Meningitis caused by a lipopolysaccharide deficient Neisseria meningitidis

The Diverse Spectrum of Invasive Meningococcal Disease in Pediatric and Adolescent Patients: Narrative Review of Cases and Case Series

INTRODUCTION

Invasive meningococcal disease (IMD) is a potentially life-threatening disease caused by Neisseria meningitidis infection. We reviewed case reports of IMD from newborns, infants, children, and adolescents, and described the real-life clinical presentations, diagnoses, treatment paradigms, and clinical outcomes.

METHODS

PubMed and Embase were searched for IMD case reports on patients aged ≤ 19 years published from January 2011 to March 2023 (search terms "Neisseria meningitidis" or "invasive meningococcal disease", and "infant", "children", "paediatric", pediatric", or "adolescent").

RESULTS

We identified 97 publications reporting 184 cases of IMD, including 25 cases with a fatal outcome. Most cases were in adolescents aged 13-19 years (34.2%), followed by children aged 1-5 years (27.6%), children aged 6-12 years (17.1%), infants aged 1-12 months (17.1%), and neonates (3.9%). The most common disease-causing serogroups were W (40.2%), B (31.7%), and C (10.4%). Serogroup W was the most common serogroup in adolescents (17.2%), and serogroup B was the most common in the other age groups, including children aged 1-5 years (11.5%). The most common clinical presentations were meningitis (46.6%) and sepsis (36.8%).

CONCLUSIONS

IMD continues to pose a threat to the health of children and adolescents. While this review was limited to case reports and is not reflective of global epidemiology, adolescents represented the largest group with IMD. Additionally, nearly half of the patients who died were adolescents, emphasizing the importance of monitoring and vaccination in this age group. Different infecting serogroups were predominant in different age groups, highlighting the usefulness of multivalent vaccines to provide the broadest possible protection against IMD. Overall, this review provides useful insights into real-life clinical presentations, treatment paradigms, diagnoses, and clinical outcomes to help clinicians diagnose, treat, and, ultimately, protect patients from this devastating disease.

Targeting LPS biosynthesis and transport in gram-negative bacteria in the era of multi-drug resistance

Gram-negative bacteria pose a major threat to human health in an era fraught with multi-drug resistant bacterial infections. Despite extensive drug discovery campaigns over the past decades, no new antibiotic target class effective against gram-negative bacteria has become available to patients since the advent of the carbapenems in 1985. Antibiotic discovery efforts against gram-negative bacteria have been hampered by limited intracellular accumulation of xenobiotics, in large part due to the impermeable cell envelope comprising lipopolysaccharide (LPS) in the outer leaflet of the outer membrane, as well as a panoply of efflux pumps. The biosynthesis and transport of LPS are essential to the viability and virulence of most gram-negative bacteria. Thus, both LPS biosynthesis and transport are attractive pathways to target therapeutically. In this review, we summarize the LPS biosynthesis and transport pathways and discuss efforts to find small molecule inhibitors against targets within these pathways.

BACTERIAL MENINGITIS PRESENTING WITH A NORMAL CEREBROSPINAL FLUID LEUKOCYTE COUNT

OBJECTIVES

We describe clinical characteristics and outcome of adults with bacterial meningitis presenting with a normal CSF leukocyte count.

METHODS

We studied community-acquired bacterial meningitis with a normal CSF leukocyte count (≤ 5 per mm³) in adults from a prospective nationwide cohort study.

RESULTS

From 2006 through 2020, 39 of 2,357 (2%) episodes presented with a normal CSF leukocyte count. Immunocompromising conditions were present in 19 of 39 patients (49%), compared to 690 of 2303 (30%) in patients with elevated leukocytes (P=0.02). The triad of fever, neck stiffness, and altered consciousness was present in 6 of 34 patients (18%). CSF protein was abnormal in 25 of 37 patients (68%). We identified 3 clinical subgroups: those with severe pneumococcal meningitis (20 patients [51%]), with mainly sepsis (8 [21%]), and a miscellaneous group (11 [28%]). All patients with severe pneumococcal meningitis presented with high CSF protein levels and 18 of 19 (95%) had bacteria in the CSF Gram stain. Outcome was unfavorable in 23 of 39 (59%) patients and 12 (31%) died.

CONCLUSION

Patients with bacterial meningitis may present with normal CSF leukocyte counts. In these patients, CSF protein levels and Gram staining are important diagnostic parameters.

