Group B Streptococci Epsilon/Zeta Toxin-antitoxin System Stabilizes Plasmid pAT29 in S. agalactiae

Group B streptococci (GBS) or Streptococcus agalactiae is the major cause of various diseases in newborns and elderly people.The genome of GBS is highly heterogeneous and contains mobile genetic elements, which play a significant role in the rapid adaptation of GBS to different ecological niches.Some mobile genetic elements are stabilized in a cell by toxin-antitoxin systems.The present study focused on investigating the putative Epsilon/Zeta toxin-antitoxin system, which is encoded by the GBS pathogenicity island PAI-A.Genes of the putative Zeta toxin, Epsilon antitoxin, and the entire Epsilon/Zeta operon from the GBS strain 07y/08 were cloned and expressed in Escherichia coli.The overexpression of the Zeta toxin gene caused temporary inhibition of E. coli growth.PAI-A's Epsilon antitoxin could bind to the Zeta toxin and eliminate its toxicity.The Epsilon/Zeta toxin-antitoxin system inserted into plasmid pAT29 stabilized an otherwise unstable plasmid in GBS.Based on the experimental data, we concluded that the biological role of this genetic element in GBS was to prevent translocation of the pathogenicity island PAI-A from the genome of S. agalactiae.Thus, the Epsilon/ Zeta toxin-antitoxin system of GBS might contribute to the adaptation of the microorganism to new ecological niches.The present research might contribute to the development of a new class of antimicrobial substances.