Bacterial endotoxins: chemical structure and biologic activity

Engineering a suite of E. coli strains for enhanced expression of bacterial polysaccharides and glycoconjugate vaccines

BACKGROUND

Glycoengineering, in the biotechnology workhorse bacterium, Escherichia coli, is a rapidly evolving field, particularly for the production of glycoconjugate vaccine candidates (bioconjugation). Efficient production of glycoconjugates requires the coordinated expression within the bacterial cell of three components: a carrier protein, a glycan antigen and a coupling enzyme, in a timely fashion. Thus, the choice of a suitable E. coli host cell is of paramount importance. Microbial chassis engineering has long been used to improve yields of chemicals and biopolymers, but its application to vaccine production is sparse.

RESULTS

In this study we have engineered a family of 11 E. coli strains by the removal and/or addition of components rationally selected for enhanced expression of Streptococcus pneumoniae capsular polysaccharides with the scope of increasing yield of pneumococcal conjugate vaccines. Importantly, all strains express a detoxified version of endotoxin, a concerning contaminant of therapeutics produced in bacterial cells. The genomic background of each strain was altered using CRISPR in an iterative fashion to generate strains without antibiotic markers or scar sequences.

CONCLUSIONS

Amongst the 11 modified strains generated in this study, E. coli Falcon, Peregrine and Sparrowhawk all showed increased production of S. pneumoniae serotype 4 capsule. Eagle (a strain without enterobacterial common antigen, containing a GalNAc epimerase and PglB expressed from the chromosome) and Sparrowhawk (a strain without enterobacterial common antigen, O-antigen ligase and chain length determinant, containing a GalNAc epimerase and chain length regulators from Streptococcus pneumoniae) respectively produced an AcrA-SP4 conjugate with 4 × and 14 × more glycan than that produced in the base strain, W3110. Beyond their application to the production of pneumococcal vaccine candidates, the bank of 11 new strains will be an invaluable resource for the glycoengineering community.

Depyrogenation using Plasmas: A Novel Approach for Endotoxin Deactivation Using a Dielectric Barrier Discharge at Atmospheric Pressure

Developing a low-cost depyrogenation process is vital in extending medical applicability of polymers that can be used in medicine. We present an overview of the plasma-based depyrogenation literature and address the need to develop a non-thermal plasma-based depyrogenation process for delicate materials such as chitosan. We present a low-cost plasma apparatus to treat chitosan powder in hermetically sealed bags. We decouple the experiments into two; depyrogenation experiments for dried standard endotoxin on glass slides, and chitosan modifications analysis through FTIR spectroscopy. We demonstrate depyrogenation efficacy with up to a 4-log reduction in endotoxin levels and discuss minor changes observed in plasma-treated chitosan.

Global and Targeted Lipid Analysis of Gemmata obscuriglobus Reveals the Presence of Lipopolysaccharide, a Signature of the Classical Gram-Negative Outer Membrane

Planctomycete bacteria possess many unusual cellular properties, contributing to a cell plan long considered to be unique among the bacteria. However, data from recent studies are more consistent with a modified Gram-negative cell plan. A key feature of the Gram-negative plan is the presence of an outer membrane (OM), for which lipopolysaccharide (LPS) is a signature molecule. Despite genomic evidence for an OM in planctomycetes, no biochemical verification has been reported. We attempted to detect and characterize LPS in the planctomycete Gemmata obscuriglobus. We obtained direct evidence for LPS and lipid A using electrophoresis and differential staining. Gas chromatography-mass spectrometry (GC-MS) compositional analysis of LPS extracts identified eight different 3-hydroxy fatty acids (3-HOFAs), 2-keto 3-deoxy-d-manno-octulosonic acid (Kdo), glucosamine, and hexose and heptose sugars, a chemical profile unique to Gram-negative LPS. Combined with molecular/structural information collected from matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) MS analysis of putative intact lipid A, these data led us to propose a heterogeneous hexa-acylated lipid A structure (multiple-lipid A species). We also confirmed previous reports of G. obscuriglobus whole-cell fatty acid (FA) and sterol compositions and detected a novel polyunsaturated FA (PUFA). Our confirmation of LPS, and by implication an OM, in G. obscuriglobus raises the possibility that other planctomycetes possess an OM. The pursuit of this question, together with studies of the structural connections between planctomycete LPS and peptidoglycans, will shed more light on what appears to be a planctomycete variation on the Gram-negative cell plan.

