Here, we describe a method to induce meningococcal meningitis through an intracisternal route of infection in adult mice. We present a step by step protocol of meningococcal infection from the preparation of inoculum to the intracisternal infection; then record the animal survival and evaluate the bacterial loads in murine tissues.
Neisseria meningitidis (meningococcus) is a narrow-host-range microorganism, globally recognized as the leading cause of bacterial meningitis. Meningococcus is a transient colonizer of human nasopharynx of approximately 10% of healthy subject. In particular circumstances, it acquires an invasive ability to penetrate the mucosal barrier and invades the bloodstream causing septicaemia. In the latest case, fulminating sepsis could arise even without the consequent development of meningitis. Conversely, bacteria could poorly multiply in the bloodstream, cross the blood brain barrier, reach the central nervous system, leading to fulminant meningitis. The murine models of bacterial meningitis represent a useful tool to investigate the host-pathogen interactions and to analyze the pathogenetic mechanisms responsible for this lethal disease. Although, several experimental model systems have been evaluated over the last decades, none of these were able to reproduce the characteristic pathological events of meningococcal disease. In this experimental protocol, we describe a detailed procedure for the induction of meningococcal meningitis in a mouse model based on the intracisternal inoculation of bacteria. The peculiar signs of human meningitis were recorded in the murine host through the assessment of clinical parameters (e.g., temperature, body weight), evaluation of survival rate, microbiological analysis and histological examination of brain injury. When using intracisternal (i.cist.) inoculum, meningococci complete delivery directly into cisterna magna, leading to a very efficient meningococcal replication in the brain tissue. A 1,000-fold increase of viable count of bacteria is observed in about 18 h. Moreover, meningococci are also found in the spleen, and liver of infected mice, suggesting that the liver may represent a target organ for meningococcal replication.
Neisseria meningitidis is a Gram negative β-proteobacterium restricted to the human host, well known for being one of the most common causes of meningitis and sepsis in the human population across the world. It colonizes the upper respiratory tract (nose and throat) of healthy and asymptomatic carriers (2-30% of the population), but the bacterium sometimes evades various host immune defenses and spreads from the bloodstream to the brain causing an uncontrolled local inflammation, known as meningococcal meningitis. A combination of host and bacterial factors appears to contribute to the transition from the commensal to the invasive behavior1.
N. meningitidis is specialized exclusively in human colonization and infection. It has a narrow host range and, therefore, has limited in vivo pathogenesis studies due to the lack of suitable animal models that reproduce the human meningococcal disease. As a result, it had led to fundamental gaps in the comprehension concerning the pathogenesis of septicemia and meningitis caused by meningococcus. In the last decades, the development of many in vitro systems allowed the identification of several meningococcal virulence factors2,3,4. Although these valuable studies provided important insights to understand the role of these factors for a successful meningococcal infection, these models did not allow assessment of the consequences of bacterial interactions with the humoral and cellular immune system and even less with the whole tissue. In vivo animal models of infection are of great relevance as well for the evaluation of protection degree conferred by vaccine formulations. As a human-tropic pathogen, meningococcus possess appropriate determinants necessary for successful infection such as surface structures (i.e., type IV pili and opacity proteins) and iron uptake systems for human receptors and transport proteins (i.e., transferrin and lactoferrin)5,6,7 to properly adhere, survive and invade the human host. Finally, the genetic variation abilities of the pathogen to evade and/or block the human immune response further contribute to the high species tropism8,9. Therefore, the absence of specific host factors, involved in the interaction, can block steps of the pathogen’s life cycle, establishing significant difficulties in the development of small animal models summarizing the meningococcal life cycle.
Over the past decades, several approaches have been developed to improve our understanding of the meningococcal infectious cycle. Infections of two animal model, mouse and rat, either intraperitoneally (i.p.) or intranasally (i.n.), were developed to reproduce meningococcal disease10,11,12,13,14,15,16,17. The laboratory mouse is probably one of the more versatile animals for inducing experimental meningococcal infection.
However, the i.p. way of infection leads to the development of severe sepsis although it does not mimic the natural route of infection, whereas the i.n. route of infection was useful to evaluate meningococcal pathogenesis, even though it may induce lung infection prior to sepsis10,11,12,13,14,15,16,17.
