serum on strains from Salmonella serogroup O48, specifically on the O-antigen length. After multiple passages in serum, three out of four tested strains became resistant to serum action. The gas-liquid chromatography/tandem mass spectrometry analysis showed that, for most of the strains, the average length of the LPS O-antigen increased. Thus, we have discovered a link between the resistance of bacterial cells to serum and the elongation of the LPS O-antigen.

Relationship of Triamine-Biocide Tolerance of Salmonella enterica Serovar Senftenberg to Antimicrobial Susceptibility, Serum Resistance and Outer Membrane Proteins

A new emerging phenomenon is the association between the incorrect use of biocides in the process of disinfection in farms and the emergence of cross-resistance in Salmonella populations. Adaptation of the microorganisms to the sub-inhibitory concentrations of the disinfectants is not clear, but may result in an increase of sensitivity or resistance to antibiotics, depending on the biocide used and the challenged Salmonella serovar. Exposure of five Salmonella enterica subsp. enterica serovar Senftenberg (S. Senftenberg) strains to triamine-containing disinfectant did not result in variants with resistance to antibiotics, but has changed their susceptibility to normal human serum (NHS). Three biocide variants developed reduced sensitivity to NHS in comparison to the sensitive parental strains, while two isolates lost their resistance to serum. For S. Senftenberg, which exhibited the highest triamine tolerance (6 × MIC) and intrinsic sensitivity to 22.5% and 45% NHS, a downregulation of flagellin and enolase has been demonstrated, which might suggest a lower adhesion and virulence of the bacteria. This is the first report demonstrating the influence of biocide tolerance on NHS resistance. In conclusion, there was a potential in S. Senftenberg to adjust to the conditions, where the biocide containing triamine was present. However, the adaptation did not result in the increase of antibiotic resistance, but manifested in changes within outer membrane proteins’ patterns. The strategy of bacterial membrane proteins’ analysis provides an opportunity to adjust the ways of infection treatments, especially when it is connected to the life-threating bacteremia caused by Salmonella species.

The Effect of Colistin Resistance-Associated Mutations on the Fitness of Acinetobacter baumannii

Acinetobacter baumannii had emerged as an important nosocomial and opportunistic pathogen worldwide. To assess the evolution of colistin resistance in A. baumannii and its effect on bacterial fitness, we exposed five independent colonies of A. baumannii ATCC 17978 to increasing concentrations of colistin in agar (4/5) and liquid media (1/5). Stable resistant isolates were analyzed using whole genome sequencing. All strains were colistin resistant after exposure to colistin. In addition to the previously reported lpxCAD and pmrAB mutations, we identified four novel putative colistin resistance genes: A1S_1983. hepA. A1S_3026, and rsfS. Lipopolysaccharide (LPS) loss mutants exhibited higher fitness costs than those of the pmrB mutant in nutrient-rich medium. The colistin-resistant mutants had a higher inhibition ratio in the serum growth experiment than that of the wild type strain in 100% serum. Minimum inhibitory concentration (MIC) results showed that the LPS-deficient but not the pmrB mutant had an altered antibiotic resistance profile. The compensatory mutations partially or completely rescued the LPS-deficient’s fitness, suggesting that compensatory mutations play an important role in the emergence and spread of colistin resistance in A. baumannii.

Glycolipid substrates for ABC transporters required for the assembly of bacterial cell-envelope and cell-surface glycoconjugates

Glycoconjugates, molecules that contain sugar components, are major components of the cell envelopes of bacteria and cover much of their exposed surfaces. These molecules are involved in interactions with the surrounding environment and, in pathogens, play critical roles in the interplay with the host immune system. Despite the remarkable diversity in glycoconjugate structures, most are assembled by glycosyltransferases that act on lipid acceptors at the cytosolic membrane. The resulting glycolipids are then transported to the cell surface in processes that frequently begin with ATP-binding cassette transporters. This review summarizes current understanding of the structure and biosynthesis of glycolipid substrates and the structure and functions of their transporters. This article is part of a Special Issue entitled: Bacterial Lipids edited by Russell E. Bishop.

