Pathogenesis of bacterial meningitis: contributions by experimental models in rabbits

Stem cells for neonatal brain injury - Lessons from the bench

Neonatal brain injury resulting from various intractable disorders including intraventricular hemorrhage and hypoxic ischemic encephalopathy still remains a major cause of mortality and morbidities with few effective treatments. Recent preclinical research results showing the pleiotropic neuroprotective effects of stem cell therapy, specifically mesenchymal stem cells (MSCs), suggest that MSCs transplantation might be a promising new therapeutic modality for neuroprotection against the currently intractable and devastating neonatal brain injury with complex multifactorial etiology. This review summarizes recent advances in preclinical stem cell research for treating neonatal brain injury with a focus on the important issues including the mechanism of neuroprotection, and determining the ideal cell source, route, timing and dose of MSCs transplantation.

Streptococcus pneumoniae Rapidly Translocate from the Nasopharynx through the Cribriform Plate to Invade the Outer Meninges

The entry routes and translocation mechanisms of microorganisms or particulate materials into the central nervous system remain obscure We report here that Streptococcus pneumoniae (pneumococcus), or polystyrene microspheres of similar size, appear in the meninges of the dorsal cortex of mice within minutes of inhaled delivery. Recovery of viable bacteria from dissected tissue and fluorescence microscopy show that up to at least 72 h, pneumococci and microspheres were predominantly found in the outer of the two meninges: the pachymeninx. No pneumococci were found in blood or cerebrospinal fluid. Intravital imaging through the skull, aligned with flow cytometry showed recruitment and activation of LysM+ cells in the dorsal pachymeninx at 5 and 10 hours following intranasal infection. Imaging of the cribriform plate suggested that both pneumococci and microspheres entered through the foramina via an inward flow of fluid connecting the nose to the pachymeninx. Our findings bring new insight into the varied mechanisms of pneumococcal invasion of the central nervous system, but they are also pertinent to the delivery of drugs to the brain and the entry of airborne particulate matter into the cranium. IMPORTANCE Using two-photon imaging, we show that pneumococci translocate from the nasopharynx to the dorsal meninges of a mouse in the absence of any bacteria found in blood or cerebrospinal fluid. Strikingly, this takes place within minutes of inhaled delivery of pneumococci, suggesting the existence of an inward flow of fluid connecting the nasopharynx to the meninges, rather than a receptor-mediated mechanism. We also show that this process is size dependent, as microspheres of the same size as pneumococci can translocate along the same pathway, while larger size microspheres cannot. Furthermore, we describe the host response to invasion of the outer meninges. Our study provides a completely new insight into the key initial events that occur during the translocation of pneumococci directly from the nasal cavity to the meninges, with relevance to the development of intranasal drug delivery systems and the investigations of brain damage caused by inhaled air pollutants.

Quorum sensing-1 signaling of N-hexanoyl-L-homoserine lactone contributes to virulence in avian pathogenic Escherichia coli

Avian pathogenic E. coli (APEC) caused avian colibacillosis is mostly common in poultry industry worldwide. APEC virulence factors lead to pathogenesis and the quorum sensing (QS) system is actively involved in the regulation of these virulence factors. Signaling molecules in QS are known as autoinducers (AIs). In QS-1, E. coli encodes a single LuxR homolog, i.e., SdiA, but does not express the LuxI homolog, an acyl-homoserine lactone (AHL) synthase of producing AI-1. Avian pathogenic E. coli (APEC) regulates its virulence genes expression in response to exogenous AHLs, but regulatory mechanisms of AHL and QS-1 are still unknown. This study targeted the APEC CE129 isolate as the reference strain, and the Yersinia enterocolitica yenI gene was expressed into APEC CE129. CE129/pyenI was conferred the ability to produce AHL signal. The CE129 SdiA mutant strain with an in-frame sdiA (AHL receptor) gene deletion was constructed by a λRed recombination system, which lost the ability to sense AHL. The goal of this study was to explore the function of QS-1 upon virulence and elucidate the regulatory effect of QS-1/AHL signals in the APEC strain. Adherence and invasion assays revealed that QS-1 affected APEC adherence and survival ability. APEC biofilm formation was also suppressed under C6HSL. Interestingly, APEC exhibited different phenotypes of acid tolerance and flagella expression when compared to enterotoxigenic E. coli or enterohemorrhagic E. coli (ETEC and EHEC, respectively). These findings enhance our understanding of the QS mechanism.

