Manno(rhamno)biose-containing capsular polysaccharides of Klebsiella pneumoniae enhance opsono-stimulation of human polymorphonuclear leukocytes

Microbial evasion of the complement system: a continuous and evolving story

The complement system is a fundamental part of the innate immune system that plays a key role in the battle of the human body against invading pathogens. Through its three pathways, represented by the classical, alternative, and lectin pathways, the complement system forms a tightly regulated network of soluble proteins, membrane-expressed receptors, and regulators with versatile protective and killing mechanisms. However, ingenious pathogens have developed strategies over the years to protect themselves from this complex part of the immune system. This review briefly discusses the sequence of the complement activation pathways. Then, we present a comprehensive updated overview of how the major four pathogenic groups, namely, bacteria, viruses, fungi, and parasites, control, modulate, and block the complement attacks at different steps of the complement cascade. We shed more light on the ability of those pathogens to deploy more than one mechanism to tackle the complement system in their path to establish infection within the human host.

Clinical Value of Glycan Changes in Cerebrospinal Fluid for Evaluation of Post-Neurosurgical Bacterial Meningitis with Hemorrhagic Stroke Patients

Post-neurosurgical bacterial meningitis (PNBM) is one of the severe complications in patients receiving neurosurgical procedures. Recent studies have found microbe-related glycans play important roles in adhesion, invasion, and toxicity toward innate immunological reactions. In this study, we aimed to investigate the glycomic profile and its potential diagnostic efficacy in post-neurosurgical bacterial meningitis (PNBM) patients with hemorrhagic stroke. A total of 136 cerebrospinal fluid (CSF) samples were recruited and divided into a PNBM group and a non-PNBM group based on the clinical diagnostic criteria. A lectin biochip-based method was established for the detection of glycans in CSF. The clinicopathological data and biochemical parameters in CSF from all patients were analyzed. Two models for multivariate analysis investigating glycan changes in the CSF were conducted, aiming at determining the specific expression and diagnostic efficacy of lectin-probing glycans (LPGs) for PNBM. In univariate analysis, we found that 8 out of 11 LPGs were significantly correlated with PNBM. Model 1 multivariate analysis revealed that PNA (p = 0.034), Jacalin (p = 0.034) and LTL (p = 0.001) were differentially expressed in the CSF of PNBM patients compared with those of non-PNBM patients. Model 2 multivariate analysis further disclosed that LTL (p = 0.021) and CSF glucose (p < 0.001) had independent diagnostic efficacies in PNBM, with areas under the curve (AUC) of 0.703 and 0.922, respectively. In summary, this study provided a new insight into the subject of CSF glycomics concerning bacterial infection in patients with hemorrhagic stroke.

Epidemiological Features and Impact of High Glucose Level on Virulence Gene Expression and Serum Resistance of Klebsiella pneumoniae Causing Liver Abscess in Diabetic Patients

Purpose

Klebsiella pneumoniae (K. pneumoniae) is a Gram-negative bacterium that is predominantly associated with liver abscesses in global diabetic patients. High levels of glucose in the surrounding of K. pneumonia increase its pathogenicity including capsular polysaccharide (CPS) and fimbriae. Other important virulent factors include outer membrane protein A (ompA) and regulator mucoid phenotype A (rmpA). The objective of this investigation was to elucidate the effects of high glucose on rmpA and ompA gene expression and serum resistance of K. pneumoniae causing liver abscess.

Patients and Methods

The clinical history of 57 patients suffering from K. pneumoniae-caused liver abscesses (KLA) was acquired and their clinical and laboratory manifestations in the presence or absence of diabetes were analyzed. The antimicrobial susceptibility, serotypes, and virulence genes were tested. Clinical isolates of 3 serotype-K1 hypervirulent K. pneumoniae (hvKP) were used to detect the effect of exogenous high glucose on rmpA, ompA, and clbB genes expression, and bacterial serum resistance.

Results

KLA patients with diabetes showed higher C-reactive protein (CRP) compared to non-diabetic KLA patients. Furthermore, the diabetic group showed increased incidences of sepsis and invasive infections, and their length of hospital stay was also prolonged. Pre-incubation of K. pneumoniae in high glucose (0.5%) concentration up-regulated rmpA, ompA, and clbB genes expression. However, cAMP supplementation, which was inhibited by environmental glucose, reversed the increase of rmpA and ompA in a cAMP-dependent manner. Moreover, hvKP strains incubated in high glucose also exhibited enhanced protection from serum killing.

