Early administration of adequate antimicrobial therapy is the cornerstone of treatment in patients with meningitis. Despite developments in antimicrobial agents, Lm meningitis is still a serious disease that carries high morbidity and mortality rates. A relevant aspect in this study is the prospective inclusion of adult patients with acute community-acquired Lm meningitis, to evaluate risk factors associated with Lm aetiology which should contribute to early appropriate antimicrobial therapy of this serious and life-threatening disease. Thus, in a large cohort of patients with Ac-ABM, our study reveals that elderly, immunocompromised patients, and a higher CSF/blood glucose ratio were independently associated with isolation of Lm as the responsible pathogen.
In our own series, the incidence of acute community-acquired Lm meningitis in adults was 16.5% (46/278). Previously, in a multicenter study  conducted in the United States, the authors reported that Lm accounted for 8% of the cases of bacterial meningitis. Other studies from Europe and North America have reported incidences of 5%-10% or more among all episodes of Ac-ABM among adults [15–18]. Over a 36-year period, Durand et al. described that Lm accounted for 11% of the episodes of Ac-ABM in adults . The probable increase in the incidence may be attributed to longer life expectancy, changes in diet or food processing, and the higher number and longer survival of immunocompromised people .
On the other hand, symptoms and signs of patients presenting with Lm meningitis were not different from those found in the general population of patients with Ac-ABM, according to previous studies .
The CSF profile in patients with Listeria infection revealed significantly fewer WBC and lower protein concentrations than patients with infection due to other pathogens, and a trend towards less hypoglycorrhachia and a lower percentage of PMNs. However, these findings do not apply to all cases . In the appropriate clinical situation, clinicians should not exclude Lm based on the degree of pleocytosis, the percentage of PMNs, or the concentration of glucose or protein in the CSF.
The experience in the present series suggests that the Gram stain is negative in two-thirds of the cases of Lm meningitis, and may be misleading in many of the remaining cases. Clinicians should be aware of these difficulties, inform the microbiology department when they suspect this pathogen, and not rule out Lm based on Gram stain alone. Furthermore, despite the diversity of serotypes of Lm, only three serotypes are responsible for >90% of human disease: 1/2a, 1/2b, and 4b. The majority of strains were serotype 4b  (23 of 28 cases; 82%), suggesting that serotype 4b of Lm is more virulent than others. Otherwise, blood cultures were positive in more than half of the cases in the present series. Some reports advocate the use of the polymerase chain reaction or immunoassays for the early detection of Lm [22, 23], but these techniques need further evaluation.
Most patients in our case series received appropriated empirical therapy, 40/43 (93%), in 11 of them associated to aminoglycosides. Our study showed that a definitive combined therapy with aminoglycosides was significantly associated with an increasing trend for an unfavourable outcome and mortality, which corresponds with a previous study . These results could be explained by Lm meningitis patients have an associated co-morbid condition, in which addition of an aminoglycoside may be harmful. For this reason, actually this combined therapy is been questioned, moreover of the drug's associated nephrotoxicity and inability to cross the blood-brain barrier. In this way, every case should be approached independently, although certain subsets of patients probably require empiric antibiotic treatment for Lm, such as patients over 45-50 years of age, patients with immunosuppression, a higher CSF/blood glucose ratio, or patients with a Gram stain of CSF revealing Gram-positive bacilli , and the associated with aminoglycosides should be avoid in order to improve patient's outcome.
A significant number of patients, 5 of 31 (16.1%) of the survivors in the present series had some degree of residual neurological deficit at hospital discharge. Although an even higher incidence of residual neurological deficits (32%) was noted among survivors enrolled in 3 previous series and deficits persisted for more than a year in 4 of 10 of those cases with adequate follow up [25–27].
Moreover, mortality due to Lm is among the highest of all causes of acute bacterial meningitis . Part of the high mortality of Listeria infection may relate to the increased number of immunocompromised and older patients affected by this pathogen [29, 30]. Although evidence for use of anticonvulsant prophylaxis in Lm meningitis is lacking, it may be considered as seizures have previously been related to increases mortality , as did we in the present series, although this was not statistically significant. Further studies are needed in order to clarify this point.
Finally, the role of adjunctive therapy with corticosteroids has been widely debated. A clinical trial showed a beneficial effect of early dexamethasone treatment in adults with bacterial meningitis . Nevertheless, there are no studies on the effect of dexamethasone in adult Listeria meningitis. We observed a higher survival rate among those patients that received dexamethasone concomitant to antibiotic, although this association did not reach statistical significance. Our results suggest that further studies are needed to evaluate the effect of corticosteroid therapy in adult patients with Lm meningitis.
The present study has several limitations. First, given the small number of patients, we were unable to develop a statistical model to identify risk factors independently associated with a poor outcome or death in patients with acute community-acquired Lm meningitis. Secondly, only patients who underwent lumbar puncture and who had a positive cerebrospinal fluid culture were included. Negative cerebrospinal fluid cultures occur in 20 percent of patients with acute community acquired bacterial meningitis. Third, delay of antimicrobial therapy was not recorded. Despite all these limitations, we consider that this study contributes to our knowledge of risk factors associated with development of acute community-acquired Lm meningitis, and to evaluating epidemiology, clinical features, management, and outcome in this homogenous population of adult patients with acute community-acquired Lm meningitis.