Developing a newborn rat model of ventriculitis without concomitant bacteremia by intraventricular injection of K1 (-) Escherichia coli

BACKGROUND

Neonatal meningitis caused by Escherichia coli results in high mortality and neurological disabilities, and the concomitant systemic bacteremia confounds its mortality and brain injury. This study developed an experimental model of neonatal ventriculitis without concomitant systemic bacteremia by determining the bacterial inoculum of K1 capsule-negative E. coli by intraventricular injection in newborn rats.

METHODS

We carried out intraventricular injections 1 × 10² (low dose), 5 × 10² (medium dose), or 1 × 10³ (high dose) colony-forming units (CFU) of K1 (-) E. coli (EC5ME) in Sprague-Dawley rats at postnatal day (P) 11. Ampicillin was started at P12. Blood and cerebrospinal fluid (CSF) cultures were performed at 6 h, 1 day, and 6 days after inoculation. Brain magnetic resonance imaging (MRI) was performed at P12 and P17. Survival was monitored, and brain tissue was obtained for histological and biochemical analyses at P12 and P17.

RESULTS

Survival was inoculum dose-dependent, with the lowest survival in the high-dose group (20%) compared with the medium- (67%) or low- (73%) dose groups. CSF bacterial counts in the low- and medium-dose groups were significantly lower than that in the high-dose group at 6 h, but not at 24 h after inoculation. No bacteria were isolated from the blood throughout the experiment or from the CSF at P17. Brain MRI showed an inoculum dose-dependent increase in the extent of brain injury and inflammatory responses.

CONCLUSIONS

We developed a newborn rat model of bacterial ventriculitis without concomitant systemic bacteremia by intraventricular injection of EC5ME.

Animal models of Streptococcus pneumoniae disease

SUMMARY

Streptococcus pneumoniae is a colonizer of human nasopharynx, but it is also an important pathogen responsible for high morbidity, high mortality, numerous disabilities, and high health costs throughout the world. Major diseases caused by S. pneumoniae are otitis media, pneumonia, sepsis, and meningitis. Despite the availability of antibiotics and vaccines, pneumococcal infections still have high mortality rates, especially in risk groups. For this reason, there is an exceptionally extensive research effort worldwide to better understand the diseases caused by the pneumococcus, with the aim of developing improved therapeutics and vaccines. Animal experimentation is an essential tool to study the pathogenesis of infectious diseases and test novel drugs and vaccines. This article reviews both historical and innovative laboratory pneumococcal animal models that have vastly added to knowledge of (i) mechanisms of infection, pathogenesis, and immunity; (ii) efficacies of antimicrobials; and (iii) screening of vaccine candidates. A comprehensive description of the techniques applied to induce disease is provided, the advantages and limitations of mouse, rat, and rabbit models used to mimic pneumonia, sepsis, and meningitis are discussed, and a section on otitis media models is also included. The choice of appropriate animal models for in vivo studies is a key element for improved understanding of pneumococcal disease.

The arthritis severity quantitative trait locus Cia7 regulates neutrophil migration into inflammatory sites

Neutrophils are required for the development of arthritis in rodents, and are the predominant cell in the synovial fluid of active rheumatoid arthritis. We hypothesized that neutrophil migration into the inflammed joint is genetically regulated. In addition, this genetic regulation would be accounted for by one of the arthritis loci that we have previously identified in an intercross between arthritis-susceptible DA and arthritis-resistant ACI rats studied for collagen-induced arthritis. We used the synovial-like air pouch model injected with carrageenan, and tested DA, ACI, and four congenic strains. ACI exudates had a significantly lower number of neutrophils compared with DA. Transfer of DA alleles at Cia7 into the ACI background, as in ACI.DA(Cia7) congenics, was enough to increase exudate neutrophil numbers to levels identical to DA, and this locus accounted for the difference between parental strains. None of the other congenic intervals explained the differences in exudate neutrophil counts. In conclusion, we have identified a novel function for Cia7, and determined that it regulates neutrophil migration into a synovial-like inflammatory site. Our data revealed no intrinsic defect in neutrophil responses to chemotactic agents, and suggest that Cia7 regulates an as yet unidentified factor central to neutrophil recruitment into inflammed tissues.

The trigeminovascular system in bacterial meningitis

Headache as a cardinal symptom of acute meningitis reflects activation of trigeminal afferents from the meninges. With their perivascular endings, these fibers form the so-called trigeminovascular system (TVS), which releases proinflammatory neuropeptides upon nociceptive stimulation. In the present article, we review a role of the TVS in enhancing the early inflammatory response of bacterial meningitis. Furthermore, we discuss inhibition of neuropeptide release from the TVS using 5HT(1B/D) agonists as a potential new anti-inflammatory treatment strategy for early bacterial meningitis.

