Neisseria meningitidis: colonisation (the norm); infection (the exception)

If you were to read some of the press around the outbreak of invasive meningococcal disease in Kent, you’d think that a) Neissera meningitidis = meningitis and b) everybody with Neisseria meningitidis has invasive meningococcal disease. Neither could be further from the truth! This blog explores N. meningitidis colonisation and infection.

Colonisation (the norm) po

Neisseria meningitidis is primarily a commensal coloniser of the human nasopharynx, carried asymptomatically by a substantial proportion of healthy individuals:

• 5–10% of the general population are colonised at any one time, with higher rates in adolescents and young adults. Rates of carriage can be 25% or more in some student groups.
• Carriage can persist for weeks to months without symptoms; humans are the only reservoir.
• Colonisation represents a stable ecological relationship, with the organism residing on mucosal surfaces without causing host damage.
• Transmission occurs via respiratory droplets or secretions, most commonly from colonised individuals rather than those with disease.

Invasive Meningococcal Disease (IMD) (the exception)

Only in a small minority of newly colonised individuals does N. meningitidis breach mucosal barriers and enter normally sterile sites (bloodstream or CSF):

• IMD typically presents as meningitis (~50%) or septicaemia (35–40%), often progressing rapidly with high mortality (8–15%).
• Invasive disease occurs in around 1 per 100,000 people annually in the UK and global prevalence ranges from <1 to 10 per 100,000 globally.
• The transition from carriage to disease is multifactorial, involving host susceptibility (e.g., complement deficiency, asplenia), microbial virulence factors (e.g., capsule), and environmental or behavioural factors (e.g., smoking, viral co‑infection).
• Some N. meningitidis strains are associated with colonisation (nongroupable or express B, Y, X, Z, or 29E capsular serogroups) and others are associated with causing invasive meningococcal disease (most express capsules of serogroups A, B, C, Y and sometimes W-135).
• Acquiring a strain with the ability to cause invasive disease does not necessarily mean that it will, although some strains are more likely to cause invasive disease than others. For example, members of the ET-37 complex (largely serogroup C) are estimated to cause invasive meningococcal disease in one in 20 to one in 400 acquisitions.

Summary

• Carriage is normal, common, and usually harmless.

• Invasive disease is uncommon but severe, requiring rapid recognition, urgent therapy, and public‑health action.

• Understanding this distinction underpins:

  • appropriate risk communication (avoiding the centre of Canterbury for fear of invasive meniongococcal disease is not warranted...!)
  • case management and contact prophylaxis,
  • IPC strategies focusing on droplet precautions for cases but not carriers,
  • vaccination policy targeting the serogroups responsible for IMD rather than carriage alone.