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Application of a Rapid Method for Gram Differentiation of Plant Pathogenic and Saprophytic Bacteria Without Staining. T. V. Suslow, Research fellow, Department of Plant Pathology, University of California, Berkely 94720; M. N. Schroth(2), and M. Isaka(3). (2)Professor, Department of Plant Pathology, University of California, Berkely 94720; (3)Professor, Department of Agriculture, Fukui Prefectural College, Ohata-Cho, Fukui, Japan. Phytopathology 72:917-918. Accepted for publication 11 December 1981. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-917.

The Gram-stain reactions of pathogenic and saprophytic bacteria isolated from plant parts were accurately characterized by mixing a concentrated droplet of cells with 3% potassium hydroxide (KOH). Compared to standard Gram-staining procedures, the KOH test was rapid, simple, and completely accurate. One hundred thirty strains representing Achromobacter, Agrobacterium, Arthrobacter, Azotobacter, Bacillus, Corynebacterium, Erwinia, Pseudomonas, Xanthomonas, and Streptomyces species were tested. Bacterial cells were aseptically removed from an agar medium with a toothpick and placed on a glass slide in a 50 μl drop of 3% KOH with rapid circular agitation. With Gram-negative strains the suspension became viscous as revealed by a mucoid thread that formed when the toothpick was lifted. Gram-positive bacteria dispersed into the drop and did not have this reaction. A few strains of Corynebacterium caused an increase in drop viscosity, but did not produce a mucoid thread.