The inhibitory effect of LPS on the expression of GPR81 lactate receptor in blood-brain barrier model in vitro

BACKGROUND

Lipopolysaccharide (LPS) is one of the main constituents of the cell wall of gram-negative bacteria. As an endotoxin, LPS induces neuroinflammation, which is associated with the blood-brain barrier impairment. Lactate is a metabolite with some significant physiological functions within the neurovascular unit/blood-brain barrier (BBB). Accumulation of extracellular and cerebrospinal fluid lactate is a specific feature of bacterial meningitis. However, the role of lactate production, transport, and sensing by lactate receptors GPR81 in the pathogenesis of bacterial neuroinflammation is still unknown.

METHODS

In this study, we analyzed effects of LPS on the expression of GPR81 and MCT-1 and proliferation of cerebral endothelial cells in the BBB model in vitro. We used molecular profiling methods to measure the expression of GPR81, MCT-1, IL-1β, and Ki67 in the cerebral endothelium after treatment with different concentrations of LPS followed by measuring the level of extracellular lactate, transendothelial electric resistance, and permeability of the endothelial cell layer.

RESULTS

Our findings showed that exposure to LPS results in neuroinflammatory changes associated with decreased expression of GPR81 and MCT-1 in endothelial cells, as well as overproduction of IL-1β and elevation of lactate concentrations in the extracellular space in a dose-dependent manner. LPS treatment reduced JAM tight junction protein expression in cerebral endothelial cells and altered BBB structural integrity in vitro.

CONCLUSION

The impairment of lactate reception and transport might contribute to the alterations of BBB structural and functional integrity caused by LPS-mediated neuroinflammation.

Expanding the paradigm for the outer membrane: Acinetobacter baumannii in the absence of endotoxin

Asymmetry in the outer membrane has long defined the cell envelope of Gram-negative bacteria. This asymmetry, with lipopolysaccharide (LPS) or lipooligosaccharide (LOS) exclusively in the outer leaflet of the membrane, establishes an impermeable barrier that protects the cell from a number of stressors in the environment. Work done over the past 5 years has shown that Acinetobacter baumannii has the remarkable capability to survive with inactivated production of lipid A biosynthesis and the absence of LOS in its outer membrane. The implications of LOS-deficient A. baumannii are far-reaching - from impacts on cell envelope biogenesis and maintenance, bacterial physiology, antibiotic resistance and virulence. This review examines recent work that has contributed to our understanding of LOS-deficiency and compares it to studies done on Neisseria meningitidis and Moraxella catarrhalis; the two other organisms with this capability.

LpxK Is Essential for Growth of Acinetobacter baumannii ATCC 19606: Relationship to Toxic Accumulation of Lipid A Pathway Intermediates

Acinetobacter baumannii is a Gram-negative pathogen for which new therapies are needed. The lipid A biosynthetic pathway has several potential enzyme targets for the development of anti-Gram-negative agents (e.g., LpxC). However, A. baumannii ATCC 19606 can grow in the absence of LpxC and, correspondingly, of lipid A. In contrast, we show that cellular depletion of LpxK, a kinase occurring later in the pathway, inhibits growth. Growth inhibition results from toxic accumulation of lipid A pathway intermediates, since chemical inhibition of LpxC or fatty acid biosynthesis rescues cell growth upon loss of LpxK. Overall, this suggests that targets such as LpxK can be essential for growth even in those Gram-negative bacteria that do not require lipid A biosynthesis per se. This strain provides an elegant tool to derive a better understanding of the steps in a pathway that is the focus of intense interest for the development of novel antibacterials.

