ISG15 and infection: Since Listeria, as other pathogenic bacteria, often targets post-translational modifications during infection (Bonazzi et al., 2008; Ribet and Cossart, 2010; Ribet et al., 2010), we were interested in investigating the interplay between the interferon-stimulated ubl ISG15 and Listeria. ISG15 plays an important role in the innate immune response to viruses.