The bacterium Listeria monocytogenes, most commonly spread through raw dairy and meat products, is able to dodge the immune response by occupying vacuoles within macrophages and replicating, a new study shows.

John Brumell and colleagues from the Hospital for Sick Children in Toronto studied severe combined immunodeficient (SCID) mice and observed L. monocytogenes replicating within vacuoles in liver granuloma macrophages.

The team has termed the compartments 'spacious Listeria-containing phagosomes' (SLAPs). The study is published in the January 17 issue of Nature.

Listeria is usually swallowed by a phagosome but is able to harness the pore-forming toxin Liseriolysin O (LLO) and escape into the cytosol, where it replicates rapidly and spreads from the host cell.

The Canadian team has discovered that LLO is also necessary for SLAP formation, where the bacteria are able to replicate, albeit at a slower rate compared to those in the cytosol.

The researchers suggest that this is a potent mechanism by which the pathogen can establish persistent infection.