Bugs that cause childhood pneumonia and meningitis have evolved to evade vaccines by swapping bits of their genome with other bacteria, according to a new study.

...the bugs were able to swap several parts of their respective genomes at once

The findings show how quickly these life-threatening pathogens can disguise themselves with borrowed genetic decoys, and how hard it is for medicine to keep up.

Diseases caused by Streptococcus pneumoniae are thought to kill over a million young children around the world each year.

Vaccines that protect against these so-called pneumoccoccal infections are designed to recognise a material on the outer surface of a bacterium’s cell called polysaccharide.

Each of over 90 serotypes of these bacteria has a different polysaccharide coating.

In 2000, a vaccine that targeted seven serotypes proved highly effective when introduced in the United States.

The same formula – which also prevented transmission from children to adults – was adopted in Britain, said the publication Nature Genetics.

Over time, however, the vaccine worked less well, so researchers led by Rory Bowden at the University of Oxford set out to discover why.

Combining cutting-edge genetic analysis with epidemiology, which examines how disease spreads, they found that the deadly pathogens escaped detection by swapping genes with other, slightly different, bacteria.

Remarkably, the exchanged genetic material came from precisely that part of the genome responsible for making the cell’s coating – the area targeted by the vaccine.

The bacteria, in other words, had kept their virulence intact but changed their outward appearance.

“Imagine that each strain of the pneumoccoccus bacteria is a class of schoolchildren all wearing the school uniform,” explained Dr Bowden.

“If a boy steals from the corner shop, a policeman – the vaccine – can easily identify which school he belongs to by his uniform.” But if the boy swaps his sweater with a friend from another school, Dr Bowden continued, the policeman will no longer know where to look and the thief, like the bacteria, will escape.

The researchers identified several such “recombined” serotypes resistant to the vaccine, and one in particular that had spread across the US from east to west over several years.

They also observed that the bugs were able to swap several parts of their respective genomes at once.