Endophytic and ectophytic potato-associated bacterial communities differ in structure and antagonistic function against plant pathogenic fungi

Endobacteria in some ectomycorrhiza of Scots pine (Pinus sylvestris)

The diversity of cultivable endobacteria associated with four different ectomycorrhizal morphotypes (Suillus flavidus, Suillus variegatus, Russula paludosa and Russula sp.) of Scots pine (Pinus sylvestris) was analysed by restriction fragment length polymorphism profiling of PCR-amplified rDNA intergenic spacer regions and by sequence analyses of 16S rRNA genes. Ectomycorrhizal root tip surface-sterilization methods were developed and assessed for their efficiencies. Bacterial communities from surface-sterilized ectomycorrhizal root tips were different from those of ectomycorrhizal root tips without surface-sterilization for all the morphotypes studied. Endobacteria belonging to the genera Pseudomonas, Burkholderia and Bacillus were isolated from more than one ectomycorrhizal morphotype, whereas species of Rahnella, Janthinobacterium and Rhodococcus were only isolated from the single morphotypes of S. variegatus, R. paludosa and Russula sp., respectively. Some of the isolated endobacteria utilized fungal sugars more readily than typical plant sugars in carbon utilization assays.

Bacterial endophytes of the wildflowerCrocus albiflorusanalyzed by characterization of isolates and by a cultivation-independent approach

The presence and taxonomy of endophytic bacteria of the entire aerial parts of crocus (Crocus albiflorus), a wildflower native in the Alps, were investigated. A combination of plating of plant macerates, isolation and sequence identification of isolates, and direct 16S rDNA PCR amplification followed by whole-community fingerprinting (T-RFLP) and by construction of a bacterial clone library was used. The results clearly indicated that a wide range of bacteria from diverse phylogenetic affiliation, mainly γ-Proteobacteria and Firmicutes, live in association with plants of C. albiflorus. The community composition of the culturable component of the microflora was remarkably different from that of the clone library. Only three bacterial divisions were found in the culture collection, which represented 17 phylotypes, whereas six divisions were identified in the clonal analysis comprising 38 phylotypes. The predominant group in the culture collection was the low G + C Gram-positive group, whereas in the clone library, the γ-Proteobacteria predominated. Interestingly, the most prominent bacterium within the uncultured bacterial community was a pseudo monad closely related to a cold-tolerant Pseudomonas marginalis strain. The results suggest that Crocus supports a diverse bacterial microflora resembling the microbial communities that have been described for other plants and containing species that have not been described in association with plants.Key words: crocus, endophytes, 16S rRNA, 16S rDNA clone library, T-RFLP analysis, community analysis.

The rhizosphere as a reservoir for opportunistic human pathogenic bacteria

During the last years, the number of human infections caused by opportunistic pathogens has increased dramatically. One natural reservoir of opportunistic pathogens is the rhizosphere, the zone around roots that is influenced by the plant. Due to a high content of nutrients, this habitat is a 'microbial hot-spot', where bacterial abundances including those with strong antagonistic traits are enhanced. Various bacterial genera, including Burkholderia, Enterobacter, Herbaspirillum, Ochrobactrum, Pseudomonas, Ralstonia, Staphylococcus and Stenotrophomonas, contain root-associated strains that can encounter bivalent interactions with both plant and human hosts. Mechanisms responsible for colonization of the rhizosphere and antagonistic activity against plant pathogens are similar to those responsible for colonization of human organs and tissues, and pathogenicity. Multiple resistances against antibiotics are not only found with clinical strains but also with strains isolated from the rhizosphere. High competition, the occurrence of diverse antibiotics in the rhizosphere, and enhanced horizontal gene transfer rates in this microenvironment appear to contribute to the high levels of natural resistances. While opportunistic bacteria from the rhizosphere have some properties in common, each of these emerging pathogens has its own features, which are discussed in detail for Burkholderia, Ochrobactrum and Stenotrophomonas.