$$\rightleftharpoonup{xx}$$
$$\longleftharp{xx}$$,
$$\longrightharp{xx}$$,
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.