The researchers found bacteria that had evaded the vaccine by swapping the region of the genome responsible for making the polysaccharide coating with the same region from a different serotype, not targeted by the vaccine. This effectively disguised the bacteria, making it invisible to the vaccine. This exchange of genome regions occurred during a process known as recombination, whereby one of the bacteria replaces a piece of its own DNA with a piece from another bacterial type.
Dr Rory Bowden, from the University of Oxford, explains: "Imagine that each strain of the pneumococcus bacteria is a class of schoolchildren, all wearing the school uniform. If a boy steals from his corner shop, a policeman in this case the vaccine can easily identify which school he belongs to by looking at his uniform. But if the boy swaps his sweater with a friend from another school, the policemen will no longer be able to recognise him and he can escape. This is how the pneumococcus bacteria evade detection by the vaccine."
Dr Bowden and colleagues identified a number of recombined serotypes that had managed to evade the vaccine. One in particular grew in frequency and spread across the US from east to west over several years. They also showed that during recombination, the bacteria also traded a number of other parts of the genome at the same time, a phenomenon never before observed in natural populations of pneumococcus. This is of particular concern as recombination involving multiple fragments of DNA allows rapid simultaneous exchange of key regions of the genome within the bug, potentially allowing it to quickly develop antibiotic resistance.
The original 7-valent vaccine in the US has now been replaced by a 13-valent vaccine, which targets thirteen different serotypes, including the particular type which had escaped the original vaccine. In the UK, the 7-valent vaccine resulted in a substantial drop in disease overall. This overall e
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