A group of six genes causes some strains of a foodborne bacterium to become highly dangerous, or virulent.

Listeria monocytogenes (pictured) can be found in many foods, including unpasteurized milk, and can cause miscarriage in pregnant women or kill infected people if the pathogen moves to the brain. All strains are currently considered to be equally virulent, but Sylvain Brisse and Marc Lecuit of the Pasteur Institute in Paris and their colleagues found differences in virulence when they analysed genomic and epidemiological data from more than 6,600 L. monocytogenes isolates taken from food and human samples. Food-associated strains mainly infected people who had weakened immune systems, but more-virulent clinical isolates were found in healthy people. The 'CC4' group of strains carried a six-gene cluster that, when deleted, made the bacteria less capable of invading brain and placental tissues in mice.

Credit: CNRI/SPL

Public-health surveillance efforts for foodborne illnesses should look out for these hypervirulent strains, the authors say.

Nature Genet. http://dx.doi.org/10.1038/ng.3501 (2016)