Conclusion

High glucose levels reflected by poor glycemic control has increased gene expression of rmpA and ompA in hvKP by the cAMP signaling pathway and enhanced its resistance to serum killing, thus providing a new and reasonable explanation for the high incidences of sepsis and invasive infections in KLA patients with diabetes.

Complement killing of clinical Klebsiella pneumoniae isolates is serum concentration dependent

Klebsiella pneumoniae is an opportunistic gram-negative pathogen causing serious infections, including sepsis. In plasma, activation of the complement cascades is important for killing bacteria. Thirty clinical Klebsiella spp. blood isolates were analyzed for serum susceptibility in 75% normal human serum (NHS). Twenty-two were serum resistant and eight were serum sensitive, and subsequently tested in 5-75% NHS. Two isolates were killed in 5% and the remaining six in 50%-75% NHS. The two 5% sensitive isolates showed binding of complement (C)4 and C3 in 5% NHS with formation of membrane attack complex (MAC). Inhibition of the classical/lectin mediated pathways (CP/LP) using a C4 specific nanobody, hC4Nb8, led to survival of both isolates in 5% NHS. Using nanobody hC3Nb1, inhibiting the alternative pathway (AP), the isolates were killed in 5% NHS, and amplification of the CP/LP by AP was not necessary for killing. Sole AP killing of these isolates when inhibiting CP/LP with hC4Nb8 was observed in 50% NHS, stressing the concentration dependent functionality of AP. For the less sensitive isolates, killing required activation of CP/LP and AP demonstrated by inhibition with nanobodies. AP inhibition resulted in no C3 deposition on the serum resistant isolate, supporting that AP was the sole activation pathway.

Complement Evasion Strategies of Human Pathogenic Bacteria

Human pathogens need to overcome an elaborate network of host defense mechanisms in order to establish their infection, colonization, proliferation and eventual dissemination. The interaction of pathogens with different effector molecules of the immune system results in their neutralization and elimination from the host. The complement system is one such integral component of innate immunity that is critically involved in the early recognition and elimination of the pathogen. Hence, under this immune pressure, all virulent pathogens capable of inducing active infections have evolved immune evasive strategies that primarily target the complement system, which plays an essential and central role for host defense. Recent reports on several bacterial pathogens have elucidated the molecular mechanisms underlying complement evasion, inhibition of opsonic phagocytosis and cell lysis. This review aims to comprehensively summarize the recent findings on the various strategies adopted by pathogenic bacteria to escape complement-mediated clearance.

Bacterial glycans and their interactions with lectins in the innate immune system

Bacterial surfaces are rich in glycoconjugates that are mainly present in their outer layers and are of great importance for their interaction with the host innate immune system. The innate immune system is the first barrier against infection and recognizes pathogens via conserved pattern recognition receptors (PRRs). Lectins expressed by innate immune cells represent an important class of PRRs characterized by their ability to recognize carbohydrates. Among lectins in innate immunity, there are three major classes including the galectins, siglecs, and C-type lectin receptors. These lectins may contribute to initial recognition of bacterial glycans, thus providing an early defence mechanism against bacterial infections, but they may also be exploited by bacteria to escape immune responses. In this review, we will first exemplify bacterial glycosylation systems; we will then describe modes of recognition of bacterial glycans by lectins in innate immunity and, finally, we will briefly highlight how bacteria have found ways to exploit these interactions to evade immune recognition.

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.

The hijackers guide to escaping complement: Lessons learned from pathogens

Pathogens that invade the human host are confronted by a multitude of defence mechanisms aimed at preventing colonization, dissemination and proliferation. The most frequent outcome of this interaction is microbial elimination, in which the complement system plays a major role. Complement, an essential feature of the innate immune machinery, rapidly identifies and marks pathogens for efficient removal. Consequently, this creates a selective pressure for microbes to evolve strategies to combat complement, permitting host colonization and access to resources. All successful pathogens have developed mechanisms to resist complement activity which are intimately aligned with their capacity to cause disease. In this review, we describe the successful methods various pathogens use to evade complement activation, shut down inflammatory signalling through complement, circumvent opsonisation and override terminal pathway lysis. This review summarizes how pathogens undermine innate immunity: 'The Hijackers Guide to Complement'.