Acinetobacter baumannii ATCC 19606 can grow without lipid A, the major component of lipooligosaccharide. However, we previously reported that depletion of LpxH (the fourth enzyme in the lipid A biosynthetic pathway) prevented growth of this strain due to toxic accumulation of lipid A pathway intermediates. Here, we explored whether a similar phenomenon occurred with depletion of LpxK, a kinase that phosphorylates disaccharide 1-monophosphate (DSMP) at the 4′ position to yield lipid IV_A. An A. baumannii ATCC 19606 derivative with LpxK expression under the control of an isopropyl β-d-1-thiogalactopyranoside (IPTG)-regulated expression system failed to grow without induction, indicating that LpxK is essential for growth. Light and electron microscopy of LpxK-depleted cells revealed morphological changes relating to the cell envelope, consistent with toxic accumulation of lipid A pathway intermediates disrupting cell membranes. Using liquid chromatography-mass spectrometry (LCMS), cellular accumulation of the detergent-like pathway intermediates DSMP and lipid X was shown. Toxic accumulation was further supported by restoration of growth upon chemical inhibition of LpxC (upstream of LpxK and the first committed step of lipid A biosynthesis) using CHIR-090. Inhibitors of fatty acid synthesis also abrogated the requirement for LpxK expression. Growth rescue with these inhibitors was possible on Mueller-Hinton agar but not on MacConkey agar. The latter contains outer membrane-impermeable bile salts, suggesting that despite growth restoration, the cell membrane permeability barrier was not restored. Therefore, LpxK is essential for growth of A. baumannii, since loss of LpxK causes accumulation of detergent-like pathway intermediates that inhibit cell growth.

IMPORTANCEAcinetobacter baumannii is a Gram-negative pathogen for which new therapies are needed. The lipid A biosynthetic pathway has several potential enzyme targets for the development of anti-Gram-negative agents (e.g., LpxC). However, A. baumannii ATCC 19606 can grow in the absence of LpxC and, correspondingly, of lipid A. In contrast, we show that cellular depletion of LpxK, a kinase occurring later in the pathway, inhibits growth. Growth inhibition results from toxic accumulation of lipid A pathway intermediates, since chemical inhibition of LpxC or fatty acid biosynthesis rescues cell growth upon loss of LpxK. Overall, this suggests that targets such as LpxK can be essential for growth even in those Gram-negative bacteria that do not require lipid A biosynthesis per se. This strain provides an elegant tool to derive a better understanding of the steps in a pathway that is the focus of intense interest for the development of novel antibacterials.

Exogenous fatty acid metabolism in bacteria

Bacterial type II fatty acid synthesis (FASII) is a target for novel antibiotic development. All bacteria encode for mechanisms to incorporate exogenous fatty acids, and some bacteria can use exogenous fatty acids to bypass FASII inhibition. Bacteria encode three different mechanisms for activating exogenous fatty acids for incorporation into phospholipid synthesis. Exogenous fatty acids are converted into acyl-CoA in Gammaproteobacteria such as E. coli. Acyl-CoA molecules constitute a separate pool from endogenously synthesized acyl-ACP. Acyl-CoA can be used for phospholipid synthesis or broken down by β-oxidation, but cannot be used for lipopolysaccharide synthesis. Exogenous fatty acids are converted into acyl-ACP in some Gram-negative bacteria. The resulting acyl-ACP undergoes the same fates as endogenously synthesized acyl-ACP. Exogenous fatty acids are converted into acyl-phosphates in Gram-positive bacteria, and can be used for phospholipid synthesis or become acyl-ACP. Only the order Lactobacillales can use exogenous fatty acids to bypass FASII inhibition. FASII shuts down completely in presence of exogenous fatty acids in Lactobacillales, allowing Lactobacillales to synthesize phospholipids entirely from exogenous fatty acids. Inhibition of FASII cannot be bypassed in other bacteria because FASII is only partially down-regulated in presence of exogenous fatty acid or FASII is required to synthesize essential metabolites such as β-hydroxyacyl-ACP. Certain selective pressures such as FASII inhibition or growth in biofilms can select for naturally occurring one step mutations that attenuate endogenous fatty acid synthesis. Although attempts have been made to estimate the natural prevalence of these mutants, culture-independent metagenomic methods would provide a better estimate.

Bacterial fatty acid metabolism in modern antibiotic discovery

Bacterial fatty acid synthesis is essential for many pathogens and different from the mammalian counterpart. These features make bacterial fatty acid synthesis a desirable target for antibiotic discovery. The structural divergence of the conserved enzymes and the presence of different isozymes catalyzing the same reactions in the pathway make bacterial fatty acid synthesis a narrow spectrum target rather than the traditional broad spectrum target. Furthermore, bacterial fatty acid synthesis inhibitors are single-targeting, rather than multi-targeting like traditional monotherapeutic, broad-spectrum antibiotics. The single-targeting nature of bacterial fatty acid synthesis inhibitors makes overcoming fast-developing, target-based resistance a necessary consideration for antibiotic development. Target-based resistance can be overcome through multi-targeting inhibitors, a cocktail of single-targeting inhibitors, or by making the single targeting inhibitor sufficiently high affinity through a pathogen selective approach such that target-based mutants are still susceptible to therapeutic concentrations of drug. Many of the pathogens requiring new antibiotic treatment options encode for essential bacterial fatty acid synthesis enzymes. This review will evaluate the most promising targets in bacterial fatty acid metabolism for antibiotic therapeutics development and review the potential and challenges in advancing each of these targets to the clinic and circumventing target-based resistance. This article is part of a Special Issue entitled: Bacterial Lipids edited by Russell E. Bishop.