IMPORTANCE

Bacterial species are classified as Gram positive or negative based on their cell envelope structure. For 25 years, the envelope of planctomycete bacteria has been considered a unique exception, as it lacks peptidoglycan and an outer membrane (OM). However, the very recent detection of peptidoglycan in planctomycete species has provided evidence for a more conventional cell wall and raised questions about other elements of the cell envelope. Here, we report direct evidence of lipopolysaccharide in the planctomycete G. obscuriglobus, suggesting the presence of an OM and supporting the proposal that the planctomycete cell envelope is an extension of the canonical Gram-negative plan. This interpretation features a convoluted cytoplasmic membrane and expanded periplasmic space, the functions of which provide an intriguing avenue for future investigation.

FtsH-mediated coordination of lipopolysaccharide biosynthesis in Escherichia coli correlates with the growth rate and the alarmone (p)ppGpp

The outer membrane is the first line of defense for Gram-negative bacteria and serves as a major barrier for antibiotics and other harmful substances. The biosynthesis of lipopolysaccharides (LPS), the essential component of the outer membrane, must be tightly controlled as both too much and too little LPS are toxic. In Escherichia coli, the cellular level of the key enzyme LpxC, which catalyzes the first committed step in LPS biosynthesis, is adjusted by proteolysis carried out by the essential and membrane-bound protease FtsH. Here, we demonstrate that LpxC is degraded in a growth rate-dependent manner with half-lives between 4 min and >2 h. According to the cellular demand for LPS biosynthesis, LpxC is degraded during slow growth but stabilized when cells grow rapidly. Disturbing the balance between LPS and phospholipid biosynthesis in favor of phospholipid production in an E. coli strain encoding a hyperactive FabZ protein abolishes growth rate dependency of LpxC proteolysis. Lack of the alternative sigma factor RpoS or inorganic polyphosphates, which are known to mediate growth rate-dependent gene regulation in E. coli, did not affect proteolysis of LpxC. In contrast, absence of RelA and SpoT, which synthesize the alarmone (p)ppGpp, deregulated LpxC degradation resulting in rapid proteolysis in fast-growing cells and stabilization during slow growth. Our data provide new insights into the essential control of LPS biosynthesis in E. coli.

Endotoxin liberation from Neisseria meningitidis correlates to their ability to induce procoagulant and fibrinolytic factors in human monocytes

Endotoxin released from different strains of Neisseria meningitidis were studied for their ability to induce procoagulant (tissue factor, TF), fibrinolytic (plasminogen activator, PA) and antifibrinolytic (plasminogen activator inhibitor 2, PAI-2) factors in human monocytes. Two meningococcal strains that liberate endotoxin (E+; 270+ and 840+) and 2 non-liberating (E-; 270- and 840-) strains were used. The endotoxin activity in culture filtrates of these strains was monitored with the Limulus amoebocyte lysate (LAL) test. There was a marked difference between E+ and E- strains in their ability to liberate endotoxin. Suspensions of whole bacteria of all 4 strains induced a significant (14-19-fold) increase in monocyte TF expression when present in concentrations > 10(5) CFU/ml. At lower concentrations (10(4) CFU/ml), E+ strains were clearly more potent stimulators of TF synthesis than E- strains. Culture filtrates of E+ strains were up to 10(4)-fold more potent in inducing TF synthesis than filtrates from E- strains. This marked difference in inducing potency between E+ and E- strains was also observed when monocyte PAI-2 synthesis was examined. The PA expression, on the other hand, was suppressed when monocytes were incubated in the presence of culture filtrates, especially filtrates from the E+ strains. The increased procoagulant and antifibrinolytic activity, together with reduced profibrinolytic activity of monocytes, was closely correlated to the amount of endotoxin measured in the culture filtrates. These changes may contribute substantially to the coagulopathic state seen during systemic meningococcal disease.

Reactive oxygen species, antiproteases, and cytokines in sepsis

Although the shock syndrome is recognized as a form of "mediator poisoning", a plethora of details is hardly converging into a coherent concept of chronological and molecular order. As a model for organ failure in septic shock, three alternative experimental approaches with a common pathology are presented: When galactosamine-sensitized mice receive either lipopolysaccharide or leukotriene D4 or tumor necrosis factor alpha they develop fulminant hepatitis within few hours with a lethal outcome within one day. Detailed pharmacological intervention studies allow to conclude that endotoxin-induced leukotriene D4 release induces a transient ischemia by the known vasoconstrictive action of this eicosanoid. A following reperfusion/reoxygenation phase gives rise to superoxide formation which inactivates alpha 1 proteinase inhibitor. Thus a serine protease becomes active which is responsible for the processing of a monocytic tumor necrosis factor alpha precursor to be released into the circulation after proteolytic cleavage. By this sequence the final central mediator of shock and sepsis becomes systematically abundant. The concept arising from these studies reconciles previously known findings and provides a link between the role of reactive oxygen species in inflammation, the balance of proteases and antiproteases in the extracellular space and the release of the cytokine tumor necrosis factor in sepsis and shock.

