Guild Composition of Root-Associated Bacteria Changes with Increased Soil Contamination.

The interaction of plants and root-associated bacteria encourage the removal of soil contaminants. Engineers and scientists have looked at this phenomenon as a possible means of soil treatment (rhizodegradation). In this study, root-associated bacteria were isolated and selected for growth on a model soil contaminant: polycyclic aromatic hydrocarbons. Isolates were compared genetically to see how plant-bacteria interactions change with soil contamination levels. Characterization of root-associated bacteria was performed using REP-PCR genetic fingerprinting and 16s rRNA gene alignments for identification. Genomic fingerprinting indicated that the composition of PAH-metabolizing bacteria ("guild") was similar among plant species at each treatment level. However, guild composition changed with contamination level and differed from that of bulk soils, suggesting a common rhizosphere effect among plant species related to PAH contamination. PAH-metabolizing bacteria were identified through 16s rRNA gene alignment as members of the α-, β-, and γ-proteobacteria, Actinobacteria, and Bacilli classes. Burkholderia and Pseudomonas spp. were the only genera of bacteria isolated from all plant types in uncontaminated controls. Bacterial species found at the highest treatment included Achromobacter xylosoxidans, Rhodococcus spp., members of the Microbacteriae, Stenotrophomonas rhizophilia, as well as other members of the alpha-proteobacteria. Given their ability to grow on PAHs and inhabit highly contaminated rhizospheres, these bacteria appear good candidates for the promotion of rhizodegradation.