The French company "Danone" (as in "Dannon Yogurt") has an excellent research program on this and has a very good collection of monographs available. I am going to attach a PDF on probiotics but will quote some as well-

"As probiotics affect the composition of the intestinal
flora, they can also affect its function.
Several possible mechanisms have been suggested
that could account for the effect of probiotics on
intestinal disorders and diarrhea, notably the produc-
tion by some lactic bacteria of antimicrobial sub-
stances (hydrogen peroxide, bacteriocins and organic
acids for example) which have a damaging effect on
the growth of pathogens, and modulate the function
of the immune system.
The probiotics could influence the ability of the flora
to hydrolyze the glucose bonds; this ability has a limi-
ting effect, for instance, on the uptake of the
isoflavones of soy (87). Some lactic bacteria are able
to reduce the activity of some enzymes of the flora,
such as ß-glucuronidase, ß-glucosidase, azoreductase
and nitroreductase, which are responsible for con-
verting procarcinogenic substances into carcinogens.
This has been demonstrated in animal species and in
Man (71;72;88)."

Darn it! The meager file sizes permitted here are so small that it won't let me post it. If anyone wants it, PM me.