The i.p. mouse model was instrumental to assess the protection from the meningococcal challenge10,11,12. The mouse model of meningococcal colonization based on the i.n. route of infection has been developed with infant mice, as they are more susceptible to meningococci, to reproduce an invasive infection mimicking the course of the meningococcal disease in humans13,14,15,16,17. Moreover, to promote meningococcal replication in the murine host, a growing number of technical strategies were also applied including the administration of the iron to the animals to improve the infection, the use of high bacterial inoculum, mouse-passaged bacterial strain as well as the employment of infant or immunocompromised animal hosts10,13,15,18,19. Expression of specific human factors like CD4620 or transferrin21 has increased the susceptibility of mice to this human-tropic bacterium; the employment of the human skin xenograft model of infection has also been useful to evaluate the adhesion ability of meningococci to human endothelium22,23. Collectively, the recent development of humanized transgenic mice has improved the understanding of the meningococcal pathogenesis and its host interactions.
Previously, we developed a murine model of meningococcal meningitis where the inoculation of bacteria was performed into the cisterna magna of adult mice with mouse-passaged bacteria24. Clinical parameters and the survival rate of infected mice demonstrated the establishment of meningitis with characteristics comparable to those seen in the human host, as well as, the microbiological and histological analyses of the brain. From these infected mice, bacteria were, also, recovered from blood, liver, and spleen, and bacterial loads from peripheral organs correlated with the infectious dose. In particular, this model was employed to evaluate the virulence of an isogenic mutant strain defective in the L-glutamate transporter GltT24. Recently, using our mouse model of meningococcal meningitis based on i.cist. route with serogroup C strain 93/42862,24 and an isogenic mutant defective in cssA gene encoding for UDP-N-acetylglucosamine 2-epimerase25, we have analyzed the role of exposed sialic acid in the establishment of disease in mice.
In this protocol, we describe a straightforward method to induce experimental meningococcal meningitis based on the i.cist. route of infection in Balb/c adult mice. This method is particularly useful for the characterization of meningococcal infection in a murine host, as well as for the assessment of the virulence between wild type reference strains and isogenic mutants. The intra-cisternal route of infection ensures complete delivery of the meningococci directly into the cisterna magna, which in turn facilitates bacterial replication in the cerebrospinal fluid (CSF) and induces meningitis with features that mimic those present in humans2,24,25,26.
This protocol was conducted to minimize animal suffering and reduce the number of mice in accordance with the European Communities Council Directive of November 24, 1986 (86/609/EEC). In vivo experiments reported in this study were approved by the Ethical Animal Care and Use Committee (Prot. number 2, 14 December 2012) and the Italian Ministry of Health (Prot. number 0000094-A-03/01/2013). All the procedures should be performed inside the Biosafety Cabinet 2 (BSC2) in a BSL2 room, and the potential infected waste should be disposed in dedicated containers.
1. Infection of Mice with N. meningitidis Serogroup C Strain
CAUTION: N. meningitidis is potentially a harmful pathogen and all necessary precautions must be taken when handling this microorganism. The entire experimentation requires Biosafety Level 2 (BSL2) containment. The researcher involved in animal studies should wear disposable personal protective equipment (PPE) for the duration of the experiment.
2. Animal Survival and CFU Counts
3. Preparation of Brain Tissues for CFU Count
Survival of mice infected with N. meningitidis wild type and isogenic mutant strains.
The Neisseria meningitidis strains used in these representative results are the serogroup C reference strain 93/4286 (ET-37) and its isogenic mutant 93/4286ΩcssA obtained by insertional inactivation of the cssA gene, coding for the UDP-N-acetylglucosamine 2-epimerase, that maps in capsule synthesis locus25. To assess the virulence degree of the cssA-defective strain in the present murine model, the lethal dose able to determine the death of 50% of infected animals (LD50) was evaluated. To this purpose three groups of animals were infected intracisternally with doses ranging from 104 to 106 CFU of the wild type strain 93/4286 and with doses of mutant strain 93/4286ΩcssA between 107 to 109 CFU. Generally, the reduction of clinical parameters (e.g., body weight and temperature) and the increasing of mortality rate happened within the first 72 h after the infection. The LD50 for the wild type strain corresponded to the meningococcal challenge of 104 CFU, whereas the mortality rate with the dose of 105 CFU was equal to 83.4% and with 106 CFU of 100% (Figure 1A). Conversely, to obtain the LD50 for the mutant strain 93/4286ΩcssA, it has been necessary a dose of 108 CFU (Figure 1B), an amount of 10,000 folds higher compared to wild type strain.
