THE CONGO RED REACTION IN BACTERIA AND ITS USEFULNESS IN THE IDENTIFICATION OF RHIZOBIA

Abstract

Congo red was found in this research to form colloidal complexes with di-and tri-valent cations in the presence of acid and alkali. The complexes formed at a pH of 2–3 were uniformly blue; those formed at a pH greater than 12 varied in color with the ion involved, a fuchsia complex being formed with magnesium. The congo red reaction in bacteria (e.g. coloration of the growth in media containing the dye) is explained as due to the adsorption of Congo red on the surface of the bacterial cell, to the ions which predominate at the surface, and to the production of acid or alkali by the bacterium during growth. The temperature of incubation and composition of the growth medium influence colony coloration by affecting the metabolism of the organism.A negative form of selection for rhizobia is suggested using a nitrogen-deficient and a nitrogen-rich carbohydrate medium containing 0.25 g Congo red per liter of agar and 0.025 g Congo red per liter of broth, with incubation at 28 °C and 37 °C respectively. Color differentiation among the rhizobial strains tested, four normal and seven mutant varieties of Rhizobium meliloti and Rhizobium trifolii, occurred on the nitrogen-rich agar medium when incubation was carried out at 25 °C to 28 °C.