"Fermentor" vs. "fermenter"

I am curious to know the correct usage of these words as it seems to be misused often. See https://homebrew.meta.stackexchange.com/q/202/59 for a related question.

From Dictionary.com and the Free Online Dictionary: fermenter: any agent or substance, such as a bacterium, mold, yeast, or enzyme, that causes fermentation; or, (also fermentor) an apparatus that maintains optimal conditions for the growth of microorganisms, used in large-scale fermentation and in the commercial production of antibiotics and hormones.

So fermenter can mean either the enzyme or the fermentation apparatus, whereas fermentor can only be used to mean the apparatus.

Fermentor is a container in which fermentation takes place; fermenter can be that container, or an organism that causes fermentation.