Comparative whole genome analysis of Listeria monocytogenes 4b strains reveals least genome diversification irrespective of their niche specificity

Listeria monocytogenes has emerged as the deadly pathogen inflicting high mortality in humans and animals. To investigate the strain-specific characteristics of L. monocytogenes, and their role(s) in virulence and ecological sustenance, we sequenced genome of three L. monocytogenes strains BHU1, 2 and 3 isolated from the Ganges river, agricultural soil and human placenta bit, respectively, and compared it with L. monocytogenes EGD-e serovar 1/2a and L. monocytogenes F2365 serovar 4b strain. The contigs of all the three strains had a similarity (>90%) in regions that aligned with EGD-e and F2365. A total of 2872 core genes on the set of strains were identified in BHU1, 2 and 3 strains. In the mice virulence assay, BHU2 and 3 strains showed pathogenicity while BHU1 was non-pathogenic. These strains were also characterized with unique genes (8 genes in BHU1, 12 in BHU 2, and 17 in BHU 3 strain). Phylogenetic analysis based on multilocus sequence typing revealed BHU1, 2 and 3 strains to be more closely related to lineage I, serotype 4b, clonal complex 1, and sequence type 328. BHU1 strain seemed to harbor nucleotide mutation from A to G in the major virulence genes i.e. hemolysin D and listeriolysin O. Strain-specific mutations 8, 5 and 2 were identified in BHU1, 2 and 3, respectively, compared to F2365. Though all the three strains were genetically very close, the observed differences may play the crucial role(s) in their virulence attributes, and also, in the prevalence of L. monocytogenes.