Comparative analysis of the genomes of Shigella dysenteriae type 2 & type 7 isolates.

Background & objectives: The four species of the genus Shigella, namely, S. dysenteriae, S. flexneri, S. boydii and S. sonnei cause a wide spectrum of illness from watery diarrhoea to severe dysentery. Genomes of these four species show great diversity. In this study, NotI, XbaI or I-CeuI restriction enzyme digested genomes of two Shigella dysenteriae isolates belonging to the serotypes 2 and 7 were extensively analyzed to find their relatedness, if any, with the whole genome sequenced strains of S. dysenteriae type 1 and S. flexneri type 2a. Methods: Pulsed-field gel electrophoresis (PFGE) technique was used to determine the diversity of Shigella genomes by rapid construction of physical maps. DNA end labelling, Southern hybridization and PCR techniques were also applied for mapping purposes. Results: The intron-coded enzyme I-CeuI cuts the bacterial genome specifically at its rrn operon. PFGE of I-CeuI digested S. dysenteriae genomes were found to carry seven rrn operons. However, I-CeuI profiles showed distinct restriction fragment polymorphism (RFLP) between the isolates as well as with the whole genome sequenced isolates. Further studies revealed that the genome sizes and I-CeuI linkage maps of the S. dysenteriae type 7 and type 2 isolates were similar to that of S. dysenteriae type 1 and S. flexneri type 2a genomes, respectively. Interpretation & conclusions: Our findings indicate that the type 7 and type 1 isolates of S. dysenteriae were probably evolved from a same precursor, while the type 2 and S. flexneri type 2a were probably evolved and diversified from a common progenitor.