I have heard that rice bran is used in a kind of pickled fish too but no reason was given.

I have also read about the use of straw (bales) in water streams/canals to keep the water clean (the experiment suggested that "straw is a good fungal/algal inhibitor").

So, not only bran (powder husks), also straw (dry stems/stalks and dry leaves) may be both used in preserving foods.

** Your experiment indicated that bran did not inhibit growth of vinegar bacteria.

What micro-organisms can be inhibited? By how much? For how long? And ... need more research work eh? ;-)