When reading disk diffusion results on Pseudomonas aeruginosa, we sometimes observe a "fringe" of growth at the periphery of the zones of inhibition. This is inside an area of heavier growth. Should this "fringe" be ignored, or should the zone size be measured from the inner "fringe"?

We assume that your quality control (QC) values for Pseudomonas aeruginosa ATCC 27853 are in range. The QC results are a good indication of how the zones should be read. This is critical because your results may reflect over-inoculating the plates, which should be indicated by the QC results. Beta-lactam drugs are more subject to inoculum effects than are other drug classes. If the QC results are in control and the fringe is obvious with your patient tests and you can readily see it without a dissecting microscope, consider it "growth". If the fringe is very faint and difficult to see, ignore it and measure the zone where growth is obvious.