The annual incidence of bacterial meningitis in the developed world is about 3 to 5 cases per 100 000. It has been estimated that over 75 per cent of all cases of bacterial meningitis occur in children under 15 years of age, and it is one of the most common life-threatening infections encountered by the medical profession.
The causative organisms will vary according to the population studied and the age of the study group, and may rise considerably, for example during epidemics of meningococcal infection. Currently, Neisseria meningitidis and Streptococcus pneumoniae are the most common causes of bacterial meningitis outside the neonatal age group.
The microbial epidemiology of meningitis is changing in older children and adults, in whom nosocomial meningitis accounts for an increasing proportion of infections. Many of these are associated with recent neurosurgical intervention or trauma. In such cases, Pseudomonas aeruginosa, enterococci, Staphylococcus aureus, and the coagulase-negative staphylococci are the most common causative organisms. Table..! lists the common causes of bacterial meningitis associated with different underlying conditions.
Table 1 Organisms associated with the development of meningitis according to different underlying conditions
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