The Phylogenomics of CRISPR-Cas System in Salmonella: an evolutionary race with housekeeping genes

Salmonellae display intricate evolutionary patterns comprising over 2500 serovars having diverse pathogenic profiles. The acquisition/exchange of various virulence factors influence the evolutionary framework. To gain insights into evolution of Salmonella as a pathogen in association with the CRISPR-Cas genes we performed phylogenetic surveillance across strains of 22 Salmonella serovars. The strains assorted into two main clades, pertaining to the differences in their CRISPR1-leader and cas operon. Considering Salmonella enterica subsp. enterica serovar Typhimurium and serovar Typhi as signature serovars, we classified the clades as CRISPR1-STM/cas-STM and CRISPR1-STY/cas-STY, respectively. Serovars of the two clades displayed better relatedness, concerning CRISPR-1 leader and cas operon, across genera than between themselves. This signifies the acquisition of CRISPR1/Cas region a horizontal gene transfer event owing to the presence of mobile genetic elements flanking CRISPR1 array. The CRISPR2 tree does not show such relation. Spacer mapping of the two CRISPR arrays suggests the construct to be canonical, with only 8.8% spacer conservation among the serovars. As opposed to broad-host-range serovars, the host-specific serovars harbor fewer spacers. All typhoidal serovars have CRISPR1-STY/cas-STY system. Comparison of CRISPR and cas phenograms with that of multilocus sequence typing (MLST) suggests differential evolution of CRISPR/Cas system implying supplementary roles beyond immunity.Competing Interest StatementThe authors have declared no competing interest.View Full Text