Biofilm frozen cross section stained to reveal spatial variation in the local growth rate.  The bulk fluid was at the top and the substratum upon which the biofilm grew was at the bottom of the image. The fluorescent color after acridine orange staining was shown to reflect the relative growth rate of the microorganisms.  Orange indicates rapid growth and green indicates slow or no growth.  Rapid growth occurs in the biofilm near the biofillm-bulk fluid interface.  In places where the biofilm is thin, microorganisms throughout the biofilm are growing rapidly.  In places where the biofilm is thicker, there are zones deep in the biofilm that experience slow growth, probably due to nutrient limitation.  Scale bar is 100 microns.  See Wentland et al., 1996 in our Resource Library collection of abstracts.