Isolation and mode of action of a cell-free bacteriocin (mutacin) from serotype g Streptococcus mutans MT3791.

A bacteriocin, mutacin MT3791, was isolated from the culture supernatant of a dialysate medium of Tryptose phosphate broth cultures of Streptococcus mutans MT3791 (serotype g), a clinical isolate from a carious lesion of a Japanese child. The mutacin was found to inhibit the growth of most indicator strains of S. mutans and S. salivarius by the drop assay method. The mutacin was purified 1,450 fold from culture supernatant of S. mutans MT3791 by 60% saturated ammonium sulfate precipitation, followed by ultracentrifugation at 300,000 X g for 18 h. Gel filtration studies using Sepharose 4B indicated that the mutacin was highly aggregated, but could be dissociated by addition of 1% (final concentration) Tween 80 or 6M urea. The mutacin was heat stable, but the activity was destroyed after autoclaving at 120 degrees C for 60 min. It was susceptible to the enzymatic action of papain and pronase, but not trypsin, lipase or nucleases. The mutacin was adsorbed to whole cells of most strains of S. mutans. Mutacin MT3791 was bactericidal for an indicator strain, S. mutans MT703R (serotype e). The mutacin inhibited the incorporation of isotope-labelled precursors of protein, DNA, and RNA. Furthermore, mutacin MT3791 selectively and markedly suppressed the growth of S. mutans present in the dental plaque obtained from active carious lesions in the teeth of children. Possible role of mutacin in vivo was discussed with special reference to oral microbial ecology.