Evaluation of N. meningitidis viable CFU in the mouse brain tissues.
To follow the kinetics of infection in the brain tissue of infected animals, a time course assay was performed with wild type or cssA mutant strain25. After the i.cist. injection with 5×105 CFU of 93/4286 or 93/4286ΩcssA strains, there was a rapid increase of wild type bacteria in the brain tissue reaching the highest numbers at around 24 h post infection (Figure 2A); conversely in the brain of mice challenged with the isogenic cssA-defective mutant, the viable counts dropped progressively over time until to 2.026 log CFU ± 1.774 72 h post infection (Figure 2A). The experiment has shown that 33.3% of the mutant challenged mice showed bacterial clearance from the infection site, whereas infection with wild type strain was never eradicated from the brain of animals.
Evaluation of meningococcal load in spleen and liver 48h post-challenge.
This experiment was performed to evaluate the clearance of bacteria from infected mice 48 h post-challenge in peripheral organs. To this aim, two groups of mice were infected with 5 x 105 CFU of either 93/4286 or 93/4286ΩcssA strains, and bacterial viable counts were evaluated in the spleen and liver of infected mice25 (Figure 2B). Following 48 h from the meningococcal injection, the cssA-defective mutant was completely cleared in spleen and liver, while the animals infected with wild type strain exhibited a persistent systemic infection framework. The mean values of CFU after 48 h were indeed still 3.212 log CFU ± 3.354 and 6.949 log CFU ± 1.37 in the spleen and liver, respectively. The difference in bacterial loads in the liver tissue of two animal groups was statistically significant (with a P < 0.001).
Figure 1: Survival of mice infected with 93/4286 wild type or cssA-defective N. meningitidis strains. (A) Three groups of Balb/c mice (n= 6/dose) were infected i.cist. with 104, 105, and 106 CFU/mouse of the wild type strain 93/4286 and (B) with 107, 108, and 109 CFU/mouse of the isogenic cssA-defective mutant. Mice were monitored for a week, and survival was recorded. Results are expressed as percent survival at different doses over time, the log rank p value was < 0.05 for mice infected with the wild type strain. This figure has been modified from Colicchio et al.25. Please click here to view a larger version of this figure.
Figure 2: Evaluation of bacterial loads over time in mice inoculated with the 93/4286 or 93/4286ΩcssA strains. (A) Time course of bacterial loads in the brain tissues following i.cist. infection. Two groups of Balb/c mice (n = 20/group) were infected by the i.cist. route with 5 x 105 CFU of either the wild type strain 93/4286 or the cssA-defective mutant. Animals were sacrificed 4, 24, 48, and 72 h after infection. Brains were harvested, mechanically homogenized in GC medium, and viable counts were determined. Results are expressed as mean ±SD log of CFU numbers per organ at different time points after inoculation. Asterisks indicate statistical significance (**, P < 0.01). (B) Bacterial loads over time in spleen and liver. Two groups of Balb/c mice (n = 5/group) were infected i.cist. with 5 x 105 CFU of either the wild type strain 93/4286 or the cssA-defective mutant. Animals were euthanized 48 h after infection. Spleens and livers were harvested, mechanically homogenized, and viable counts were determined. Results are expressed as log CFU numbers per organ. Horizontal bars indicate mean logs of bacterial titers. Asterisks indicate statistical significance (***, P < 0.001). This figure has been modified from Colicchio et al.25. Please click here to view a larger version of this figure.
In this study, we describe an experimental protocol to induce meningococcal meningitis in adult mice by i.cist. inoculation of meningococcal bacteria. To our knowledge, no other model of meningococcal meningitis has been developed in laboratory mice infected by i.cist. route; in the past, this way has been explored to provide models of meningococcal meningitis in both rat31 and rabbit32. It is well-known that the highest rate of meningococcal disease is found between young children, adolescents, and young adults33,34,35; for this reason, in our meningitis mouse model, instead of focusing on neonatal or infant animals, 8 week-old immunocompetent animals were employed.
