BACKGROUND
bacterial meningitis is an important infection of the central nervous system , and the three major responsible bacteria are neisseria meningitidis, haemophilus influenzae and streptococcus pneumoniae  <cit> .
despite the use of antimicrobial therapy, pneumococcal meningitis  has the highest case-fatality rate  for bacterial meningitis, and in  <dig> % of cases, it leads to serious neurological sequelae, including cognitive impairment  <cit> .
development of pm generally starts from pneumococcal colonisation of the nasopharynx, which is the natural reservoir of s. pneumoniae in humans and especially in children  <cit> .
the pathogenic steps leading to pm include invasion of the bloodstream from the nasopharyngeal mucosa, survival in the blood, and subsequent entry into the cns by crossing the blood-brain-barrier   <cit> .
however, pm can also be caused by either contiguous spread of pneumococci infecting the sinuses or the middle ear, or accidental traumatic inoculation of bacteria into the cns  <cit> .
a recent paper showed that non-hematogenous invasion of the brain by s. pneumoniae in mice may also occur through retrograde axonal transport along olfactory neurons  <cit> .
once the pneumococcus starts replicating in the cerebrospinal fluid , severe inflammation occurs in cerebral vessels and subarachnoid space, and damage to the brain parenchyma is produced  <cit> .
animal models of pm have been developed in order to:  characterise the pathogenesis of meningitis,  analyse the role of pneumococcal virulence factors in the disease,  understand the host immune response to s. pneumoniae infection, and  test the efficacy of novel antibiotics and vaccine candidates.
both infant and adult rats  <cit> , and also adult rabbits  <cit>  have largely been employed as animal models to characterise pm induced by intracisternal inoculation of bacteria.
infant rats have also been used to study haematogenous meningitis following intraperitoneal infection with s. pneumoniae  <cit> .
however, models of pm have also been developed in the mouse by using the following routes of infection:  intraperitoneal   <cit> ,  intranasal   <cit> , or  intracranial  parenchymal  <cit>  or cisternal  <cit> .
haematogenous murine meningitis models  allow to study pm pathogenesis, and i.n.
models are particularly useful as they mimic the natural infection route of s. pneumoniae in humans.
however, those models present the disadvantage that pm is induced in about half of the animals, while the remaining mice may die of sepsis without developing meningitis.
models of meningitis induced by the i.p.
route were employed to carry out therapeutic studies  <cit>  and investigations on pm pathogenesis  <cit> .
the i.n.
model by zwijnenburg et al.
<cit>  was employed in interleukin - <dig>  il- <dig>  and il- <dig> receptor deficient mice to investigate the role of different cytokines in pm  <cit> .
direct induction  of pm mimics meningitis caused by contiguous spread from the sinuses or traumatic entrance of pneumococci into the cns and allows to study the host-parasite interaction in the brain.
besides an early model of i.c.-induced meningitis in mice  <cit>  employed in therapeutic studies  <cit> , the model by gerber et al.
is an useful and reliable system for causing pm in the mouse  <cit> .
following infection of c57bl/ <dig> inbred mice into the right lobe of the brain with type  <dig> pneumococci, bacterial enumeration in different organs, brain histology, behavioural tests, and clinical scores were performed  <cit> .
a model of intracisternal infection was described by koedel et al., who induced meningitis via inoculation of s. pneumoniae  into the cisterna magna of c57bl/ <dig> mice and investigated the function of nitric oxide in the disease  <cit> .
both models, largely employed in studies on the roles of both pneumococcal  <cit>  and host factors  <cit>  in pm, rely on the use of inbred mouse strains and type  <dig> pneumococci.
in the present study, we describe an experimental model of pm in outbred mice based on the direct inoculation of bacteria into the subarachnoid space through a soft point located  <dig>  mm rostral from the bregma.
both the technique employed for infection and the anatomical coordinates of the inoculation site are accurately described.
the model was tested with pneumococcal strains of three different capsular serotypes and it was characterised in detail by using tigr <dig>  as a model strain.
the proposed method is precise, simple, cost-effective, fast and reproducible, and the disease induced closely resembles pm in humans.