Klebsiella pneumoniae infection biology: living to counteract host defences

Klebsiella species cause a wide range of diseases including pneumonia, urinary tract infections (UTIs), bloodstream infections and sepsis. These infections are particularly a problem among neonates, elderly and immunocompromised individuals. Klebsiella is also responsible for a significant number of community-acquired infections. A defining feature of these infections is their morbidity and mortality, and the Klebsiella strains associated with them are considered hypervirulent. The increasing isolation of multidrug-resistant strains has significantly narrowed, or in some settings completely removed, the therapeutic options for the treatment of Klebsiella infections. Not surprisingly, this pathogen has then been singled out as an 'urgent threat to human health' by several organisations. This review summarises the tremendous progress that has been made to uncover the sophisticated immune evasion strategies of K. pneumoniae. The co-evolution of Klebsiella in response to the challenge of an activated immune has made Klebsiella a formidable pathogen exploiting stealth strategies and actively suppressing innate immune defences to overcome host responses to survive in the tissues. A better understanding of Klebsiella immune evasion strategies in the context of the host-pathogen interactions is pivotal to develop new therapeutics, which can be based on antagonising the anti-immune strategies of this pathogen.

Phage-Borne Depolymerases Decrease Klebsiella pneumoniae Resistance to Innate Defense Mechanisms

Klebsiella pneumoniae produces capsular polysaccharides that are a crucial virulence factor protecting bacteria against innate response mechanisms of the infected host. Simultaneously, those capsules are targeted by specific bacteriophages equipped with virion-associated depolymerases able to recognize and degrade these polysaccharides. We show that Klebsiella phage KP32 produces two capsule depolymerases, KP32gp37 and KP32gp38, with a high specificity for the capsular serotypes K3 and K21, respectively. Together, they determine the host spectrum of bacteriophage KP32, which is limited to strains with serotype K3 and K21. Both depolymerases form a trimeric β-structure, display moderate thermostability and function optimally under neutral to alkaline conditions. We show that both depolymerases strongly affect the virulence of K. pneumoniae with the corresponding K3 and K21 capsular serotypes. Capsule degradation renders the otherwise serum-resistant cells more prone to complement-mediated killing with up to four log reduction in serum upon exposure to KP32gp37. Decapsulated strains are also sensitized for phagocytosis with a twofold increased uptake. In addition, the intracellular survival of phagocytized cells in macrophages was significantly reduced when bacteria were previously exposed to the capsule depolymerases. Finally, depolymerase application considerably increases the lifespan of Galleria mellonella larvae infected with K. pneumoniae in a time- and strain-dependent manner. In sum, capsule depolymerases are promising antivirulence compounds that act by defeating a major resistance mechanism of K. pneumoniae against the innate immunity.

Evasion Mechanisms Used by Pathogens to Escape the Lectin Complement Pathway

The complement system is a crucial defensive network that protects the host against invading pathogens. It is part of the innate immune system and can be initiated via three pathways: the lectin, classical and alternative activation pathway. Overall the network compiles a group of recognition molecules that bind specific patterns on microbial surfaces, a group of associated proteases that initiates the complement cascade, and a group of proteins that interact in proteolytic complexes or the terminal pore-forming complex. In addition, various regulatory proteins are important for controlling the level of activity. The result is a pro-inflammatory response meant to combat foreign microbes. Microbial elimination is, however, not a straight forward procedure; pathogens have adapted to their environment by evolving a collection of evasion mechanisms that circumvent the human complement system. Complement evasion strategies features different ways of exploiting human complement proteins and moreover features different pathogen-derived proteins that interfere with the normal processes. Accumulated, these mechanisms target all three complement activation pathways as well as the final common part of the cascade. This review will cover the currently known lectin pathway evasion mechanisms and give examples of pathogens that operate these to increase their chance of invasion, survival and dissemination.