Gram-negative sepsis. Background, clinical features, and intervention

Gram-negative sepsis remains an urgent medical problem, with more than 200,000 cases occurring each year in the United States and an associated mortality rate of 20 to 50 percent. Since the onset of shock greatly worsens prognosis and to encourage early intervention, the term sepsis syndrome was developed to describe the features of a preshock septic state. Early clinical and metabolic indicators are discussed, and current therapy is reviewed. Better understanding of the pathophysiology of endotoxin release from Gram-negative bacteria and advances in biotechnology have led to the development of potential new treatments for sepsis. One such development--monoclonal antibodies to endotoxin--has shown great promise in the effort to block the progression to septic shock, reduce mortality, and decrease the overall costs of sepsis to the patient and to the national economy.

Endotoxin-inducible cytotoxicity in liver cell cultures--I

It is known that rodents challenged with a combination of galactosamine and endotoxin develop a fulminant hepatitis within several hours. Until now, no in-vitro correlate for this organ-specific lesion has been described. Here, in-vitro conditions have been developed which allow examination of lipopolysaccharide (endotoxin)-inducible cell injury to hepatocytes. Under these in-vitro conditions (RPMI 1640 supplemented with 10% calf serum, 40% oxygen tension) which require the presence of functionally intact Kupffer cells, a concentration-dependent lactate dehydrogenase release is inducible by different lipopolysaccharides in hepatocyte cultures from Fischer rats. It can be abrogated by polymyxin B. These co-cultures secreted tumor necrosis factor-alpha into the medium upon a lipopolysaccharide stimulus. The presence of a tumor necrosis factor-alpha antiserum reduced the major part of the endotoxin-inducible cytotoxicity. Similarities in vitro and in vivo of the cytotoxic potency of various endotoxin species and the different responsiveness of hepatocytes from two different rat strains support that this co-culture system might be useful for studying endotoxin-inducible lesions in vitro.

Endotoxin levels measured by a chromogenic assay in portal, hepatic and peripheral venous blood in patients with cirrhosis

Endotoxin concentrations were measured in the portal, hepatic and peripheral venous blood of two groups of patients with cirrhosis using a limulus-based chromogenic assay. The high sensitivity of chromogenic detection allowed measurement of endotoxin as low as 10 to 15 pg per ml, an order of magnitude greater than previously possible by gelation studies. Group 1 consisted of 56 patients with cirrhosis undergoing angiographic evaluation. In this group, there was wide variability in hepatic venous concentration [73 +/- 110 pg per ml (mean +/- S.D.)] and peripheral venous concentration [31 +/- 58 pg per ml]. However, paired t test showed peripheral venous concentration was significantly (p less than 0.001) lower than hepatic venous concentration. Neither hepatic or peripheral venous endotoxin levels correlated significantly with a variety of clinical, biochemical or radiological parameters. Group 2 consisted of 21 patients with cirrhosis undergoing shunt surgery. Endotoxin levels again showed a wide range, with portal venous concentration (142 +/- 167 pg per ml) and simultaneous peripheral venous concentration (82 +/- 150 pg per ml). Paired t test in this group showed a significant (p less than 0.001) portal to peripheral venous gradient. This study showed the feasibility of measuring endotoxin in plasma to low concentrations by a chromogenic assay technique. It supports the concept of relatively high levels of endotoxin in the portal circulation. In the presence of liver disease, systemic endotoxemia occurs, which is augmented by stressful situations.

[Causes and therapy of organ failure: mediators, their role and therapeutic implications as exemplified by the infected patient]

Mediators from the coagulation system, the complement system, the kallikrein-kinin system and the arachidonic acid metabolism are made responsible for the pathogenesis of organ failure (ARDS) following sepsis. Products from these systems influence, directly or indirectly, vascular tone and permeability, especially in the pulmonary circulation. Besides these mediators, toxic oxygen species and proteases released from activated granulocytes and macrophages injure endothelial cells and provoke vascular leakage.

Current perspectives on septic shock

Although septic shock may be initiated by invading microbes, it is the metabolic and immunologic host responses that determine the true pathophysiology of this common critical care illness. Currently, septic shock therapeutics emphasize empiric and symptomatic treatment. Biochemical elucidation of the septic process will ultimately result in specific interventions for this ominous intensive care syndrome.