Models of experimental bacterial meningitis. Role and limitations

The seriousness of bacterial meningitis has encouraged the development of animal models that characterize complex pathogenetic and pathophysiologic mechanisms, provide evaluation of pharmacokinetic and antimicrobial effects of antibiotics (especially since the worldwide emergence of multiresistant bacteria), and establish new adjuvant treatment strategies (e.g., use of anti-inflammatory agents). The information obtained from an animal model depends on the site of inoculation. For example, using intranasal, intravenous, subcutaneous, or intraperitoneal inoculation, it is the bacterial and host factors that determine the development of bacteremia and the potential for a pathogen to invade the central nervous system that primarily are studied. In contrast, experimental models using direct inoculation into the cerebrospinal fluid can reliably produce lethal infections over a predictable time course. Furthermore, because adult animals will not reliably develop meningitis after intranasal or intraperitoneal challenge, infant animals are used. Because these models bypass the natural dissemination of bacteria from the intravascular compartment to the central nervous system, the pathogenesis is artificial. These models, however, are extremely useful for the study of pathogen and host factors leading to meningeal inflammation and resulting complications, and for evaluating potentially useful agents for treatment therapy. During the past decade, the design of clinical studies has been stimulated by findings obtained from these animal models.

Pathogenesis of bacterial meningitis

Bacterial meningitis is fatal in 5% to 40% of patients and causes neurologic sequelae in up to 30% of survivors. Much has been learned recently about the mechanisms that lead to brain injury during meningitis. Once bacteria have gained access to the central nervous system, their multiplication triggers a complex host response consisting of humoral and cellular immune mediators, reactive oxygen intermediates, matrix-metalloproteinases, and other host-derived factors. Alterations of the cerebral vasculature, with disruption of the blood brain barrier and global and focal ischemia, ultimately lead to functional and structural brain damage. This article reviews current concepts of the pathophysiology of bacterial meningitis and emphasizes possible therapeutic strategies to prevent its harmful consequences.

Effect of thalidomide on the inflammatory response in cerebrospinal fluid in experimental bacterial meningitis

In experimental bacterial meningitis in rabbits, the inflammatory process is largely mediated by cytokines such as IL-1 and TNF-alpha. Since thalidomide has been shown to inhibit TNF-alpha production, experiments were carried out to determine whether the drug can modulate the inflammatory response to either lysates of H. influenzae (gram negative) or heat killed S. pneumoniae (gram positive) in rabbits. The introduction of a lysate of H. influenzae into the CSF of rabbits causes a very acute inflammatory response, as indicated by a rapid increase in TNF-alpha levels in the CSF and a concomitantly rapid leukocytosis. In contrast, the introduction of heat killed S. pneumoniae, induces a more indolent inflammatory response which also wanes more slowly. Thalidomide treatment reduces TNF-alpha production in both experimental systems, but has a greater effect on the more indolent gram positive inflammatory response in which peak TNF-alpha levels in the CSF are reduced by > 50%. Also, a sustained inhibition of leukocytosis is observed in the inflammatory response to heat-killed gram positive bacteria. In meningeal inflammation induced by the Gram negative lysate, treatment with thalidomide results in only a 29% inhibition of TNF-alpha release into the CSF. In contrast to the drug effect on TNF-alpha, thalidomide treatment does not significantly affect IL-1 levels in these models of rabbit bacterial meningitis.

Mediators of meningitis: therapeutic implications

Despite the availability of potent antibiotics, bacterial meningitis is still a major clinical problem. Mortality is high, and up to a third of survivors are left with neurologic sequelae that may range from mild behavioral disorders to mental retardation or deafness. New therapeutic approaches to meningeal inflammation, however, are reducing the neurologic risks.

Early synergistic killing activity at concentrations attainable in CSF of amoxicillin or cefotaxime and aminoglycosides against Haemophilus influenzae

Rapid eradication of bacteria from the CSF is critical for the outcome of Haemophilus influenzae meningitis in children. In 15 patients studied, the mean H. influenzae colony counts in CSF were 10(5) CFU/ml (range: 10(2) to 10(9) CFU/ml). Time-kill curves were determined for amoxicillin and cefotaxime alone and in combination with gentamicin or amikacin, against 60 clinical isolates of H. influenzae at concentrations equivalent to those found in CSF following systemic administration. Against beta-lactamase-negative strains (n = 44) a bactericidal effect was observed at 18 h for amoxicillin alone, at 5 h for amoxicillin plus aminoglycosides and at 2.5 h for cefotaxime with or without aminoglycosides. Against beta-lactamase-positive strains a bactericidal effect was observed at 18 h for cefotaxime, at 5 h for amoxicillin plus aminoglycosides and at 2.5 h for cefotaxime plus aminoglycosides. It appears that despite low concentrations of gentamicin or amikacin in the CSF, the accelerated killing of H. influenzae provides a rationale for the initial use of the combination of cefotaxime and aminoglycosides in the initial treatment of H. influenzae meningitis.

