Bacterial meningitis

Many different bacteria may cause meningitis but those with the highest incidence are Neisseria meningitidis, Streptococcus pneumoniae and Haemophilus influenzae.

Neisseria meningitidis

N. meningitidis (meningococcus) is a Gram-negative bacterium found in the nasopharynx of healthy adults and children (carriers).  People of any age can carry this bacterium without becoming ill and it is estimated that around 5–10% of the population in developed countries are carriers of meningococcal bacteria, with higher carriage rates in developing countries.¹  N. meningitidis is particularly important as this organism has the potential to cause meningitis epidemics (see section on Incidence).

There are at least thirteen different serogroups of N. meningitidis, defined on the basis of the immunochemistry of the capsular polysaccharide.  Only five of the identified serogroups (serogroups A, B, C, W-135 and Y) usually have pathogenic potential.

Streptococcus  pneumoniae

S. pneumoniae, also known as pneumococcus, is a Gram-positive bacterium found in the back of the nose and throat.  Virtually all children become carriers of this organism at some time during their lives and carrier state has been found to build natural immunity to this bacterium.  There are over 90 serogroups of S. pneumoniae, differentiated by the chemical composition of the polysaccharide capsule that surrounds the bacterium.  Only a small number of the serogroups cause disease. 

Pneumococcal infections are a major cause of morbidity and mortality, globally.  In addition to causing meningitis, the pneumococcus can cause other serious infections such as pneumonia, endocarditis, septic arthritis and septicaemia, as well as less serious infections such as otitis media, glue ear, sinusitis and chest infections.  Pneumococcal meningitis can affect any age group, but those at most risk are babies and young children less than 18 months of age, who do not yet have fully developed immune systems.  The elderly and people with conditions that affect their immune systems are also at increased risk.  S. pneumoniae is the most common cause of meningitis in patients who have suffered head trauma.

Haemophilus influenzae

H. influenzae type b (Hib) is a Gram-negative bacterium commonly found in the nasopharynx of healthy adults and children.  H. influenzae used to be a major cause of bacterial meningitis in Europe but the number of cases has fallen dramatically since the introduction of routine vaccination against Hib disease.  Meningitis caused by this bacterium is now rarely seen.