Insights into the CRISPR/Cas system of Gardnerella vaginalis

Background: Gardnerella vaginalis is identified as the predominant colonist of the vaginal tracts of women diagnosed with bacterial vaginosis (BV). G. vaginalis can be isolated from healthy women, and an asymptomatic BV state is also recognised. The association of G. vaginalis with different clinical phenotypes could be explained by different cytotoxicity of the strains, presumably based on disparate gene content. The contribution of horizontal gene transfer to shaping the genomes of G. vaginalis is acknowledged. The CRISPR loci of the recently discovered CRISPR/Cas microbial defence system provide a historical view of the exposure of prokaryotes to a variety of foreign genetic elements. Results: The CRISPR/Cas loci were analysed using available sequence data from three G. vaginalis complete genomes and 18 G. vaginalis draft genomes in the NCBI database, as well as PCR amplicons of the genomic DNA of 17 clinical isolates. The cas genes in the CRISPR/Cas loci of G. vaginalis belong to the E. coli subtype. Approximately 20% of the spacers had matches in the GenBank database. Sequence analysis of the CRISPR arrays revealed that nearly half of the spacers matched G. vaginalis chromosomal sequences. The spacers that matched G. vaginalis chromosomal sequences were determined to not be self-targeting and were presumably neither constituents of mobile-element-associated genes nor derived from plasmids/viruses. The protospacers targeted by these spacers displayed conserved protospacer-adjacent motifs. Conclusions: The CRISPR/Cas system has been identified in about one half of the analysed G. vaginalis strains. Our analysis of CRISPR sequences did not reveal a potential link between their presence and the virulence of the G. vaginalis strains. Based on the origins of the spacers found in the G. vaginalis CRISPR arrays, we hypothesise that the transfer of genetic material among G. vaginalis strains could be regulated by the CRISPR/Cas mechanism. The present study is the first attempt to determine and analyse the CRISPR loci of bacteria isolated from the human vaginal tract.