In vivo microevolution of Neisseria meningitidis during progression from harmless commensal to invasive pathogen.
Most people who encounter meningococcal bacteria, e.g. through the coughs and sneezes of others or intimate kissing, do not develop meningitis or septicaemia. Approximately 10% of people unknowingly carry the organism in their throats from where they may pass it onto others until such time as it is cleared by the immune system. What causes the bacteria to invade and cause disease in a small number of individuals is usually unknown but is likely to be a combination of human, bacterial and environmental factors.
The blood is a hostile environment for germs due to increased temperatures, altered nutrient availability and attack from the body’s immune system. The meningococcus adapts to this by switching different genes on and off through spontaneous reversible changes in its DNA. For example, its sugary coating (capsule) is required to survive in the blood but may be switched off in the throat to make it easier for the germ to be carried. Many other surface components undergo such switching, meaning an almost unlimited number of variations with different genes in the ‘on’ or ‘off’ state.
What the research team did
In this study the research team compared the genetic make-up of invasive meningococci taken from the blood and/or cerebrospinal fluid of a patient with that of meningococci concurrently taken from their throat. The throat isolates represent the close relatives left behind by the single bacterium as it invaded into the bloodstream. Any unique changes taking place in the invading bacterium prior to, during, or after invasion to aid it in the disease process would be identified during the comparisons.
Summary and impact of results
This research highlighted many differences between meningococcal bacteria living in the back of the throat and those that have invaded the bloodstream or cerebrospinal fluid, reflecting the ability of the bacteria to switch genes on and off.
The genetic changes responsible for switching the capsule ON (as observed on live bacteria in the laboratory) were flagged up in each of the corresponding comparisons indicating their effectiveness. In addition to the genes which control the capsule, a number of other genes, coding for various proteins, were also identified and were known or predicted to have various functions including:
- a protein implicated in adhesion, colonisation and persistence within the throat
- two adhesion proteins that help the bacteria adhere to and invade host cells
Survival within host
- three iron acquisition proteins that help the bacteria battle the human host for the essential nutrient, iron
- two proteins that are released from the meningococcal surface and are implicated in evasion of host immunity e.g. by chopping up host defensive molecules
- five proteins that modify other proteins by attaching sugars to them e.g. to hide from the immune system or to stick to host cells
- one protein involved in non-capsular surface sugar assembly – these sugars are highly variable and, along with associated fatty molecules, are implicated in various aspects of colonisation, invasion and disease. They are also a target for the immune system
- three proteins that chemically modify bacterial DNA e.g. to regulate the production of other proteins or to protect against invading viruses
- a major surface protein that is a vaccine antigen and a target of the host immune system
- four theoretical proteins with as-yet unknown functions, along with a predicted regulator for one of these
NB. Several of these proteins probably fall into multiple of these categories.
Among the affected proteins with known functions, each was an important component of the process by which the bacteria ultimately cause harm to the patient. Many were surface proteins and are being, or have already been considered, as potential vaccine components. The complexity of the host-bacterial relationship was made clear, however, by inconsistencies in the switching of each of the affecting proteins. With the exception of the capsule, which was always ON in the invasive isolates, each of the affected proteins was found to be switched on in some patients whilst being switched OFF in others during disease. This further highlights the advantage of using multiple proteins in a vaccine formulation, especially when some proteins may compensate for the loss of another e.g. in the case of the iron acquisition proteins.
The genome sequences generated will form a unique and invaluable resource, effectively a library, for global meningococcal experts to consult. This will help them to better understand the roles of these genes in the transition of meningococci from being harmless colonisers to potentially deadly invaders.
By improving understanding of how meningococcal bacteria cause disease, Dr Lucidarme’s work may lead to new, more effective vaccines against meningococcal disease.