A normally harmless bacterial biofilm can take a dangerous turn when exposed to a virus.

Streptococcus pneumoniae can trigger bacterial pneumonia, but also colonizes the nose or throat passages of up to 15% of healthy adults. To learn how it might become pathogenic, researchers led by Anders Hakansson at the State University of New York in Buffalo grew films of S. pneumoniae on a layer of human epithelial cells of the type that normally lines airways.

Infecting the epithelial cells with influenza virus caused bacteria to disperse from the biofilms, as did mimicking flu-induced conditions such as fever. Dispersed S. pneumoniae invaded normally uncolonized sites including the lung and middle ear in mouse studies, where they caused oedema and inflammation. They also had a more virulent gene-expression pattern than bacteria growing in biofilms or in standard laboratory conditions. 'Interkingdom signalling' could be key to inducing disease, the authors say.

mBio 4, e00438-13 (2013)