What makes meningococci turn bad?

What is it about?

Neisseria meningitidis can cause meningococcal disease, a fast-moving, often fatal disease. The same bacteria can also live in healthy carriers, within their nasal passages. Progression to meningococcal disease is a relatively rare event, associated with high-risk "hypervirulent" strains. The basis of this hypervirulence is not well understood. During a long outbreak of memingococcal disease in New Zealand, a household carriage study was carried out, in which healthy household contacts of meningococcal disease patients were tested to see if they carried N. meningitis, and if so, how closely related their strains were to those carried by the sick patient. We found that in many cases the strains were related, and had likely transmitted between household members. We set out to characterise the genetic differences between the strains carried by healthy carriers and patients, to see whether they could explain the difference in outcomes. We found that, despite the fact that pili are generally believed to be essential for carriage, a number of our carriage strains had lost them, and that this loss was correlated with a decreased inflammatory response in respiratory cells, possibly providing an advantage for these strains.

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Why is it important?

While we noted an array of genetic differences between invasive and carriage strains, we didn't identify any genetic differences that would appear to give invasive strains an advantage in causing severe disease, suggesting that infection outcome was likely more due to differences in the host. In contrast, we did find a loss of pili as a possible advantage gained by some strains carried by healthy individuals.