RESULTS
inoculation site and technique
the bregma is the intersection of the coronal and sagittal sutures of the skull and can be recognised in mice by visual examination of the exposed skull .
we inoculated mice by the i.c.
route through a soft point located along the skull midline  <dig>  mm rostral to the bregma .
the stereotaxic coordinates of such inoculation point are  <dig> mm ,  <dig>  mm rostral  and  <dig> mm ventral  from the bregma  <cit> .
a preliminary study was carried out to trace diffusion of the inoculation material from the point of injection into the brain.
three animals were injected with  <dig> μl of trypan blue and sacrificed  <dig> minutes after inoculation.
following decapitation, skulls were sectioned into coronal planes, and diffusion of trypan blue was observed .
the dye rapidly spread from the injection site into the subarachnoid and ventricular spaces ; hence, this infection route is referred to as i.c.
subarachnoidal.
histological analysis of the brain sections confirmed that the inoculation needle crossed the mouse frontal lobes and reached the subarachnoid space .
in order to assess whether the inoculation technique was traumatic to animals, another experiment was performed by inoculating three control mice with saline.
animals recovered soon after injection and did not present any neurological problem  for several weeks after inoculation .
these data allowed localisation of the anatomical coordinates of the inoculation site and proved the suitability of the i.c.
subarachnoidal infection technique.
meningitis induction by type  <dig>   <dig> and  <dig> s. pneumoniae
after characterisation of the inoculation site and technique, mice were infected with pneumococci, and the establishment of pm was evaluated and clearly evidenced by histological analysis .
in order to test the model with different pneumococcal serotypes, dose-dependent survival studies were performed, and the lethal doses killing 50% of animals  were calculated.
we chose three commonly used s. pneumoniae strains, such as d <dig>  hb <dig>  and tigr <dig>  the d <dig> strain  is the encapsulated parent of the rough type  <dig> r36a strain used by avery  <cit> .
the hb <dig> strain  is a streptomycin-resistant derivative of the a <dig> strain used by avery  <cit> .
the serotype  <dig> tigr <dig> is the genome strain sequenced by the institute for genomic research  <cit> .
the survival patterns of mice inoculated with d <dig> and hb <dig> were almost identical, with ld <dig> of  <dig>  ×  <dig> and  <dig>  ×  <dig> colony forming units , respectively .
the tigr <dig> strain was less virulent in the i.c.
subarachnoidal infection model compared to d <dig> and hb <dig>  as its ld <dig> was  <dig>  ×  <dig> cfu .
animal survival and bacterial titres after infection with the tigr <dig> strain
to describe the features of pm in detail following i.c.
subarachnoidal infection of mice, we chose to characterise pm induced by the tigr <dig> genome strain.
for this purpose, we performed time-dependent survival studies, bacterial counts in different organs/tissues, and histological analysis of brain and spleen .
in order to study animal survival, five groups of mf <dig> mice were infected with doses of tigr <dig> increasing from  <dig> to  <dig> cfu per mouse.
the percentage of animals surviving over time at each bacterial dose was analysed by a kaplan-meier curve .
after inoculating  <dig> cfu, only one mouse out of eight died  <dig> hours after infection .
at the doses of  <dig> cfu and  <dig> cfu,  <dig> % of mice survived pneumococcal challenge, whereas the remaining  <dig> % died within  <dig> hours from infection.
survival further decreased from  <dig> % in mice infected with  <dig> cfu to  <dig> % following infection with  <dig> cfu, which induced severe symptoms and subsequent death of all mice within the first  <dig> hours after challenge .
the median time-to-death of the groups injected with  <dig>   <dig>   <dig>   <dig>  and  <dig> cfu were  <dig>   <dig>   <dig>   <dig>  and  <dig> hours, respectively.
from these results, the median survival time of animals did not vary at intermediate doses , while it considerably decreased at the highest dose  leading to rapid death of all mice.
to determine the number of pneumococci in brain, spleen and blood at the final stages of the disease, moribund mice infected with  <dig> cfu of tigr <dig> were sacrificed, samples collected and appropriately treated, and viable counts were carried out.
