The formation of cluster roots by plants represents a highly efficient strategy for acquisition of sparingly available phosphate. This particular root type is characterized by a densely branched structure and high exudation of organic acids and protons, which are likely to influence the resident bacterial community. Until now, the identity of the bacterial populations living in cluster roots has not been investigated. We applied cultivation-dependent and cultivation-independent methods to characterize the dominant bacterial genera inhabiting the growing cluster roots of white lupin. We observed a high relative abundance of Burkholderia species (up to 58% of all isolated strains and 44% of all retrieved 16S rRNA sequences) and a significant enrichment with increasing cluster root age. Most of the sequences retrieved clustered together with known plant-or fungus-associated Burkholderia species, while only one of 98 sequences was affiliated with the Burkholderia cepacia complex. In vitro assays revealed that Burkholderia strains were much more tolerant to low pH than non-Burkholderia strains. Moreover, many strains produced large amounts of siderophores and were able to utilize citrate and oxalate as carbon sources. These features seem to represent important traits for the successful colonization and maintenance of Burkholderia species in white lupin cluster roots.To access sparingly available nutrients such as phosphate, plants have evolved several strategies, e.g., mycorrhizal association and cluster root formation. Cluster roots are very densely branched root structures with a particular excretion physiology (20,21,25). They occur in many species of the Proteaceae family and occasionally in the Mimosaceae, Casuarinaceae, or Fabaceae (7). White lupin is the only cluster-rooted species of agricultural importance and has thus been extensively studied (9,17,26,32,33,41,42). In white lupin, cluster root development follows a well-defined pattern: at the juvenile stage, cluster roots secrete small amounts of malate; at the mature stage, high quantities of citrate and protons are excreted, leading to drastic rhizosphere acidification; and at the senescent stage, organic acid excretion decreases. Besides citrate and malate, oxalate and fumarate have also been reported to be exuded by soil-grown lupin plants (6, 41). The close vicinity of growing cluster roots constitutes a highly selective environment, owing to rapid changes in pH and carbon availability. The abundance of bacteria, as well as richness and diversity, has been shown to decrease temporarily in mature cluster roots (40). However, the identity of the populations repressed or enriched during cluster root development has yet to be elucidated. We analyzed the bacterial communities living in the direct vicinity of cluster roots (root surface and inner tissues) by sequencing isolated strains and clone libraries constructed from root-extracted DNA. We then tested relevant physiological properties of isolates to better understand which metabolic abilities might ena...

Twenty-eight novel clorobiocin derivatives obtained from mutasynthesis experiments were investigated for their inhibitory activity towards Escherichia coli DNA gyrase and for their antibacterial activities towards clinically relevant gram-positive and gram-negative bacteria in comparison to novobiocin and clorobiocin. Clorobiocin was the most active compound both against E. coli DNA gyrase in vitro and against bacterial growth. All tested modifications of the 3-dimethylallyl-4-hydroxybenzoyl moiety reduced biological activity. The highest activities were shown by compounds containing a hydrophobic alkyl substituent at position 3 of the 4-hydroxybenzoyl moiety. Polar groups in this side chain, especially amide functions, strongly reduced antibacterial activity. Replacement of the alkyl side chain with a halogen atom or a methoxy group at the same position markedly reduced activity. Transfer of the pyrrole carboxylic acid moiety from O-3؆ to O-2؆ of L-noviose moderately reduced activity, whereas the complete absence of the pyrrole carboxylic acid moiety led to a loss of activity. Desclorobiocin derivatives lacking the chlorine atom at C-8 of the 3-amino-4,7-dihydroxycoumarin moiety also showed low activity. Lack of a methyl group at O-4؆ of L-noviose resulted in an inactive compound. From these findings it appears that clorobiocin represents a "highly evolved" structure optimized for bacterial transport and DNA gyrase inhibition.

Members of the genus Burkholderia are highly versatile bacteria that can be beneficial as well as pathogenic for their eukaryotic hosts. Furthermore, many strains exhibit a remarkable biotechnological potential. To study the ecosystem function and lifestyle of B. terricola, we analysed the interactions with plants and survival in soil as well as the mechanisms behind it. We used a combination of in vitro and ad planta assays to study Burkholderia-plant interaction and assess the role of poly-β-hydroxybutyrate (PHB). Additionally, DsRedlabelled bacteria were analysed by confocal laser scanning microscopy (CLSM) to study root colonisation. B. terricola ZR2-12 treatment resulted in enhanced growth of sugar beet plants with a more than doubled biomass relative to the non-treated control. The strain was a remarkable good root coloniser, which was found in rhizosphere as well as endorhiza of sugar beet up to 10 log 10 CFU g −1 .Using CLSM, we observed that ZR2-12 cells form large micro-colonies along the apoplastic spaces of the root. Xylem vessels were colonised by smaller aggregates and single cells, whereas in root tips mainly single cells were present. The colonisation patterns differed strongly between older and younger parts of the roots. PHB production of ZR2-12 (up to 70% (w/w) of cell dry mass) provided a competitive advantage for rhizosphere colonisation. B. terricola ZR2-12 belongs to the plant-associated Burkholderia cluster with biotechnological potential due to its excellent root colonisation and plant growth promotion.

BAL19403 exemplifies a new family of macrolide antibiotics with excellent in vitro activity against propionibacteria. MICs indicated that BAL19403 was very active against erythromycin-resistant and clindamycinresistant propionibacteria with mutations in the region from positions 2057 to 2059 (Escherichia coli numbering) of the 23S rRNA, although it is less active against those rare clinical isolates in which a methyltransferase, ErmX, confers macrolide and lincosamide resistance by dimethylation of the adenine moiety at position 2058. BAL19403 was predominantly bacteriostatic toward the propionibacteria, and population analyses indicated resistance selection frequencies for BAL19403 and the comparator drugs (erythromycin, clindamycin) in the range 10 ؊8 to 10 ؊9 for cutaneous propionibacteria with diverse antibiotic resistance profiles. On the basis of its antipropionibacterial activity and its high anti-inflammatory activity, BAL19403 represents a promising topical treatment for mild to moderate inflammatory acne vulgaris.

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