In our experimental model, we decided to use i.cist. inoculum as it ensures the releasing of the meningococci directly into cisterna magna so facilitating bacterial replication in the CSF. This route of inoculation is physiologically more accessible28 and less traumatic than the intracranial subarachnoidal route, already used for the development of meningitis due to Streptococcus spp.36,37. Although it does not represent the natural way of infection of meningococcus, the injection of bacteria in this area was instrumental for the induction of meningococcal meningitis, as shown by mouse survival, bacteria loads, clinical parameters, and also by histological analysis24,25,26. Interestingly, reference strain 93/4286 induced meningitis with histopathologic features mimicking those observed in human disease24,25,26.
To establish a standardized murine infection and to guarantee the safety of researchers, it was preferred to start from a titrated frozen stock of bacteria rather than a fresh bacterial growth24,25,30,38, moreover, we decided to use a sub-lethal dose of live meningococci with the aim to limit a rapid fatal outcome and permit the development of brain damage2,24,25,26. Nevertheless, to determine an appropriate infectious dose for different strains, preliminary experiments must be performed. In the present study, we tested a serogroup C reference strain 93/4286 and an isogenic mutant strain 93/4286ΩcssA with a dose of 5 x 105 CFU/ mice.
Although this model does not mimic the initial phases of colonization and invasion of meningococcus, the bacterial isolate grows well not only in the CSF, but it is also able to remain in the spleen and liver compartments. During the hypoferremic phase of neisserial infection, most of heme-derived iron remains combined with liver ferritin. As ferritin can be used meningococci to obtain iron39, the liver constitutes a target organ for bacterial replication.
In this experimental procedure, the inbred Balb/c mouse strain was used in replacement of the outbred CD-1 strain that was originally employed to develop the meningococcal meningitis model24. Outbred mice were characterized by a wide genetic variability that may be more appropriate to reveal several effects in a variable cohort such as human population40,41. However, this variability needs a higher sample size to obtain sufficient statistical significance and may interfere with the standardization of procedures and targeted studies.
Despite the narrow host range of meningococcus, to ensure the replication of bacteria in murine host and enhance the virulence of meningococci, iron dextran was administered to animals before the infection6,14. Finally, compared to other experimental model of meningitis, animals were not treated with any antibiotics, to not affect the course of disease and/or profile of the inflammatory response26.
To date, our studies have highlighted that the i.cist. model is functional to induce both meningitis and invasive meningococcal disease compared to i.p. or i.n. models of infection, which are characterized by the occurrence of sepsis and bacteremia before meningitis is established10,11,12,13,14,15,16,17. Therefore, this infection model based on the induction of meningitis in murine host, could be useful not only to evaluate innovative therapeutic strategy to prevent bacterial replication directly into CSF but also to analyze the efficacy of possible passive immune therapy against human pathogens.
However meningococcal infection is a multistep process, that includes nasopharynx colonization, access to bloodstream, crossing of the blood brain barrier and finally uncontrolled proliferation in the CSF, our model only reproduces some aspects of the meningococcal infection, in order to subvert in part these limitations transgenic animal model may be useful to mimic the human pathogenesis of meningococcal disease.
The authors have nothing to disclose.
The studies were supported in part by PRIN 2012 [grant number 2012WJSX8K]: “Host-microbe interaction models in mucosal infections: development of novel therapeutic strategies” and by PRIN 2017 [2017SFBFER]: “An integrated approach to tackle the interplay among adaptation, stressful conditions and antimicrobial resistance of challenging pathogens”.
