Comparison of Widely Used Listeria monocytogenes Strains EGD, 10403S, and EGD-e Highlights Genomic Differences Underlying Variations in Pathogenicity
Authors: Bécavin Christophe , Bouchier Christiane , Lechat Pierre , Archambaud Cristel , Creno Sophie , Gouin Edith , Wu Zongfu , Kühbacher Andreas , Brisse Sylvain , Pucciarelli M. Graciela , García-del Portillo Francisco , Hain Torsten , Portnoy Daniel A. , Chakraborty Trinad , Lecuit Marc , Pizarro-Cerdá Javier , Moszer Ivan , Bierne Hélène , Cossart Pascale 
Published: March 2014

For nearly 3 decades, listeriologists and immunologists have used mainly three strains of the same serovar (1/2a) to analyze the virulence of the bacterial pathogenListeria monocytogenes. The genomes of two of these strains, EGD-e and 10403S, were released in 2001 and 2008, respectively. Here we report the genome sequence of the third reference strain, EGD, and extensive genomic and phenotypic comparisons of the three strains. Strikingly, EGD-e is genetically highly distinct from EGD (29,016 single nucleotide polymorphisms [SNPs]) and 10403S (30,296 SNPs), and is more related to serovar 1/2c than 1/2a strains. We also found that while EGD and 10403S strains are genetically very close (317 SNPs), EGD has a point mutation in the transcriptional regulator PrfA (PrfA*), leading to constitutive expression of several major virulence genes. We generated an EGD-e PrfA* mutant and showed that EGD behaves like this strainin vitro, with slower growth in broth and higher invasiveness in human cells than those of EGD-e and 10403S. In contrast, bacterial counts in blood, liver, and spleen during infection in mice revealed that EGD and 10403S are less virulent than EGD-e, which is itself less virulent than EGD-e PrfA*. Thus, constitutive expression of PrfA-regulated virulence genes does not appear to provide a significant advantage to the EGD strain during infectionin vivo, highlighting the fact thatin vitroinvasion assays are not sufficient for evaluating the pathogenic potential ofL. monocytogenesstrains. Together, our results pave the way for deciphering unexplained differences or discrepancies in experiments using differentL. monocytogenesstrains.

IMPORTANCEOver the past 3 decades,Listeriahas become a model organism for host-pathogen interactions, leading to critical discoveries in a broad range of fields, including bacterial gene regulation, cell biology, and bacterial pathophysiology. Scientists studyingListeriause primarily three pathogenic strains: EGD, EGD-e, and 10403S. Despite many studies on EGD, it is the only one of the three strains whose genome has not been sequenced. Here we report the sequence of its genome and a series of important genomic and phenotypic differences between the three strains, in particular, a critical mutation in EGD’s PrfA, the main regulator ofListeriavirulence. Our results show that the three strains display differences which may play an important role in the virulence differences observed between the strains. Our findings will be of critical relevance to listeriologists and immunologists who have used or may useListeriaas a tool to study the pathophysiology of listeriosis and immune responses.