Characterization of CRISPR-Cas system in clinical Staphylococcus epidermidis strains revealed its potential association with bacterial infection sites

Abstract

Staphylococcus epidermidis is considered as a major cause of nosocomial infections, bringing an immense burden to healthcare systems. Virulent phages have been confirmed to be efficient in combating the pathogen, but the prensence of CRISPR-Cas system, which is a bacterial immune system eliminating phages was reported in few S. epidermidis strains. In this study, the CRISPR-Cas system was detected in 12 from almost 300 published genomes in GenBank and by PCR of cas6 gene in 18 strains out of 130 clinical isolates obtained in Copenhagen. Four strains isolated in 1965–1966 harboured CRISPR elements confirming that this immunity system was not recently acquired by S. epidermidis. In these CRISPR-positive strains, 44 and 12 spacers were found to belong to CRISPR1 and CRISPR2 elements, respectively. However, only 15 spacers displayed homology to reported phages and plasmids DNA. Interestingly, 5 different spacers located in the CRISPR1 locus with homolgy to virulent phage 6ec DNA sequences, and 19 strains each carrying 2 or 3 different spacers recognizing this phage, implied that the CRISPR-Cas immunity could be abrogated by nucleotide mismatch between the spacer and its target phage sequence, while new spacers obtained from the evolved phage could recover the CRISPR interference. In addition, phylogenetic analysis of the 29 CRISPR-positive isolates divided them into four lineages, with 81% human blood isolates as a distinct sub-lineage, suggesting that the CRISPR difference is closely related to diverse habitats. Knowledge of CRISPR and its prevalence may ultimately be applied in the understanding of origin and evolution of CRISPR-positive S. epidermidis strains.