Picture:  Fragments of mold mycelia in a beer deposit.  Magnification about 200X.

Some forms of mold grow in beer.  The most likely means of mold being introduced into the beer is by exposure to the atmosphere.  Small amounts of beer surrounding a beer faucet or tap assembly can easily support the growth of molds. 

Molds will generally appear as brownish or blackish colored filamentous material which can be present on both the outside of the faucets and taps, and within the beer lines if these are not properly cleaned and maintained.

Note: The information in this section was provided by and reproduced with the permission of The Coors Brewing of Golden, Colorado.