moribund animals showed comparable bacterial counts in the brain .
similarly, dissemination from the brain to vital organs occurred and was consistent in all animals, with bacterial counts of  <dig>  ×  <dig> ±  <dig>  ×  <dig> cfu and  <dig>  ×  <dig> ±  <dig>  ×  <dig> cfu in the spleen and blood, respectively .
histological characterisation of the pm model
in order to prove the establishment of pm and study the features of the disease, we performed histological analysis on the brain of moribund mice inoculated with several doses of the tigr <dig> strain and sacrificed at various time-points after infection.
animals at the final stages of the disease showed typical signs of meningitis , but they did not develop hemiparesis or plegia.
moribund mice that had received  <dig>   <dig>   <dig>  and  <dig> cfu were humanely killed at  <dig>   <dig>   <dig>  and  <dig> hours post-infection, respectively.
two animals for each pneumococcal dose and three additional control mice were sacrificed.
brains and spleens were excised and treated for both haematoxilin-eosin and gram staining.
in infected mice, no cerebral abscesses were observed, but only granulocytic infiltrations involving the subarachnoid and ventricular spaces.
differently, control animals injected with saline showed no histological changes following inoculation.
brains of moribund mice following infection with the tigr <dig> strain showed different degrees of inflammatory changes.
inflammation was regarded as mild in the presence of marked congestion of leptomeningeal blood vessels with margination of polymorphonuclear cells , edema, and wisps of fibrin .
inflammation was considered severe when the subarachnoid space  and/or the ventricular spaces   contained cellular exudates composed of pmns entrapped in a dense fibrin net.
no large areas of cerebral necrosis were found; however, in some cases, brain damage represented by neuronal shrinkage was observed in the hippocampus .
gram staining of brain sections of infected mice revealed the presence of short chains  of gram-positive dark blue bacteria in the subarachnoid space; pneumococci were located mainly extracellularly in a background of pmns .
analysis of the spleen of animals infected with s. pneumoniae revealed histological changes in both the white and red pulp with a massive congestion of the red pulp , compared to the spleen of control mice .
these data demonstrate that the i.c.
subarachnoidal route of infection is an effective and reliable way for inducing pm in the mouse.
discussion
s. pneumoniae is one of the causative agents of bacterial meningitis responsible for death and sequelae worldwide.
the mouse has largely proved to be a reliable animal model for studying two major pneumococcal diseases such as pneumonia  <cit>  and sepsis  <cit> .
in the case of pm, the rat  <cit>  and the rabbit  <cit>  have often been preferred to the mouse.
with the exception of an initial work describing an i.c.
infection procedure for inducing pm in mice in outbred mice  <cit> , only recently, models of pm based on i.c.
inoculation were made available in inbred mice  <cit> .
in the present work, starting from an experimental method used to study meningitis caused by cryptococcus neoformans  <cit> , we developed a model of pm based on the inoculation of bacteria into the subarachnoid space of outbred mice.
this infection route  mimics bacterial entrance into the cns from the sinuses or the middle ear, or following a trauma.
we chose to inoculate mice into the subarachnoid space through a soft point located on the skull  <dig> mm lateral,  <dig>  mm rostral, and  <dig> mm ventral from the bregma.
such point easily allows inoculation of bacteria from the skull through the frontal lobes into the subarachnoid space, as shown by a preliminary experiment in which trypan blue was used to localise the injection site and trace the inoculum within the brain .
the finding that the inoculation technique did not cause any trauma to animals can be explained by the fact that frontal lobectomy is tolerated in both humans  <cit>  and rats  <cit> , as frontal lobes are mainly committed to behavioural and cognitive functions.
we decided to use mf <dig> outbred mice because this strain is well-known for its susceptibility to both intranasal  <cit>  and intravenous  <cit>  challenge with s. pneumoniae, and because the use of outbred strains is cost-effective.
another research group had previously used cd- <dig> outbred mice in a study on the efficacy of clinafloxacin against pm; however, the authors did not provide a detailed description of the model  <cit> .
our i.c.