1,8 Skirted Cryovial With external thread | Starlab | E3090-6222 | |
50ml Polypropylene Conical Tube | Falcon | 352070 | 30 x 115mm |
Adson Forceps | F.S.T. | 11006-12 | Stainless Steel |
Alarm-Thermometer | TESTO | 9000530 | |
BactoTM Proteose Peptone | BD | 211693 | |
BD Micro Fine syringe | BD | 320837 | U-100 Insulin |
BD Plastipak syringe 1ml 25GA 5/8in | BD | 300014 | 05x16mm |
BD Plastipak syringe 5ml | BD | 308062 | 07 x 30mm |
BIOHAZARD AURA B VERTICAL LAMINAR FLOW CABINET | Bio Air s.c.r.l. | Aura B3 | |
BioPhotometer | Eppendorf | Model #6131 | |
Bottle D | Tecniplast | D | Graduated up to:400ml, Total Volume 450ml, 72x72x122mm |
C150 CO2 Incubator | Binder | 9040-0078 | |
Cage Body Eurostandard Type II | Tecniplast | 1264C | 267x207x140mm, Floor area 370cm2 |
Cell Culture Petri Dish With Lid | Thermo Scientific | 150288 | Working Volume: 5mL |
Centrifuge | Eppendorf | Microcentrifuge 5415R | |
Cuvetta semi-micro L. Form | Kartell S.p.A. | 01938-00 | |
di-Potassium hydrogen phosphate trihydrate | Carlo erba | 471767 | |
di-Sodium hydrogen phosphate anhydrous ACS-for analysis | Carlo Erba | 480141 | g1000 |
Diete Standard Certificate | Mucedola s.r.l. | 4RF21 | Food pellet for animal |
Dumont Hp Tweezers 5 Stainless Steel | F.S.T. by DUMONT | AGT5034 | 0,10 x 0,06 mm tip |
Electronic Balance | Gibertini | EU-C1200 | Max 1200g, d=0,01g, T=-1200g |
Eppendorf Microcentrifuge tube safe-lock | Eppendorf | T3545-1000EA | |
Erythromycin | Sigma-Aldrich | E-6376 | 25g |
Extra Fine Bonn Scissors | F.S.T. | 14084-08 | Stainless Steel |
Filter Top (mini- Isolator), H-Temp with lock clamps | Tecniplast | 1264C400SUC | |
GC agar base | OXOID | CM0367 | |
Gillies Forceps 1 x2 teeth | F.S.T. | 11028-15 | Stainless Steel |
Glicerin RPE | Carlo Erba | 453752 | 1L |
Graefe Forceps | F.S.T. | 11052-10 | Serrated Tip Width: 0.8mm |
Inner lid | Tecniplast | 1264C116 | |
Iron dextran solution | Sigma-Aldrich | D8517-25ML | |
Ketamine | Intervet | ||
Microbiological Safety Cabinet BH-EN and BHG Class II | Faster | BH-EN 2004 | |
Microcentrifuge tubes 1.5ml | BRAND | PP780751 | screw cap PP, grad |
Mouse Handling Forceps | F.S.T. | 11035-20 | Serrated rubber; Gripping surface:15 x 20 mm |
Mucotit-F2000 | MERZ | 61846 | 2000ml |
Natural Latex Gloves | Medica | M101 | |
New Brunswick Classic C24 Incubator Shaker | PBI international | C-24 Classic Benchtop Incubator Shaker | |
Petri PS Dishes | VWR | 391-0453 | 90X14.2MM |
Pipetman Classic P20 | Gilson | F123600 | 2-20microL |
Pipetman Classic P200 | Gilson | F123601 | 20-200microL |
Pipetman Classim P1000 | Gilson | F123602 | 200-1000microL |
Polyvitox | OXOID | SR0090A | |
Potassium Chloride | J.T. Baker Chemicals B.V. | 0208 | 250g |
Potassium Dihydrogen Phosphate | J.T. Baker Chemicals B.V. | 0240 | 1Kg |
PS Disposible forceps | VWR | 232-0191 | |
Removable Divider | Tecniplast | 1264C812 | |
Round-Bottom Polypropylene Tubes | Falcon | 352063 | 5ml |
Sodium Chloride | MOLEKULA | 41272436 | |
SS retainer and Polyester FilterSheet | Tecniplast | 1264C | |
Standard Pattern Forceps | F.S.T. | 11000-12 | Stainless |
Stevens Tenotomy Scissors | F.S.T. | 14066-11 | Stainless Steel |
Surgical Scissor – ToughCut | F.S.T. | 14130-17 | Stainless |
Touch N Tuff disposible nitrile gloves | Ansell | 92-500 | |
Ultra Low Temperature (ULT) Freezer | Haier | DW-86L288 | Volume= 288L |
Wagner Scissors | F.S.T. | 14070-12 | Stainless Steel |
Xylazine | Intervet |