LACK OF SURFACE-RECEPTORS NOT RESTRICTION-MODIFICATION SYSTEM DETERMINES F4 PHAGE RESISTANCE IN STREPTOCOCCUS-BOVIS II 1/

The resistance of Streptococcus bovis strain II/1, the producer of Sbv I restriction endonuclease, to F4 phage infection was demonstrated by the double-agar-layer method. Despite the presence of restriction endo nuclease SbvI which can cleave F4 phage DNA to numerous fragments in v itro, the evidence that adsorption inhibition is the most important de fence mechanism in phage resistance of S. bovis II/1 strain was obtain ed by adhesion experiments in vivo. Electron microscopy of phage-host mixtures showed many phage particles on the bacterial surface of phage -sensitive S. bovis 47/3 control strain in comparison with no phage pa rticles seen on S. bovis II/1 (phage-resistant) strain surface.