subarachnoidal infection model was tested by using a range of bacterial doses of three different s. pneumoniae strains.
the strains chosen are the serotypes routinely employed by researchers in the pneumococcal field, and have proved to be highly pathogenic in different mouse infection models  <cit> .
the use of several bacterial doses is also important, as it allows a more accurate evaluation of virulence for each strain, as well as establishing the most appropriate dose to be employed in different studies.
the model proved to be suitable for use with pneumococci of different serotypes, as type  <dig> d <dig>  type  <dig> hb <dig>  and type  <dig> tigr <dig> were all able to cause pm with ld <dig> ranging from  <dig>  ×  <dig> to  <dig>  ×  <dig> cfu.
then, pm was further characterised and standardised by using tigr <dig> as a model strain.
kaplan-meier survival analysis of animals inoculated with different doses of tigr <dig> showed that  <dig> cfu was lethal for all mice within  <dig> hours from infection , suggesting it as the appropriate dose inducing pm similar to hyperacute meningitis in humans.
moribund mice with acute meningitis after infection with  <dig> cfu were also septicaemic, as pneumococci could also be recovered from the blood and spleen.
bacterial titres were consistent in each organ/tissue of every mouse examined, underlining the reproducibility of data obtainable by using our model.
dissemination of pneumococci after infection with lethal doses is also in agreement with other i.c.
murine models of pm  <cit> .
the actual induction and subsequent characterisation of pm caused by the tigr <dig> strain was then carried out by histological analysis of the brain tissue from moribund animals.
we chose not to analyse pm by viable counting of pneumococci in the csf, due to the difficulty of sample collection  <cit>  and to the necessity of using pooled csf samples  <cit> .
histological examination of the brain showed both cases of mild meningeal inflammation and cases of severe granulocytic effusion in the subarachnoid and ventricular spaces  .
cerebral abscesses were not observed, further confirming that the our i.c.
subarachnoidal model is indeed a meningitis and not an encephalitis model of infection.
neither inflammatory changes nor death were observed following injection of saline into control mice.
inflammation and pmns distribution in the brain of moribund mice closely mimicked the histopathology of meningitis in humans  <cit> .
post-mortem examination of brains from patients who rapidly  died due to hyperacute meningitis generally reveals the presence of mild lesions consisting of a sparse leptomeningeal exudate with vessel congestion and pmn margination, in contrast to patients who survived for two or more days, who often exhibit a severe inflammation with fibrin and pmns in subarachnoid and ventricular spaces  <cit> .
a detailed analysis of pm largely resembling human meningitis was also reported by gerber et al., who injected c57bl/ <dig> inbred mice into the right lobe of the brain  <cit> .
in that model, the inoculation site was characterised by a large purulent infiltrate present in both meninges and ventricula, and necrosis was observed in all investigated brain regions  <cit> .
in another model, proposed by koedel et al., pneumococci were given by transcutaneous injection directly into the cisterna magna  <cit> .
in that study, brain lesions occurred in all mice  <dig> hours after infection, and histopathological examination revealed intense granulocytic infiltrations in the subarachnoid and ventricular spaces, and absence of cortical necrosis  <cit> .
this finding differs from the model by gerber et al., who instead reported the presence of extensive cerebral necrotic processes  <cit> .
in our model, we could not observe large areas of necrosis, but we found some signs of neuronal damage  in the hippocampus of a few animals.
CONCLUSIONS
the present work proposes a method to induce experimental pm in outbred mice by using an i.c.
subarachnoidal route of infection.
the stereotaxic coordinates of the injection site are provided to allow easy recognition of the inoculation point in the mouse.
the model is simple and fast, and the technique assures the development of meningitis, as demonstrated by histological analysis, survival data, and microbiological parameters.
no significant differences were observed in the ability of the three pneumococcal strains used to cause disease, emphasising the value of the model.
it is worth noting that the use of outbred mice still results in data reproducibility, as replicates in this model closely paralleled each other in terms of survival, cfu counts per organ, and histopathological features.
in addition, experiments in outbred mice are cost-effective and can be performed in larger animal groups thereby improving statistical significance.
this experimental pm model may be particularly useful for all researchers involved in studies that will investigate the host-pathogen interaction at the cerebral level, with emphasis on both pathogen-associated virulence factors and host-specific brain defences.

