Scientists have discovered how a type of flesh-eating bug disables a key part of the body's immune system.
Muscle infected with the strep bacteria
A team from Hammersmith Hospitals NHS Trust looked at why the streptococcus A bacteria can be deadly in some cases.
It causes conditions ranging from mild throat infections to the flesh-eating bug necrotising fasciitis.
The Hammersmith team found group A strep can destroy a key immune system messenger which should alert the body to the bacteria's presence.
Many people carry the bacteria harmlessly on their skin and in their nose and throat.
But it can cause severe bloodstream infections, toxic shock syndrome - and necrotising fasciitis.
Over 2,000 cases of invasive disease caused by strep A are reported every year. Up to one in five patients die, if the bacteria get into the blood.
'Shooting the messenger'
The team's research began when a patient admitted to the hospital died from necrotising fasciitis, caused by a very virulent form of strep A.
Experts looked at post mortem samples from the patient and found that, despite the presence of the bacteria in muscle tissue, there were hardly any white blood cells, which would normally be the body's first line of defence.
The strain affecting the patient was isolated and grown in the lab, and biochemical analysis found that the bacteria prevented the immune system responding as normal by "shooting the messenger".
The bacteria interacts with a molecule called IL-8 which should act as the messenger to rally other parts of the immune system to attack the "invader".
But these messages are blocked, meaning the bacteria is unchecked.
The researchers then looked closely at its make-up.
They found a particular piece on the surface of the bacteria, which they have called SpyCEP, is responsible for blocking the action of IL-8.
Strains of strep A with the highest levels of SpyCEP were most effective at inhibiting the body's immune response.
Dr Shiranee Sriskandan, who led the research, said the finding was important because more analysis of the SpyCEP fragment could help in the development of a vaccine against strep A.
She said: "There is currently no vaccine against all types of group A strep.
"We are hopeful that this is the first step to developing one that is broadly effective and safe."
Professor Ron Cutler, an infectious diseases expert at the University of East London, said: "This is an exciting finding."
He added: "It's a very technical finding, but we could potentially have a vaccine against streptococcus necrotising fasciitis."
Professor Cutler said much more research was needed to develop any vaccine and to see how effective it could be, and who should be receive it.
The research has been published in the Journal of Infectious Diseases.