[yorwba]

The beneficial mutation is not secretion of mutacin-1140, but absence of lactic acid production. Transplanting only the original mutacin-1140-producing strain wouldn't help anyone, since it still produces lactic acid that would destroy your teeth all the same.

david_l_lin upthread is stressing that lactic acid is ordinary Streptococcus mutans' primary defense against other bacteria and it wouldn't be able to survive without it.

The significance of mutacin-1140 is that it provides an alternative means of defense against other bacteria (or maybe offense would be a better word) which potentially makes lactic acid redundant.