 bacterial meningitis is an important infection of the central nervous system, and the three major responsible bacteria are neisseria meningitidis, haemophilus influenzae and streptococcus pneumoniae. despite the use of antimicrobial therapy, pneumococcal meningitis has the highest case-fatality rate for bacterial meningitis, and in % of cases, it leads to serious neurological sequelae, including cognitive impairment. development of pm generally starts from pneumococcal colonisation of the nasopharynx, which is the natural reservoir of s. pneumoniae in humans and especially in children. the pathogenic steps leading to pm include invasion of the bloodstream from the nasopharyngeal mucosa, survival in the blood, and subsequent entry into the cns by crossing the blood-brain-barrier. however, pm can also be caused by either contiguous spread of pneumococci infecting the sinuses or the middle ear, or accidental traumatic inoculation of bacteria into the cns. a recent paper showed that non-hematogenous invasion of the brain by s. pneumoniae in mice may also occur through retrograde axonal transport along olfactory neurons. once the pneumococcus starts replicating in the cerebrospinal fluid, severe inflammation occurs in cerebral vessels and subarachnoid space, and damage to the brain parenchyma is produced. animal models of pm have been developed in order to: characterise the pathogenesis of meningitis, analyse the role of pneumococcal virulence factors in the disease, understand the host immune response to s. pneumoniae infection, and test the efficacy of novel antibiotics and vaccine candidates. both infant and adult rats, and also adult rabbits have largely been employed as animal models to characterise pm induced by intracisternal inoculation of bacteria. infant rats have also been used to study haematogenous meningitis following intraperitoneal infection with s. pneumoniae. however, models of pm have also been developed in the mouse by using the following routes of infection: intraperitoneal, intranasal, or intracranial parenchymal or cisternal. haematogenous murine meningitis models allow to study pm pathogenesis, and i.n. models are particularly useful as they mimic the natural infection route of s. pneumoniae in humans. however, those models present the disadvantage that pm is induced in about half of the animals, while the remaining mice may die of sepsis without developing meningitis. models of meningitis induced by the i.p. route were employed to carry out therapeutic studies and investigations on pm pathogenesis. the i.n. model by zwijnenburg et al. was employed in interleukin - il- and il- receptor deficient mice to investigate the role of different cytokines in pm. direct induction of pm mimics meningitis caused by contiguous spread from the sinuses or traumatic entrance of pneumococci into the cns and allows to study the host-parasite interaction in the brain. besides an early model of i.c.-induced meningitis in mice employed in therapeutic studies, the model by gerber et al. is an useful and reliable system for causing pm in the mouse. following infection of c57bl/ inbred mice into the right lobe of the brain with type pneumococci, bacterial enumeration in different organs, brain histology, behavioural tests, and clinical scores were performed. a model of intracisternal infection was described by koedel et al., who induced meningitis via inoculation of s. pneumoniae into the cisterna magna of c57bl/ mice and investigated the function of nitric oxide in the disease. both models, largely employed in studies on the roles of both pneumococcal and host factors in pm, rely on the use of inbred mouse strains and type pneumococci. in the present study, we describe an experimental model of pm in outbred mice based on the direct inoculation of bacteria into the subarachnoid space through a soft point located mm rostral from the bregma. both the technique employed for infection and the anatomical coordinates of the inoculation site are accurately described. the model was tested with pneumococcal strains of three different capsular serotypes and it was characterised in detail by using tigr as a model strain. the proposed method is precise, simple, cost-effective, fast and reproducible, and the disease induced closely resembles pm in humans.  