Differential fluorescence-based genetic screens to identify novel Listeria monocytogenes virulence determinants

Abstract

Listeria monocytogenes is a Gram-positive, facultative intracellular pathogen that causes gastroenteritis, which in the young, the elderly, and the immunocompromised can progress to severe invasive disease with high mortality. While previous studies have largely elucidated the bacterial and host mechanisms necessary for the bacterium to access its replicative niche in the host cell cytosol, the L. monocytogenes factors required for adaptation to life within this restrictive environment are poorly understood. In this dissertation, I describe a fluorescence-activated cell sorting (FACS)-based differential fluorescence genetic screening technique for the identification of L. monocytogenes genes necessary for optimal intracellular replication. Bacteria harboring deletions in identified genes were defective for intracellular replication, plaque formation, and in vivo virulence, validating the ability of the screening method to identify novel intracellular replication-defective mutants. Minor alteration of the FACS-based screening strategy allowed the detection and differentiation of bacterial mutants displaying varying severities of actin-based motility defects. A preliminary FACS-based genetic screen to identify actin-based motility mutants isolated multiple independent insertions within internal control genes, demonstrating the potential utility of FACS-based differential fluorescence genetic screening methods for the identification of L. monocytogenes genes important for multiple virulence phenotypes. Lastly, my characterization of the X-prolyl aminopeptidase PepP, a novel virulence factor identified by the FACS-based genetic screen to discover genes necessary for optimal intracellular replication, revealed this enzyme plays an unexpected role in L. monocytogenes virulence gene regulation.