A cariogenic strain, Streptococcus mutans PK 1, has been demonstrated to have prophage by observation of phage particles with an electron microscope and by induction with mitomycin C. The phage particles could not be detected in a mutant strain which lost the characteristic adhesive nature on glass surfaces and exhibited diminished ability to synthesize insoluble polysaccharide. By infecting the mutant cells with the phages or with free phage deoxyribonucleic acid isolated from the parent strain of S. mutans PK1, the mutant cells were transformed to the cariogenic strain with adhesive nature. The transformants retained the general characteristics of S. mutans PK 1, but in addition all transformants showed a new character; namely, the transformant cell could produce ammonia from arginine, whereas neither the parent nor mutant strains of S. mutans PK 1 had such a property;
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