inoculation site and technique the bregma is the intersection of the coronal and sagittal sutures of the skull and can be recognised in mice by visual examination of the exposed skull. we inoculated mice by the i.c. route through a soft point located along the skull midline mm rostral to the bregma. the stereotaxic coordinates of such inoculation point are mm, mm rostral and mm ventral from the bregma. a preliminary study was carried out to trace diffusion of the inoculation material from the point of injection into the brain. three animals were injected with μl of trypan blue and sacrificed minutes after inoculation. following decapitation, skulls were sectioned into coronal planes, and diffusion of trypan blue was observed. the dye rapidly spread from the injection site into the subarachnoid and ventricular spaces; hence, this infection route is referred to as i.c. subarachnoidal. histological analysis of the brain sections confirmed that the inoculation needle crossed the mouse frontal lobes and reached the subarachnoid space. in order to assess whether the inoculation technique was traumatic to animals, another experiment was performed by inoculating three control mice with saline. animals recovered soon after injection and did not present any neurological problem for several weeks after inoculation. these data allowed localisation of the anatomical coordinates of the inoculation site and proved the suitability of the i.c. subarachnoidal infection technique. meningitis induction by type and s. pneumoniae after characterisation of the inoculation site and technique, mice were infected with pneumococci, and the establishment of pm was evaluated and clearly evidenced by histological analysis. in order to test the model with different pneumococcal serotypes, dose-dependent survival studies were performed, and the lethal doses killing 50% of animals were calculated. we chose three commonly used s. pneumoniae strains, such as d hb and tigr the d strain is the encapsulated parent of the rough type r36a strain used by avery. the hb strain is a streptomycin-resistant derivative of the a strain used by avery. the serotype tigr is the genome strain sequenced by the institute for genomic research. the survival patterns of mice inoculated with d and hb were almost identical, with ld of × and × colony forming units, respectively. the tigr strain was less virulent in the i.c. subarachnoidal infection model compared to d and hb as its ld was × cfu. animal survival and bacterial titres after infection with the tigr strain to describe the features of pm in detail following i.c. subarachnoidal infection of mice, we chose to characterise pm induced by the tigr genome strain. for this purpose, we performed time-dependent survival studies, bacterial counts in different organs/tissues, and histological analysis of brain and spleen. in order to study animal survival, five groups of mf mice were infected with doses of tigr increasing from to cfu per mouse. the percentage of animals surviving over time at each bacterial dose was analysed by a kaplan-meier curve. after inoculating cfu, only one mouse out of eight died hours after infection. at the doses of cfu and cfu, % of mice survived pneumococcal challenge, whereas the remaining % died within hours from infection. survival further decreased from % in mice infected with cfu to % following infection with cfu, which induced severe symptoms and subsequent death of all mice within the first hours after challenge. the median time-to-death of the groups injected with and cfu were and hours, respectively. from these results, the median survival time of animals did not vary at intermediate doses, while it considerably decreased at the highest dose leading to rapid death of all mice. to determine the number of pneumococci in brain, spleen and blood at the final stages of the disease, moribund mice infected with cfu of tigr were sacrificed, samples collected and appropriately treated, and viable counts were carried out. moribund animals showed comparable bacterial counts in the brain. similarly, dissemination from the brain to vital organs occurred and was consistent in all animals, with bacterial counts of × ± × cfu and × ± × cfu in the spleen and blood, respectively. histological characterisation of the pm model in order to prove the establishment of pm and study the features of the disease, we performed histological analysis on the brain of moribund mice inoculated with several doses of the tigr strain and sacrificed at various time-points after infection. animals at the final stages of the disease showed typical signs of meningitis, but they did not develop hemiparesis or plegia. moribund mice that had received and cfu were humanely killed at and hours post-infection, respectively. two animals for each pneumococcal dose and three additional control mice were sacrificed. brains and spleens were excised and treated for both haematoxilin-eosin and gram staining. in infected mice, no cerebral abscesses were observed, but only granulocytic infiltrations involving the subarachnoid and ventricular spaces. differently, control animals injected with saline showed no histological changes following inoculation. brains of moribund mice following infection with the tigr strain showed different degrees of inflammatory changes. inflammation was regarded as mild in the presence of marked congestion of leptomeningeal blood vessels with margination of polymorphonuclear cells, edema, and wisps of fibrin. inflammation was considered severe when the subarachnoid space and/or the ventricular spaces contained cellular exudates composed of pmns entrapped in a dense fibrin net. no large areas of cerebral necrosis were found; however, in some cases, brain damage represented by neuronal shrinkage was observed in the hippocampus. gram