The Reduced Bacteria Needed (RBN) effect changes how many Bacteria are needed to take down a planet.
For example, if a player attacks a planet with 100k Bacteria and has +100% bonus to RBN, it will be just as effective as attacking with 200k Bacteria and +0% to RBN.
RBN can be increased via Planetary Research.
NB!
RBN does not reduce the price of Bacteria Colonies and Upgrades.