Frozen cross section through a Pseudomonas aeruginosa biofilm.  The substratum was at the bottom and the flowing nutrient medium at the top.  Red is general biomass stain that indicates the total extent of the biofilm.  Green corresponds to alkaline phosphatase activity, which was induced by subjecting the biofilm to phosphate starvation.  Our interpretation of this pattern is that only the upper sixth of the biofilm was capable of de novo protein synthesis; the lower five-sixths was in an inactive state.  Scale bar 100 microns.  Image courtesy of K.D. Xu.  See Xu et al., 1998 in our Resource Library collection of abstracts.