Characterization And Evolutionary Study Of Mobile Genetic Elements In Different Serogroups Of Vibrio Cholerae

Vibrio cholerae is the causative agent of the acute diarrheal disease cholera. To date, more than 200 serotypes have been reported, but only toxigenic lineages of serogroups O1 and 0139 cause epidemic and pandemic cholera. The pathogenicity of epidemic V. cholerae is associated with the critical virulence factor, cholera toxin,encoded by the temperate bacteriophage CTX . Several other common mobile genetic elements, such as VSP-?/? and VPI-?/?, are associated with the 7th pandemic.In addition, an SXT element encoding antibiotic resistance genes has been discovered.These elements could transfer horizontally and bring new functions and characteristics, which is an important mechanism in the evolution. So, the research in the mobile genetic elements of V. cholerae is of significant value for understanding the population structure, evolution, and mechanism of pathogenicity and antibiotic resistance of this pathogen.In this study, an approach was established to identify genomic islands based on GC content and comparative genomic analysis, and analyzed the distribution and variation of mobile genetic elements in different serogroups of V. cholerae. Using PERL programming language, an approach was established to identify genomic islands based on GC content. Based on comparative genomic analysis using BLAST,the other approach was established to compare these elements. These methods and approaches can treat large genomic data set parallelizedly and rapidly. From the National Center for Biotechnology Information, 178 V. cholerae genomes were downloaded including 114 O1 isolates, 4 0139 isolates, 50 isolates of other serogroups and 10 of unknown serogroups. Using 01 serogroup El Tor strain N16961 as reference, single nucleotide polymorphisms were analyzed and the phylogenetic relationships were reconstructed at the whole genome level. Thirteen wbe sequences of O1, 0139, 022, 05, 08, 037,0108 and non O1, non 0139 isolates were obtained.By sequence alignment and cluster analysis, serogroup specific sequences were identified.By GC analysis, 13213 genomic islands were identified and different patterns were observed in 01 epidemic and non epidemic isolates, and non 01, non 0139 isolates. In the 178 V. cholerae genomes, VSP-I were screened and the sequence variations were analyzed. All of the 7th pandemic isolates carried VSP-I sequences and the variations were all SNPs. In non 01, non 0139 isolates and other species of the genus of Vibrio, VSP-I sequences were also identified and 7 insertion sequences were found in these isolates. By searching of SXT element,all the V. cholerae genomes showed a positive rate of 65.2%, including 92 01 isolates, 3 0139 isolates and 21 of other serogroups. Phylogenetic analysis showed that SXT sequences from 01 and 0139 isolates were clustered together, and the ones from non O1, non 0139 isolates were in the middle of 01/0139 and other bacteria. These results revealed that non 01, non 0139 V. cholerae may play an important role in the SXT acquisition of 01 and 0139 isolates from other bacteria. By searching of TTSS2, 5 01 non epidemic isolates and 14 of other serogroups showed positive results. By similarity analysis with other 8 TTSS2 sequences from Vibrio parahaemolyticus isolates, 3 groups of TTSS2 were clustered, including TTSS2a, TTSS2b and TTSS2c, and the TTSS2 sequences of V. cholerae were from the last two groups.There are lots of genomic isleands in the genome of V. cholerae, most of which have the ability of horizontal transfer. The sequence variations in the wbe were obvious and sequence recombination were identified,which may be the key factor for the amount of serogroups. The VSP-?/? elements were identified in non 01, non 0139 serogroups an other species of Vibrio, which are not the specific sequences of 7th pandemic isolates. TTSS2 is an important virulence factor of non 01, non 0139 V.cholerae isolates, which could obtain from different sources. Non 01, non 0139 V.cholerae may play an important role in the SXT acquisition of O1 and 0139 isolates,which is worth further research.