Mutation requires two Bacteria of equal level and always gives one Bacteria of a higher level as a reward. The rarity of the mutated Bacteria is the same as the rarity of the used Bacteria. If they are of different rarities, the mutated Bacteria will have a random rarity in the same range, so it is not possible to obtain a higher rarity Bacteria through mutation.
Mutation is instantaneous and it requires no Scientists. However, after it is complete, the player has to clean the lab before it can be again used for mutations.