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Comparison of media used to evaluate Rhizobium leguminosarum bivar viciae for phosphate-solubilizing ability

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Rhizobium leguminosarum is well known for its ability to fix nitrogen (N). In addition, its capacity to solubilize phosphate (Ph) has been receiving attention in recent years. Our ultimate objective was to select a R. leguminosarum bv. viciae isolate with superior Ph-solubilizing ability. The first step was to identify a culture medium that is sensitive and effective in identifying the ability of R. leguminosarum bv. viciae isolates to solubilize Ph. Thirty isolates were evaluated for Ph solubilization in broth and on solid formulations of three media: yeast mannitol extract (YEM), National Botanical Research Institute phosphate nutrient medium (MNBRI), and Pikovskaya phosphate medium (PVK). All media contained 5 g/L CaHPO4 as the only phosphorus (P) source. All 30 isolates increased the Ph concentration in liquid cultures, but the amount of Ph released into solution by individual isolates varied from one medium to another. In contrast, only a subset of the 30 isolates solubilized Ph on the solid cultures. Furthermore, some of the isolates that were able to solubilize Ph were only able to do so on a single medium. Regression analysis revealed no relationship between the Ph concentration in the liquid media and the zones of clearing on the solid media (p > 0.05). Although the pH of all of the liquid media dropped after 12 days of growth of the isolates, a relationship between Ph concentration and pH existed only for the MNBRI medium (r2 = 0.485, p < 0.001). Increasing the amount of N in the MNBRI medium from 0.1 g/L of (NH4)2SO4 to 0.5 g/L of (NH4)2SO4 did not affect the amount of Ph in solution, but it profoundly reduced the survival of the R. leguminosarum by approximately 50-fold. Consequently, the surviving bacteria were either more efficient at solubilizing Ph in the high N media or organic acids released from the lysis of the dead cells solubilized the CaHPO4 source.

Rhizobium leguminosarum est reconnu pour sa capacité de fixer l’azote (N). En plus, sa capacité de solubilizer le phosphate a reçu une certaine attention au cours des dernières années. Notre objectif premier était de sélectionner un isolat de R. leguminosarum bv. viciae possédant une capacité supérieure de solubilization de phosphate (Ph). La première étape a consisté à identifier un milieu de culture sensible et efficace pour mesurer la capacité de solubilization de Ph des isolats de R. leguminosarum. La capacité de solubilization du Ph de 30 isolats a été évaluée dans des préparations liquides et solides de trois milieux : le YEM (« yeast mannitol extract »), le MNBRI (« National Botanical Research Institute phosphate nutrient ») et le PVK (« Pikovskaya phosphate »). Tous les milieux contenaient 5 g/L de CaHPO4 comme seule source de phosphore (P). Tous les isolats augmentaient la concentration de phosphate inorganique (Ph) des cultures liquides, mais la quantité de Ph secrété en solution par chaque isolat variait d’un milieu à l’autre. En revanche, seul un sous-groupe d’isolats parmi les 30 solubilizait le Ph en milieu de culture solide. Qui plus est, les quelques isolats capables de solubilizer le Ph ne le faisaient que sur un seul milieu. Une analyse de régression n’a révélé aucune relation entre la concentration de Ph en milieu liquide et les zones de clarification en milieu solide (p > 0,05). Même si le pH de tous les milieux liquides chutait après 12 jours de croissance des isolats, une relation entre la concentration de Ph et le pH n’était observée que dans le milieu MNBRI seulement (r2 = 0,485, p < 0,001). Une augmentation de la quantité de N dans le milieu MNBRI de 0,1 g/L (NH4)2SO4 à 0,5 g/L (NH4)2SO4 n’affectait pas la quantité de Ph en solution, mais réduisait de façon importante la survie de R. leguminosarum d’environ 50 fois. Conséquemment, les bactéries survivantes étaient soit plus efficaces à solubilizer le Ph dans le milieu contenant une plus grande quantité de N, ou les acides organiques secrétés lors de la lyse des cellules mortes solubilizaient le CaHPO4.

Document Type: Research Article

Publication date: 2009-07-01

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  • Published since 1954, this monthly journal contains new research in the field of microbiology including applied microbiology and biotechnology; microbial structure and function; fungi and other eucaryotic protists; infection and immunity; microbial ecology; physiology, metabolism and enzymology; and virology, genetics, and molecular biology. It also publishes review articles and notes on an occasional basis, contributed by recognized scientists worldwide.
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