In this study, the bacterial communities associated with the rhizospheres of pioneer plants Bahia xylopoda and Viguiera linearis were explored. These plants grow on silver mine tailings with high concentration of heavy metals in Zacatecas, Mexico. Metagenomic DNAs from rhizosphere and bulk soil were extracted to perform a denaturing gradient gel electrophoresis analysis (DGGE) and to construct 16S rRNA gene libraries. A moderate bacterial diversity and twelve major phylogenetic groups including Proteobacteria, Acidobacteria, Bacteroidetes, Gemmatimonadetes, Chloroflexi, Firmicutes, Verrucomicrobia, Nitrospirae and Actinobacteria phyla, and divisions TM7, OP10 and OD1 were recognized in the rhizospheres. Only 25.5% from the phylotypes were common in the rhizosphere libraries and the most abundant groups were members of the phyla Acidobacteria and Betaproteobacteria (Thiobacillus spp., Nitrosomonadaceae). The most abundant groups in bulk soil library were Acidobacteria and Actinobacteria, and no common phylotypes were shared with the rhizosphere libraries. Many of the clones detected were related with chemolithotrophic and sulfur-oxidizing bacteria, characteristic of an environment with a high concentration of heavy metal-sulfur complexes, and lacking carbon and organic energy sources.

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The aim of this work is to investigate the effect of the crystalline and the double helical structures on the resistant starch (RS) fraction of autoclaved corn starch with different amylose content. Normal (NS) and high amylose (HS) corn starches are modified by autoclaving at 105, 120, and 135°C. Autoclaving causes structure disorganization in NS or structural rearrangements in HS. The RS increases from 2.1% to 5.5% in NS autoclaved at 135°C and from 12.7% to 30.2% in HS autoclaved at 120°C. The RS is correlated to T e in NS (r = 0.73) and HS (r = 0.79), indicating that the double helical crystallites with high thermal stability (T e = 176.8-193.6°C) are responsible for the increase of RS. In autoclaved HS, the RS is correlated to the degree of order (DO) (r = 0.82) suggesting that the strengthened associations between double helices contribute to the RS formation. The results lead to the possibility of controlling the resistance by modifying the starch structure using autoclaving process. This research highlights that the organization of the double helical structure is more important than the crystalline order to form resistant corn starch.

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