Toxic Accumulation of LPS Pathway Intermediates Underlies the Requirement of LpxH for Growth of Acinetobacter baumannii ATCC 19606

The lipid A moiety of lipopolysaccharide (LPS) is the main constituent of the outer leaflet of the Gram-negative bacterial outer membrane (OM) and is essential in many Gram-negative pathogens. An exception is Acinetobacter baumannii ATCC 19606, where mutants lacking enzymes occurring early in lipid A biosynthesis (LpxA, LpxC or LpxD), and correspondingly lacking LPS, can grow. In contrast, we show here that LpxH, an enzyme that occurs downstream of LpxD in the lipid A biosynthetic pathway, is essential for growth in this strain. Multiple attempts to disrupt lpxH on the genome were unsuccessful, and when LpxH expression was controlled by an isopropyl β-d-1-thiogalactopyranoside (IPTG) inducible promoter, cell growth under typical laboratory conditions required IPTG induction. Mass spectrometry analysis of cells shifted from LpxH-induced to uninduced (and whose growth was correspondingly slowing as LpxH was depleted) showed a large cellular accumulation of UDP-2,3-diacyl-GlcN (substrate of LpxH), a C14:0(3-OH) acyl variant of the LpxD substrate (UDP-3-O-[(R)-3-OH-C₁₄]-GlcN), and disaccharide 1-monophosphate (DSMP). Furthermore, the viable cell counts of the LpxH depleted cultures dropped modestly, and electron microscopy revealed clear defects at the cell (inner) membrane, suggesting lipid A intermediate accumulation was toxic. Consistent with this, blocking the synthesis of these intermediates by inhibition of the upstream LpxC enzyme using CHIR-090 abrogated the requirement for IPTG induction of LpxH. Taken together, these data indicate that LpxH is essential for growth in A. baumannii ATCC19606, because, unlike earlier pathway steps like LpxA or LpxC, blockage of LpxH causes accumulation of detergent-like pathway intermediates that prevents cell growth.

Protection of rainbow trout against yersiniosis by lpxD mutant Yersinia ruckeri

Yersinia ruckeri is a Gram negative bacteria causing yersiniosis in freshwater and marine fish. Lipid A, important for pathogenesis of Gram negative bacteria, biosynthesis pathway requires nine enzyme catalyzed steps. Although there are nine genes encoding lipid A biosynthesis in bacteria, biosynthesis of lipopolysaccharides relies on lpxD gene that encodes the third pathway enzyme. The roles of LpxD in Y. ruckeri virulence have not been studied. In the present study, in-frameshift deletion of lpxD gene and their role in Y. ruckeri virulence in rainbow trout were determined. For this purpose, 92% of the Y. ruckeri lpxD genes were deleted by homologous recombination. After running in SDS-PAGE and staining with silver stain, no LPS was detectable in the Y. ruckeri ΔlpxD mutant. Virulence and immunogenicity of the Y. ruckeri ΔlpxD mutant (YrΔlpxD) were determined in rainbow trout. Rainbow trout immunized with YrΔlpxD with immersion, or intraperitoneal injection method displayed superior protection (relative percentage survival ≥ 84%) after exposure to wild type Y. ruckeri. In conclusion, our results indicated that deletion of the lpxD gene causes significant attenuation of Y. ruckeri in rainbow trout, and LPS deficient YrΔlpxD could be used as a live attenuated vaccine against Y. ruckeri in rainbow trout. This vaccine can protect fish and it can be applied to fish with different methods such as immersion or injection.

