Exploring the antimicrobial potential of streptococcal fratricins

Many streptococci are competent for natural transformation, during which they express a murein hydrolase that lyses and kills closely related species, thereby making homologous DNA available for uptake by competent cells. This process, termed fratricide, is mediated by enzymes known as fratricins. The most extensively studied fratricin, CbpD from Streptococcus pneumoniae, is highly specific, efficient, and rapid in its killing, highlighting the potential of fratricins as targeted antimicrobial agents. Here, we screened nine putative fratricins to evaluate their antimicrobial activity. Of these, Zoocin A from Streptococcus agalactiae, ScrM from Streptococcus dysgalactiae, and CbpD from Streptococcus vestibularis were successfully purified. Zoocin A exhibited the broadest target range, whereas ScrM and CbpD-ves showed somewhat narrower activity. All three enzymes predominantly targeted streptococci. Notably, Zoocin A was active against Streptococcus mutans and Streptococcus sobrinus, species associated with dental caries. The enzymes were also active against S. agalactiae, S. dysgalactiae, and S. pyogenes, which are associated with severe infections in humans and animals. We characterised the activity and stability of Zoocin A and ScrM and found that both retained activity under physiologically relevant conditions of pH, temperature, and ionic strength. ScrM activity was independent of divalent cations, whereas Zoocin A activity required cations and was observed with both Ca2+ and Mn2+. For both enzymes, peptidoglycan cleavage was localised to the septal region of dividing cells. Zoocin A shows promise as an antimicrobial agent against S. mutans, but its efficacy against biofilms would likely benefit from further optimisation or combination with biofilm-disrupting agents.