Etiology

Causative agents can vary according to age group, country, or region. Globally, the most common pathogens causing bacterial meningitis are Neisseria meningitidis and Streptococcus pneumoniae, accounting for 9.1% to 36.2% and 2.1% to 41.2% of bacterial meningitis cases, respectively.[26] In children, S pneumoniae is the most common pathogen, causing 22.5% (Europe) to 41.1% (Africa) of cases. In adults, S pneumoniae causes 9.6% (Western Pacific) to 75.2% (Africa) of cases.[26][27]

N meningitidis serogroup B is the predominant cause of meningococcal meningitis in Europe, the US, and the Western Pacific, although with rates of serogroup Y increasing in the US since 2022.[24]​ Meningococcal serogroups C and W are most common in most of Africa and Latin America. Serogroup Y cases mostly occur in Nordic countries.[26]

The routine use of Haemophilus influenzae type b (Hib) and conjugate pneumococcal vaccines in the UK and other developed countries has markedly reduced the overall incidence of bacterial meningitis.[8][11][12][28][29]​​​​ However, because of limited economic resources and poor living conditions, many developing countries continue to have high rates of disease.

In neonates, Escherichia coli and Streptococcus agalactiae (group B streptococci) are major causative agents of bacterial meningitis.[26]

Group A streptococci (GAS), caused by Streptococcus pyogenes, has traditionally been believed to account for <2% of all cases of meningitis, but the incidence may be increasing.[30][31]​​ It is known for its high morbidity and mortality rate, especially in children.[32]

Gram-negative enteric bacilli (e.g., E coli, Serratia, Acinetobacter, Klebsiella, and Pseudomonas aeruginosa) are a rare cause of community-acquired bacterial meningitis but are an important cause of healthcare-associated ventriculitis and meningitis. Patients with both localized and disseminated strongyloidiasis (a chronic parasitic infection) are also predisposed to bacterial meningitis with enteric organisms such as Klebsiella pneumoniae (K pneumoniae), E coli, and Enterococcus spp.[33][34]​​​​​

Listeria monocytogenes can cause bacterial meningitis in patients using immunosuppressive drugs, in people who misuse alcohol, in neonates, in patients ages older than 50 years, and in patients with diabetes.[35][36]

Zoonotic causes of bacterial meningitis are rare, but important to consider when there is a history of exposure to animals. Streptococcus suis, most prevalent in Southeast Asia, is associated with raw pork meat and pig farming. Other zoonotic pathogens include Capnocytophaga canimorsus (associated with dogs), Campylobacter fetus (cattle), and Streptococcus equi (horses).[37][38][39]

Pathophysiology

Bacteria reach the central nervous system either by hematogenous spread (the most common route) or by direct extension from a contiguous site such as the paranasal sinuses or the mastoid bone. Neonates can acquire pathogens from nonsterile maternal genital secretions, through the placenta, or from their surroundings.[9] The bacteria multiply quickly once they have entered the subarachnoid space. Bacterial components in the cerebrospinal fluid induce the production of various inflammatory mediators, which in turn enhance the influx of leukocytes into the cerebrospinal fluid.[40] The inflammatory cascade leads to cerebral edema and increased intracranial pressure, which contribute to neurologic damage and even death. [Figure caption and citation for the preceding image starts]: Ventral view of human brain showing purulent basilar meningitis infection due to Streptococcus pneumoniaeImage provided by the CDC Public Health Image Library [Citation ends].com.bmj.content.model.Caption@755ea4c3

Classification

Common types of bacterial meningitis

Pneumococcal meningitis

  • Caused by Streptococcus pneumoniae

Meningococcal meningitis

  • Caused by Neisseria meningitidis

Hib meningitis

  • Caused by Haemophilus influenzae type b

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