E.coli likes an acidic environment.  My guess is that DDG increases the acidity of the rumen thus making it more hospitable for the bacteria.  If present they will increase their concentrations. Our digestive system is also acidic that is why E.coli is such a problem.  Lowering the PH of the cattle rumen will kill off (lessen) the E.coli.  Read a study once that feeding hay only for several days before slaughter drastically reduced E.coli numbers and thus lessened our risk of exposure to the bacteria.

my $0.02