Experimental meningitis in the rabbit. II. Cerebral energy metabolism in relation to increased cerebrospinal fluid concentrations of lactate

We have analyzed cerebral energy metabolism in rabbits with Streptococcus pneumoniae or Escherichia coli meningitis aiming at an increased understanding of the cerebrospinal fluid (CSF) lactacidosis observed in this disease. After intracisternal inoculation of bacteria the lactate concentration in the CSF increased to 9.7 +/- 0.7 (mean +/- SE) mmol/l compared to control values of 3.2 +/- 0.2 mmol/l. Simultaneously sampled brain tissue from parietal cortex, caudate nucleus, and thalamus showed no increase in lactate concentrations. The high-energy phosphate content decreased only marginally, phosphocreatine levels by 11-17% in the cortex and in the caudate nucleus, and adenosine triphosphate concentrations by 15%, but only in the caudate nucleus. Our results indicate that the CSF lactate increase in bacterial meningitis is not primarily linked to cerebral lactacidosis. The decreased concentrations of high-energy phosphates in diseased animals need further study but may be due to increased intracranial pressure and reduced capillary blood flow.

Experimental bacterial meningitis in the rabbit: cerebrospinal fluid changes and its relation to leukocyte response

This study was focused on cerebrospinal fluid (CSF) manifestations in experimental Streptococcus pneumoniae and Escherichia coli meningitis in rabbits. An increased (p less than 0.001) CSF lactate concentration was found in infected animals, mostly not accompanied by a decrease in CSF glucose concentrations. Despite a marked difference in CSF cellular response between the 2 etiological groups no significant difference in CSF lactate levels was found. Neither did CSF lactate levels correlate to CSF polymorphonuclear cell counts. CSF concentrations of albumin were with large variations above control levels in all infected animals. Also a small or moderate increase in CSF albumin levels was generally associated with a marked increase in CSF lactate concentration. The concentration of total amino acids in the CSF was above control values (mean + 2 SD) in 9/21 infected animals. Halothane/N2O anesthesia for 25 min increased (p less than 0.05) CSF levels of glucose, partly independent of alterations in plasma glucose concentrations, in both infected rabbits and in controls.

Treatment of bacterial meningitis

Bacterial meningitis remains a life-threatening infection at any age, and prompt, adequate treatment is of great prognostic importance. Due to the special features of the meninges as a site of infection, many problems are associated with antimicrobial therapy. Approximately 80% of the patients with bacterial meningitis belong to the pediatric age group, and the three principal pathogens are Haemophilus influenzae type b, Neisseria meningitidis and Streptococcus pneumoniae. The diagnostic requirements essential for satisfactory management are defined and the antibiotic therapy of meningitis is discussed in detail. Data obtained from both experimental and clinical meningitis are indicative that a minimum bactericidal titre of 1:10 should be achieved in the cerebrospinal fluid for optimal therapeutic results. Recommendations are given for specific antimicrobial treatment according to patient's age and causative organism, as well as guidelines for further lumbar punctures, duration of therapy and prophylaxis, with emphasis on the important role of the newer cephalosporin compounds in treatment of meningitis.

Characterization of Raji cell binding IgG in patients with metastatic breast cancer

The aim of the present study was to isolate and characterize the immune complexes detected by the Raji cell assay in metastatic breast cancer. However, the Raji cell binding material could not be separated from monomeric IgG by Sephacryl S-300 gel chromatography in any of the 16 sera investigated. The parallel elution profile of monomeric IgG and the Raji cell binding activity suggested that anti-Raji cell antibodies, rather than immune complexes of low molecular weight, were present in these sera. This was further substantiated by pepsin digestion of the Raji cell binding IgG fractions. The binding of F(ab')2 fragments was quantitatively comparable to the binding of undigested IgG, and the bound F(ab')2 fragments could be visualized by immunofluorescence at the cell membrane. The presence of antibodies against Raji cells was further confirmed by the complement-dependent cytotoxicity assay. These antibodies did not react with untransformed lymphocytes and there was no correlation with anti-Epstein-Barr virus antibodies. The incidence of cytotoxic anti-Raji cell antibodies in breast cancer was 12% compared to 20% in malignant lymphoma, to 0% in normals and patients with degenerative cardiovascular diseases, and to 5% in patients with auto-immune diseases.