Activation of Exogenous Fatty Acids to Acyl-Acyl Carrier Protein Cannot Bypass FabI Inhibition in Neisseria

Neisseria is a Gram-negative pathogen with phospholipids composed of straight chain saturated and monounsaturated fatty acids, the ability to incorporate exogenous fatty acids, and lipopolysaccharides that are not essential. The FabI inhibitor, AFN-1252, was deployed as a chemical biology tool to determine whether Neisseria can bypass the inhibition of fatty acid synthesis by incorporating exogenous fatty acids. Neisseria encodes a functional FabI that was potently inhibited by AFN-1252. AFN-1252 caused a dose-dependent inhibition of fatty acid synthesis in growing Neisseria, a delayed inhibition of growth phenotype, and minimal inhibition of DNA, RNA, and protein synthesis, showing that its mode of action is through inhibiting fatty acid synthesis. Isotopic fatty acid labeling experiments showed that Neisseria encodes the ability to incorporate exogenous fatty acids into its phospholipids by an acyl-acyl carrier protein-dependent pathway. However, AFN-1252 remained an effective antibacterial when Neisseria were supplemented with exogenous fatty acids. These results demonstrate that extracellular fatty acids are activated by an acyl-acyl carrier protein synthetase (AasN) and validate type II fatty acid synthesis (FabI) as a therapeutic target against Neisseria.

Genomic epidemiology of age-associated meningococcal lineages in national surveillance: an observational cohort study

Background

Invasive meningococcal disease (IMD) is a worldwide health issue that is potentially preventable with vaccination. In view of its sporadic nature and the high diversity of Neisseria meningitidis, epidemiological surveillance incorporating detailed isolate characterisation is crucial for effective control and understanding the evolving epidemiology of IMD. The Meningitis Research Foundation Meningococcus Genome Library (MRF-MGL) exploits whole-genome sequencing (WGS) for this purpose and presents data on a comprehensive and coherent IMD isolate collection from England and Wales via the internet. We assessed the contribution of these data to investigating IMD epidemiology.

Methods

WGS data were obtained for all 899 IMD isolates available for England and Wales in epidemiological years 2010–11 and 2011–12. The data had been annotated at 1720 loci, analysed, and disseminated online. Information was also available on meningococcal population structure and vaccine (Bexsero, GlaxoSmithKline, Brentford, Middlesex, UK) antigen variants, which enabled the investigation of IMD-associated genotypes over time and by patients' age groups. Population genomic analyses were done with a hierarchical gene-by-gene approach.

Findings

The methods used by MRF-MGL efficiently characterised IMD isolates and information was provided in plain language. At least 20 meningococcal lineages were identified, three of which (hyperinvasive clonal complexes 41/44 [lineage 3], 269 [lineage 2], and 23 [lineage 23]) were responsible for 528 (59%) of IMD isolates. Lineages were highly diverse and showed evidence of extensive recombination. Specific lineages were associated with IMD in particular age groups, with notable diversity in the youngest and oldest individuals. The increased incidence of IMD from 1984 to 2010 in England and Wales was due to successive and concurrent epidemics of different lineages. Genetically, 74% of isolates were characterised as encoding group B capsules: 16% group Y, 6% group W, and 3% group C. Exact peptide matches for individual Bexsero vaccine antigens were present in up to 26% of isolates.

Interpretation

The MRF-MGL represents an effective, broadly applicable model for the storage, analysis, and dissemination of WGS data that can facilitate real-time genomic pathogen surveillance. The data revealed information crucial to effective deployment and assessment of vaccines against N meningitidis.

Funding

Meningitis Research Foundation, Wellcome Trust, Public Health England, European Union.

Biosynthetically engineered lipopolysaccharide as vaccine adjuvant

Lipopolysaccharide (LPS), a dominant component of the Gram-negative bacterial outer membrane, is a strong activator of the innate immune system, and thereby an important determinant in the adaptive immune response following bacterial infection. This adjuvant activity can be harnessed following immunization with bacteria-derived vaccines that naturally contain LPS, and when LPS or molecules derived from it are added to purified vaccine antigens. However, the downside of the strong biological activity of LPS is its ability to contribute to vaccine reactogenicity. Modification of the LPS structure allows triggering of a proper immune response needed in a vaccine against a particular pathogen while at the same time lowering its toxicity. Extensive modifications to the basic structure are possible by using our current knowledge of bacterial genes involved in LPS biosynthesis and modification. This review focuses on biosynthetic engineering of the structure of LPS and implications of these modifications for generation of safe adjuvants.