Intravacuolar persistence in neutrophils facilitates spread to co-cultured cells.

Unlabelled: The bacterium () causes listeriosis in humans and ruminants. Acute lesions are predominantly infiltrated by polymorphonuclear neutrophils (PMNs), considered to be the efficient bactericidal arm of innate immunity. However, recent evidence suggests that PMNs cannot achieve antilisterial sterilizing immunity and that may persist within PMNs. Despite this, interactions between PMNs and remain poorly understood. In this study, we characterized the listericidal activity and interaction dynamics of bovine PMNs with . Phagocytosed failed to escape into the PMN cytosol and was primarily targeted by phagolysosomal mechanisms. However, PMNs enabled prolonged intravacuolar survival of a resilient subpopulation, largely as viable but non-culturable (VBNC) bacteria. This resilient population could spread from PMNs to a cell line, resuscitate, and complete its canonical life cycle, thereby perpetuating the infection. Therefore, we identify PMNs as a mobile niche for survival and provide evidence that PMNs harbor VBNC bacteria, potentially facilitating dissemination within the host.

Importance: () is a significant foodborne pathogen responsible for high hospitalization rates in humans, especially vulnerable groups such as the elderly, pregnant women, and immunocompromised individuals. In animals like ruminants, infection leads to severe disease manifestations, notably brainstem encephalitis. This study uncovers a novel mechanism by which bovine neutrophils (PMNs) harbor in a viable but non-culturable (VBNC) state, enabling the bacteria to hide in the host. PMNs, traditionally viewed as bacteria killers, may serve as Trojan horses, allowing to persist and spread within the host. This discovery has broad implications for understanding 's persistence, its role in recurrent infections, and the development of new therapeutic strategies targeting VBNC forms of to improve treatment outcomes and disease control.