Comparison of the infectivity of isolates of Listeria monocytogenes following intragastric and intravenous inoculation in mice

The infectivity of 19 haemolytic isolates of Listeria monocytogenes from different sources (clinical and environmental) and representative isolates from Listeria ivanovii and Listeria innocua was compared following intragastric (i.g.) and intravenous (i.v.) inoculation in immunocompetent male BALB/c mice. There was marked variation in the infectivity of the different isolates by either route but when isolates were ranked in descending order by spleen count, following i.g. administration, the strains fell into four groups. Infectivity of some isolates also differed when i.v. inoculation was compared with i.g. administration, so that assessment of virulence by spleen counts only following i.v. inoculation might fail to detect isolates of poor infectivity by the i.g. route. These results suggest that intragastric inoculation of normal immunocompetent mice is a useful model for detecting strains of L. monocytogenes that are poorly invasive via the gut even though they are relatively virulent by intravenous inoculation.