Salmonella Extracellular Matrix Components Influence Biofilm Formation and Gallbladder Colonization

Salmonella enterica serovar Typhi, the causative agent of typhoid fever in humans, forms biofilms encapsulated by an extracellular matrix (ECM). Biofilms facilitate colonization and persistent infection in gallbladders of humans and mouse models of chronic carriage. Individual roles of matrix components have not been completely elucidated in vitro or in vivo To examine individual functions, strains of Salmonella enterica serovar Typhimurium, the murine model of S Typhi, in which various ECM genes were deleted or added, were created to examine biofilm formation, colonization, and persistence in the gallbladder. Studies show that curli contributes most significantly to biofilm formation. Expression of Vi antigen decreased biofilm formation in vitro and virulence and bacterial survival in vivo without altering the examined gallbladder pro- or anti-inflammatory cytokines. Oppositely, loss of all ECM components (ΔwcaM ΔcsgA ΔyihO ΔbcsE) increased virulence and bacterial survival in vivo and reduced gallbladder interleukin-10 (IL-10) levels. Colanic acid and curli mutants had the largest defects in biofilm-forming ability and contributed most significantly to the virulence increase of the ΔwcaM ΔcsgA ΔyihO ΔbcsE mutant strain. While the ΔwcaM ΔcsgA ΔyihO ΔbcsE mutant was not altered in resistance to complement or growth in macrophages, it attached and invaded macrophages better than the wild-type (WT) strain. These data suggest that ECM components have various levels of importance in biofilm formation and gallbladder colonization and that the ECM diminishes disseminated disease in our model, perhaps by reducing cell attachment/invasion and dampening inflammation by maintaining/inducing IL-10 production. Understanding how ECM components aid acute disease and persistence could lead to improvements in therapeutic treatment of typhoid fever patients.

Role of Fimbriae, Flagella and Cellulose on the Attachment of Salmonella Typhimurium ATCC 14028 to Plant Cell Wall Models

Cases of foodborne disease caused by Salmonella are frequently associated with the consumption of minimally processed produce. Bacterial cell surface components are known to be important for the attachment of bacterial pathogens to fresh produce. The role of these extracellular structures in Salmonella attachment to plant cell walls has not been investigated in detail. We investigated the role of flagella, fimbriae and cellulose on the attachment of Salmonella Typhimurium ATCC 14028 and a range of isogenic deletion mutants (ΔfliC fljB, ΔbcsA, ΔcsgA, ΔcsgA bcsA and ΔcsgD) to bacterial cellulose (BC)-based plant cell wall models [BC-Pectin (BCP), BC-Xyloglucan (BCX) and BC-Pectin-Xyloglucan (BCPX)] after growth at different temperatures (28°C and 37°C). We found that all three cell surface components were produced at 28°C but only the flagella was produced at 37°C. Flagella appeared to be most important for attachment (reduction of up to 1.5 log CFU/cm2) although both cellulose and fimbriae also aided in attachment. The csgD deletion mutant, which lacks both cellulose and fimbriae, showed significantly higher attachment as compared to wild type cells at 37°C. This may be due to the increased expression of flagella-related genes which are also indirectly regulated by the csgD gene. Our study suggests that bacterial attachment to plant cell walls is a complex process involving many factors. Although flagella, cellulose and fimbriae all aid in attachment, these structures are not the only mechanism as no strain was completely defective in its attachment.

What's on the Outside Matters: The Role of the Extracellular Polymeric Substance of Gram-negative Biofilms in Evading Host Immunity and as a Target for Therapeutic Intervention

Biofilms are organized multicellular communities encased in an extracellular polymeric substance (EPS). Biofilm-resident bacteria resist immunity and antimicrobials. The EPS provides structural stability and presents a barrier; however, a complete understanding of how EPS structure relates to biological function is lacking. This review focuses on the EPS of three Gram-negative pathogens: Pseudomonas aeruginosa, nontypeable Haemophilus influenzae, and Salmonella enterica serovar Typhi/Typhimurium. Although EPS proteins and polysaccharides are diverse, common constituents include extracellular DNA, DNABII (DNA binding and bending) proteins, pili, flagella, and outer membrane vesicles. The EPS biochemistry promotes recalcitrance and informs the design of therapies to reduce or eliminate biofilm burden.

Model for the Controlled Synthesis of O-Antigen Repeat Units Involving the WaaL Ligase

The Wzx/Wzy O-antigen pathway involves synthesis of a repeat unit (O unit) consisting of 3 to 8 sugars on an inner-membrane-embedded lipid carrier. These O units are translocated across the membrane to its periplasmic face by Wzx, while retaining linkage to the carrier, and then polymerized by Wzy to O-antigen polymer, which WaaL ligase transfers to a lipopolysaccharide precursor to complete lipopolysaccharide synthesis, concomitantly releasing the lipid carrier. This lipid carrier is also used for peptidoglycan assembly, and sequestration is known to be toxic. Thus, O-unit synthesis must involve precise regulation to meet demand but avoid overproduction. Here we show that loss of WaaL reverses a known growth defect in a Salmonella mutant that otherwise accumulates O-unit intermediates and propose that WaaL is also involved in a novel feedback mechanism to regulate O-unit synthesis, based on the availability of O units on the periplasmic face of the membrane.