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.

Characterization of high temperature-tolerant rhizobia isolated from Prosopis juliflora grown in alkaline soil.

“…Nodules were disinfected, crushed in a small amount of saline (0.85% NaCl), and suspension streaked on yeast extract mannitol (YEM) plates. Rhizobium strains unless otherwise stated were grown on YEM plates and maintained on YEM slants that contain per liter 15 g agar, 10 g mannitol, 1 g yeast extract, 0.5 g K 2 HPO 4 , 0.2 g MgSO 4 · 7H 2 O, and 0.1 g NaCl with 0.025% (w/v) Congo red (Hahn, 1966). The pH of the medium was adjusted to 7.2, and the cultures were grown as described earlier (Surange et al, 1997).…”

Biocontrol of Aspergillus root rot by tripartite symbiosis in groundnut

“…Using a sterile glass rod, the nodules were gently crushed in a sterile yeast extract mannitol broth. An aliquot was plated onto yeast extract mannitol agar containing 0.025% Congored (Hahn, 1966) and incubated at 25±1 o C. The white, translucent, glistening, elevated and small colony with entire margin was transferred to YMA slants.…”

“…The purity of the culture was periodically ascertained using congo red reaction (Hahn, 1966). Seven days old seedlings of Arachis hypogaea L. were inoculated with 2 ml thick suspension of peanut Rhizobium (approximately 1x10 9 cells / ml) prepared from 72hr old cultures.…”

“…Evidently, the treatment AT 13 was best among other treatments. 32.6±1.90 AT 7 19.3±1.83 AT 8 22.2±1.38 AT 9 43.1±2.12 AT 10 35.1±2.01 AT 11 30.9±0.32 AT 12 37.4±1.56 AT 13 15.9±0.49…”

Impact of commonly used agrochemicals on bacterial diversity in cultivated soils

“…Congo red yeast extract mannitol agar (YMA) [17] was inoculated with 1g of each soil treatment type. Incubation was at 30°C for 5 days.…”