Characterization of the actinomycetes from the rhizosphere of a desert shrub, big sagebrush (Artemisia tridentata), focusing on their production of novel antifungal antibiotics and bioactive secondary metabolites

This dissertation describes investigations of the microbial ecology and antibiotic producing abilities of actinomycetes of a little studied desert habitat, the rhizospheres and soils associated with the desert shrub Big Sagebrush (Artemisia tridentata). The increase in the incidence of antibiotic resistance by pathogenic microbes, both in medicine and agriculture, has created a great need for the discovery of novel antibiotics. Novel bacteria from little explored environments are one such source of new antibiotics. One such habitat is the rhizosphere of Big Sagebrush. Based upon preliminary data accumulated in our laboratory, we hypothesized that this environment could be a reservoir of novel antibiotic producing actinomycetes highly adapted to that environment and producing a broad spectrum of antimicrobial bioactive secondary metabolites. The present studies confirmed our hypothesis that the rhizosphere and nearby soils associated with Big Sagebrush populations are a prolific source of actinomycetes, particularly Streptomyces species that produce broad spectrum antifungal compounds.; There were no significant differences between the numbers of culturable actinomycete and nonactinomycete bacteria between the rhizosphere and nearby bulk soils. However, using the molecular ecology technique denaturing gel electrophoresis to examine banding patterns of 16S rRNA genes from the soil microbial populations, we observed that there was a qualitative difference in the populations between the rhizosphere and bulk soils. There was also an abundance of endospore formers in these desert soils. The actinomycetes were well adapted to this oligotropic environment, as evidenced by their physiological and antibiotic producing abilities. Their physiological diversity as saprophytic bacteria was great, and their antibiotic-production targeted their primary competitors (fungi and gram-positive in this dry, nutrient poor habitat).; These actinomycetes also appear to have a mutualistic relationship with sagebrush roots, a relationship we observed via root colonization studies. We hypothesize that the actinomycetes receive nutrients from the plant while in return protecting the plants from invasive fungal pathogens and aiding the plant to assimilate soil nutrients. (Abstract shortened by UMI.)...