A novel hydroxamic acid-containing antibiotic produced by a Saharan soil-living Streptomyces strain.

During screening for potentially antimicrobial actinobacteria, a highly antagonistic strain, designated WAB9, was isolated from a Saharan soil of Algeria. A polyphasic approach characterized the strain taxonomically as a member of the genus Streptomyces. The strain WAB9 exhibited a broad spectrum of antimicrobial activity toward various multidrug-resistant micro-organisms. A PCR-based assay of genomic potential for producing bioactive metabolites revealed the presence of PKS-II gene. After 6 days of strain fermentation, one bioactive compound was extracted from the remaining aqueous phase and then purified by HPLC. The chemical structure of the compound was determined by spectroscopic (UV-visible, and (1)H and (13)C NMR) and spectrometric analysis. The compound was identified to be 2-amino-N-(2-amino-3-phenylpropanoyl)-N-hydroxy-3-phenylpropanamide, a novel hydroxamic acid-containing molecule. The pure molecule showed appreciable minimum inhibitory concentration values against a selection of drug-resistant bacteria, filamentous fungi and yeasts. Significance and impact of the study: This study presents the isolation of a Streptomyces strain, named WAB9, from a Saharan soil in Algeria. This strain was found to produce a new hydroxamic acid-containing molecule with interesting antimicrobial activities towards various multidrug-resistant micro-organisms. Although hydroxamic acid-containing molecules are known to exhibit low toxicities in general, only real evaluations of the toxicity levels could decide on the applications for which this new molecule is potentially most appropriate. Thus, this article provides a new framework of research.

Characterization and antagonistic properties of Streptomyces strains isolated from Saharan soils, and evaluation of their ability to control seedling blight of barley caused by Fusarium culmorum.

UNLABELLED: During a screening for potential plant disease control actinomycetes, a total of 133 strains were isolated from Saharan soil samples of seven Algerian regions by dilution technique on chitin-vitamins agar medium. Screening for antagonistic properties using streak assay method showed that 25% of isolates demonstrated strong activities against a wide range of plant pathogenic fungi. Due to their strong anti-Fusarium activities, six of these isolates were selected and subsequently related to Streptomyces species by polyphasic analysis. These isolates were evaluated for their biocontrol ability against Fusarium culmorum, a serious pathogenic fungus of cereals crops related to damping-off and seedling blight resulting in yield loss. Barley seeds were chosen as cereal plant model. Surface bacterized seeds with TW3, RI3 and TW2 strains expressed the highest performances and permit to reduce significantly both the disease occurrence on seedlings (62-76%) and the extent of seedling blight symptoms (over than 95%). However, a negative effect on plant establishment was observed for RI3 treatment. SIGNIFICANCE AND IMPACT OF THE STUDY: The genus Fusarium is considered to be one of the most problematic phytopathogenic fungi for crop culture worldwide. Inside this genus, F. culmorum is the aetiological agent of seedling blight in various monocotyledonous plants such as barley and cause extensive yield and quality losses in humid and semi-humid regions. Biological control may be a successful alternative to chemical control, particularly with the controversy surrounding the use of the fungicides and the limited obtained results to control F. culmorum. This study highlights the effectiveness of some antagonistic Streptomyces isolated from Algerian Saharan soils to control F. culmorum by the reduction in disease occurrence and disease severity suggesting their use on microbial biocontrol formulation against soilborne diseases.

In vitro and in vivo antagonism of actinomycetes isolated from Moroccan rhizospherical soils against Sclerotium rolfsii: a causal agent of root rot on sugar beet (Beta vulgaris L.).

AIMS: To evaluate the ability of the isolated actinomycetes to inhibit in vitro plant pathogenic fungi and the efficacy of promising antagonistic isolates to reduce in vivo the incidence of root rot induced by Sclerotium rolfsii on sugar beet. METHODS AND RESULTS: Actinomycetes isolated from rhizosphere soil of sugar beet were screened for antagonistic activity against a number of plant pathogens, including S. rolfsii. Ten actinomycetes out of 195 screened in vitro were strongly inhibitory to S. rolfsii. These isolates were subsequently tested for their ability to inhibit sclerotial germination and hyphal growth of S. roflsii. The most important inhibitions were obtained by the culture filtrate from the isolates J-2 and B-11, including 100% inhibition of sclerotial germination and 80% inhibition of hyphal growth. These two isolates (J-2 and B-11) were then screened for their ability to protect sugar beet against infection of S. rolfsii induced root rot in a pot trial. The treatment of S. rolfsii infested soil with a biomass and culture filtrate mixture of the selected antagonists reduced significantly (P < or = 0.05) the incidence of root rot on sugar beet. Isolate J-2 was most effective and allowed a high fresh weight of sugar beet roots to be obtained. Both antagonists J-2 and B-11 were classified as belonging to the genus Streptomyces species through morphological and chemical characteristics as well as 16S rDNA analysis. CONCLUSION: Streptomyces isolates J-2 and B-11 showed a potential for controlling root rot on sugar beet and could be useful in integrated control against diverse soil borne plant pathogens. SIGNIFICANCE AND IMPACT OF THE STUDY: This investigation showed the role, which actinomycete bacteria can play to control root rot caused by S. rolfsii, in the objective to reduce treatments with chemical fungicides.

An early Ca2+ influx is a prerequisite to thaxtomin A-induced cell death in Arabidopsis thaliana cells.

The pathogenicity of various Streptomyces scabies isolates involved in potato scab disease was correlated with the production of thaxtomin A. Since calcium is known as an essential second messenger associated with pathogen-induced plant responses and cell death, it was investigated whether thaxtomin A could induce a Ca2+ influx related to cell death and to other putative plant responses using Arabidopsis thaliana suspension cells, which is a convenient model to study plant-microbe interactions. A. thaliana cells were treated with micromolar concentrations of thaxtomin A. Cell death was quantified and ion flux variations were analysed from electrophysiological measurements with the apoaequorin Ca2+ reporter protein and by external pH measurement. Involvement of anion and calcium channels in signal transduction leading to programmed cell death was determined by using specific inhibitors. These data suggest that this toxin induces a rapid Ca2+ influx and cell death in A. thaliana cell suspensions. Moreover, these data provide strong evidence that the Ca2+ influx induced by thaxtomin A is necessary to achieve this cell death and is a prerequisite to early thaxtomin A-induced responses: anion current increase, alkalization of the external medium, and the expression of PAL1 coding for a key enzyme of the phenylpropanoid pathway.