Epidemiology and Virulence of Klebsiella pneumoniae

Strains of Klebsiella pneumoniae are frequently opportunistic pathogens implicated in urinary tract and catheter-associated urinary-tract infections of hospitalized patients and compromised individuals. Infections are particularly difficult to treat since most clinical isolates exhibit resistance to several antibiotics leading to treatment failure and the possibility of systemic dissemination. Infections of medical devices such as urinary catheters is a major site of K. pneumoniae infections and has been suggested to involve the formation of biofilms on these surfaces. Over the last decade there has been an increase in research activity designed to investigate the pathogenesis of K. pneumoniae in the urinary tract. These investigations have begun to define the bacterial factors that contribute to growth and biofilm formation. Several virulence factors have been demonstrated to mediate K. pneumoniae infectivity and include, but are most likely not limited to, adherence factors, capsule production, lipopolysaccharide presence, and siderophore activity. The development of both in vitro and in vivo models of infection will lead to further elucidation of the molecular pathogenesis of K. pneumoniae. As for most opportunistic infections, the role of host factors as well as bacterial traits are crucial in determining the outcome of infections. In addition, multidrug-resistant strains of these bacteria have become a serious problem in the treatment of Klebsiella infections and novel strategies to prevent and inhibit bacterial growth need to be developed. Overall, the frequency, significance, and morbidity associated with K. pneumoniae urinary tract infections have increased over many years. The emergence of these bacteria as sources of antibiotic resistance and pathogens of the urinary tract present a challenging problem for the clinician in terms of management and treatment of individuals.

Complement resistance mechanisms of Klebsiella pneumoniae

The current emergence of antibiotic-resistant bacteria causes major problems in hospitals worldwide. To survive within the host, bacterial pathogens exploit several escape mechanisms to prevent detection and killing by the immune system. As a major player in immune defense, the complement system recognizes and destroys bacteria via different effector mechanisms. The complement system can label bacteria for phagocytosis or directly kill Gram-negative bacteria via insertion of a pore-forming complex in the bacterial membrane. The multi-drug resistant pathogen Klebsiella pneumoniae exploits several mechanisms to resist complement. In this review, we present an overview of strategies used by K. pneumoniae to prevent recognition and killing by the complement system. Understanding these complement evasion strategies is crucial for the development of innovative strategies to combat K. pneumoniae.

Interaction of non-human primate complement and antibodies with hypermucoviscous Klebsiella pneumoniae

Emergent hypermucoviscosity (HMV) phenotypes of Klebsiella pneumoniae have been associated with increased invasiveness and pathogenicity in primates. In this study, we investigated the interaction of African green monkeys (AGM) (Chlorocebus aethiops sabaeus) complement and antibody with HMV and non-HMV isolates as in vitro models of primate infection. Significantly greater survival of HMV isolates was evident after incubation in normal serum or whole blood (p < 0.05) of AGM donors when compared to non-HMV strains. Greater survival of HMV strains (p < 0.05) was found after incubation in whole blood and serum from seropositive donors when compared to seronegative donor samples. Additionally, significantly greater amounts of K. pneumoniae were phagocytozed by AGM leukocytes when complement was active (p < 0.05), but no difference in uptake was observed when serum from seropositive or seronegative animals was used in challenged cells utilizing flow cytometry. Results demonstrate that interaction of cellular and humoral immune elements play a role in the in vitro killing of K. pneumoniae, particularly HMV isolates. Neither AGM serum, nor washed whole blood effectively killed HMV isolates; however, assays using heparinized whole blood of seronegative donors significantly reduced viability of HMV and non-HMV strains. The lack of bacterial killing observed in seropositive donors treatments could be at least partially associated with low IgG2 present in these animals. A better understanding of the pathogenesis of klebsiellosis in primates and host immune response is necessary to identify surface molecules that can induce both opsonizing and bactericidal antibody facilitating killing of Klebsiella, and the development of vaccines in human and animals.

Pushing the Bacterial Envelope

Over the past three decades, a powerful array of techniques has been developed for expressing heterologous proteins and saccharides on the surface of bacteria. Surface-engineered bacteria, in turn, have proven useful in a variety of settings, including high-throughput screening, biofuel production, and vaccinology. In this chapter, we provide a comprehensive review of methods for displaying polypeptides and sugars on the bacterial cell surface, and discuss the many innovative applications these methods have found to date. While already an important biotechnological tool, we believe bacterial surface display may be further improved through integration with emerging methodology in other fields, such as protein engineering and synthetic chemistry. Ultimately, we envision bacterial display becoming a multidisciplinary platform with the potential to transform basic and applied research in bacteriology, biotechnology, and biomedicine.