Restriction-modification systems and phage resistance of enterococci from ewe milk

Currently there are limited data available on the occurrence of restriction-modification systems (RMS) in the enterococci from ewe milk and their role in protection against phage infection during the fermentation process. In our study sixty-one bacterial strains from ewe milk identified as Enterococcus spp. using MALDI-TOF analysis of protein fingerprints were screened for the presence of site-specific restriction-modification activity and resistance to bacteriophages. Eight strains featured endonuclease activity detectable in crude cell extracts. The recognition sequences and optimal reaction conditions for eight of them were determined. All enzymes were detected to be isoschizomers of type-II endonucleases (SduI (in 3 strains); HaeIII (in 4 strains) and NciI). Optimal reaction activity was found in a wide range of salt concentrations, and all enzymes featured adequate activity at 37 °C. No clear correlation was observed between the presence of RMS and resistance to bacteriophage infection, indicating that the feature of RMS is not the main mechanism of bacteriophage resistance in enterococci from ewe milk. This is the first report on the relation between RMS and phage resistance in enterococci from ewe milk.