staining of brain sections of infected mice revealed the presence of short chains of gram-positive dark blue bacteria in the subarachnoid space; pneumococci were located mainly extracellularly in a background of pmns. analysis of the spleen of animals infected with s. pneumoniae revealed histological changes in both the white and red pulp with a massive congestion of the red pulp, compared to the spleen of control mice. these data demonstrate that the i.c. subarachnoidal route of infection is an effective and reliable way for inducing pm in the mouse. discussion s. pneumoniae is one of the causative agents of bacterial meningitis responsible for death and sequelae worldwide. the mouse has largely proved to be a reliable animal model for studying two major pneumococcal diseases such as pneumonia and sepsis. in the case of pm, the rat and the rabbit have often been preferred to the mouse. with the exception of an initial work describing an i.c. infection procedure for inducing pm in mice in outbred mice, only recently, models of pm based on i.c. inoculation were made available in inbred mice. in the present work, starting from an experimental method used to study meningitis caused by cryptococcus neoformans, we developed a model of pm based on the inoculation of bacteria into the subarachnoid space of outbred mice. this infection route mimics bacterial entrance into the cns from the sinuses or the middle ear, or following a trauma. we chose to inoculate mice into the subarachnoid space through a soft point located on the skull mm lateral, mm rostral, and mm ventral from the bregma. such point easily allows inoculation of bacteria from the skull through the frontal lobes into the subarachnoid space, as shown by a preliminary experiment in which trypan blue was used to localise the injection site and trace the inoculum within the brain. the finding that the inoculation technique did not cause any trauma to animals can be explained by the fact that frontal lobectomy is tolerated in both humans and rats, as frontal lobes are mainly committed to behavioural and cognitive functions. we decided to use mf outbred mice because this strain is well-known for its susceptibility to both intranasal and intravenous challenge with s. pneumoniae, and because the use of outbred strains is cost-effective. another research group had previously used cd- outbred mice in a study on the efficacy of clinafloxacin against pm; however, the authors did not provide a detailed description of the model. our i.c. subarachnoidal infection model was tested by using a range of bacterial doses of three different s. pneumoniae strains. the strains chosen are the serotypes routinely employed by researchers in the pneumococcal field, and have proved to be highly pathogenic in different mouse infection models. the use of several bacterial doses is also important, as it allows a more accurate evaluation of virulence for each strain, as well as establishing the most appropriate dose to be employed in different studies. the model proved to be suitable for use with pneumococci of different serotypes, as type d type hb and type tigr were all able to cause pm with ld ranging from × to × cfu. then, pm was further characterised and standardised by using tigr as a model strain. kaplan-meier survival analysis of animals inoculated with different doses of tigr showed that cfu was lethal for all mice within hours from infection, suggesting it as the appropriate dose inducing pm similar to hyperacute meningitis in humans. moribund mice with acute meningitis after infection with cfu were also septicaemic, as pneumococci could also be recovered from the blood and spleen. bacterial titres were consistent in each organ/tissue of every mouse examined, underlining the reproducibility of data obtainable by using our model. dissemination of pneumococci after infection with lethal doses is also in agreement with other i.c. murine models of pm. the actual induction and subsequent characterisation of pm caused by the tigr strain was then carried out by histological analysis of the brain tissue from moribund animals. we chose not to analyse pm by viable counting of pneumococci in the csf, due to the difficulty of sample collection and to the necessity of using pooled csf samples. histological examination of the brain showed both cases of mild meningeal inflammation and cases of severe granulocytic effusion in the subarachnoid and ventricular spaces. cerebral abscesses were not observed, further confirming that the our i.c. subarachnoidal model is indeed a meningitis and not an encephalitis model of infection. neither inflammatory changes nor death were observed following injection of saline into control mice. inflammation and pmns distribution in the brain of moribund mice closely mimicked the histopathology of meningitis in humans. post-mortem examination of brains from patients who rapidly died due to hyperacute meningitis generally reveals the presence of mild lesions consisting of a sparse leptomeningeal exudate with vessel congestion and pmn