Streptococcus zooepidemicus and Streptococcus equi evolution: the role of CRISPRs.

This study explores the evolution of Streptococcus equi, a bacterium that causes a prevalent horse illness, from Streptococcus zooepidemicus, an opportunistic pathogen in animals. The researchers found that alterations in the bacterium’s genetic content, particularly the removal of a genetic feature known as CRISPR, has influenced its ability to become pathogenic.

Pathogenic Evolution of Streptococcus equi

• The study begins by discussing the bacterium Streptococcus equi, which is the perpetrator of equine strangles – a common horse infection that causes abscesses in the neck and head lymph nodes and has substantial welfare and economic implications.
• Scientists believe that S. equi evolved from Streptococcus zooepidemicus, a bacterium that opportunistically affects horses and other animals. This postulation initiated the research.

Genomic Comparison and Findings

• The researchers compared the genomes of S. equi and S. zooepidemicus. They discovered instances of gene loss and gain which they believe have shaped S. equi’s ability to be a specialized pathogen.
• Specifically, the deletion of the CRISPR locus, a portion of DNA known for its unique organization of palindromic repeated sections, in the ancestor of S. equi appears to have allowed the bacterium to get and incorporate new genetic material into its genome.

Role of Mobile Genetic Elements

• This new genetic material includes four prophages and a new kind of integrative conjugative element, both of which are mobile genetic elements. Mobile genetic elements are genes or sets of genes that can move around within a genome which contributes to genetic reshuffling and evolution.
• The study suggests that the collection of virulence-related genes carried by these mobile genetic elements may have played a role in shaping S. equi’s ability to cause equine strangles.

Diversity of S. zooepidemicus and Influence of CRISPRs

• Another finding from the sequencing of S. zooepidemicus is the substantial diversity of this opportunistic bacterium. The researchers have proposed CRISPRs as influential factors in its evolution.
• CRISPRs may balance the need for genome stability with the acquisition of new genes. The analysis of spacer sequences (variable sequences between each repeat in the DNA’s CRISPR region) implies these pathogens may be vulnerable to a limited variety of phages.
• This provides evidence of genetic material exchange between multiple species, including S. pyogenes, S. agalactiae, and S. dysgalactiae, which suggests a broader ecological influence.