Synthesis of bacteriocins in liquid cultures of Streptococcus mutans

Strains of Streptococcus mutans synthesized bacteriocins in agar plates, but synthesis of detectable bacteriocins in liquid media took place only under certain culture conditions. The composition of the medium proved to be crucial. Trypticase Soy Broth with 4% Yeast Extract meeting the requirements. The effect of the Yeast Extract is obscure, for some strains also formed detectable bacteriocins in a special Trypticase medium without this agent. It was noted that the broth should be filter-sterilized rather than autoclaved and only a few days old. Attempts at liberating cell-bound bacteriocins from washed cells were unsuccessful, even when they were treated with ultrasound, EDTA, or various chemicals followed by ultrasound. On the basis of size and sensitivity to heat the bacteriocins could be divided into two groups, while their resistance to ether and chloroform and to trypsin did not follow this pattern. Dependence on plasmids could not be demonstrated by attempts at curing with acridine orange or ethidium bromide; and the involvement of phages was unlikely, since the inhibition was not transmissible and phage-like structures were not observed in the electron microscope.