margination, in contrast to patients who survived for two or more days, who often exhibit a severe inflammation with fibrin and pmns in subarachnoid and ventricular spaces. a detailed analysis of pm largely resembling human meningitis was also reported by gerber et al., who injected c57bl/ inbred mice into the right lobe of the brain. in that model, the inoculation site was characterised by a large purulent infiltrate present in both meninges and ventricula, and necrosis was observed in all investigated brain regions. in another model, proposed by koedel et al., pneumococci were given by transcutaneous injection directly into the cisterna magna. in that study, brain lesions occurred in all mice hours after infection, and histopathological examination revealed intense granulocytic infiltrations in the subarachnoid and ventricular spaces, and absence of cortical necrosis. this finding differs from the model by gerber et al., who instead reported the presence of extensive cerebral necrotic processes. in our model, we could not observe large areas of necrosis, but we found some signs of neuronal damage in the hippocampus of a few animals.  the present work proposes a method to induce experimental pm in outbred mice by using an i.c. subarachnoidal route of infection. the stereotaxic coordinates of the injection site are provided to allow easy recognition of the inoculation point in the mouse. the model is simple and fast, and the technique assures the development of meningitis, as demonstrated by histological analysis, survival data, and microbiological parameters. no significant differences were observed in the ability of the three pneumococcal strains used to cause disease, emphasising the value of the model. it is worth noting that the use of outbred mice still results in data reproducibility, as replicates in this model closely paralleled each other in terms of survival, cfu counts per organ, and histopathological features. in addition, experiments in outbred mice are cost-effective and can be performed in larger animal groups thereby improving statistical significance. this experimental pm model may be particularly useful for all researchers involved in studies that will investigate the host-pathogen interaction at the cerebral level, with emphasis on both pathogen-associated virulence factors and host-specific brain defences. 
bacteria were injected into the subarachnoid space through a soft point located mm rostral from the bregma. experimental animal models of pneumococcal meningitis are important to study the pathogenesis of meningitis, the host immune response induced after infection, and the efficacy of novel drugs and vaccines. lethal doses killing % of animals infected with type and s. pneumoniae were × × and × colony forming units, respectively. pneumococcal meningitis is associated with the highest mortality among bacterial meningitis and it may also lead to neurological sequelae despite the use of antibiotic therapy. in the present work, we describe in detail a simple, reproducible and efficient method to induce pneumococcal meningitis in outbred mice by using the intracranial subarachnoidal route of infection. the proposed method for inducing pneumococcal meningitis in outbred mice is easy-to-perform, fast, cost-effective, and reproducible, irrespective of the serotype of pneumococci used. histological analysis of the brain of animals infected with type s. pneumoniae proved the induction of meningitis closely resembling the disease in humans. streptococcus pneumoniae is the leading cause of bacterial meningitis.
 streptococcus pneumoniae is the leading cause of bacterial meningitis. pneumococcal meningitis is associated with the highest mortality among bacterial meningitis and it may also lead to neurological sequelae despite the use of antibiotic therapy. experimental animal models of pneumococcal meningitis are important to study the pathogenesis of meningitis, the host immune response induced after infection, and the efficacy of novel drugs and vaccines.  in the present work, we describe in detail a simple, reproducible and efficient method to induce pneumococcal meningitis in outbred mice by using the intracranial subarachnoidal route of infection. bacteria were injected into the subarachnoid space through a soft point located mm rostral from the bregma. the model was tested with several doses of pneumococci of three capsular serotypes, and mice survival was recorded. lethal doses killing % of animals infected with type and s. pneumoniae were × × and × colony forming units, respectively. characterisation of the disease caused by the type strain showed that in moribund mice systemic dissemination of pneumococci to blood and spleen occurred. histological analysis of the brain of animals infected with type s. pneumoniae proved the induction of meningitis closely resembling the disease in humans.  the proposed method for inducing pneumococcal meningitis in outbred mice is easy-to-perform, fast, cost-effective, and reproducible, irrespective of the serotype of pneumococci used. 