Detection of growth of Escherichia coli on two-dimensional diffusion gradient plates

One drawback of using two‐dimensional diffusion gradient plates, the subjective visual assessment of microbial growth, has been overcome. Growth of Escherichia coli was detected with pH indicators in the medium or by staining growth with a biochemical stain, L‐alanine‐p‐nitroanilide, or a respiratory dye, 2‐(p‐iodophenyl)‐3‐(p‐nitrophenyl)‐5‐phenyl tetrazolium chloride. Stained growth was scanned with a laser densitometer and traces combined in a computer to give a three‐dimensional semi‐quantitative representation of growth over the gradient plate.