Development of a competitive index assay to evaluate the virulence of Listeria monocytogenes actA mutants during primary and secondary infection of mice

We developed a competitive index assay for murine listeriosis that tests th e virulence of Listeria monocytogenes strains in different organs and at va rious times postinoculation. Studies presented here demonstrate the reprodu cibility of this assay during primary and secondary infection of inbred and outbred mice. We verified the validity of this assay by performing competi tive index analysis of a well-characterized strain of L. monocytogenes lack ing the actA gene. In addition, we found that while L. monocytogenes strain s unable to recruit vasodilator-stimulated phosphoprotein (VASP) to their s urface exhibit a 10-fold virulence attenuation in the livers of naive anima ls, they display a 50-fold survival defect in the liver during secondary li steriosis.