Einflu� von Eisen und Sauerstoff auf die Pigmentbildung bei verschiedenen Pseudomonas-Spezies

The Potential for Redox-Active Metabolites To Enhance or Unlock Anaerobic Survival Metabolisms in Aerobes

Classifying microorganisms as "obligate" aerobes has colloquially implied death without air, leading to the erroneous assumption that, without oxygen, they are unable to survive. However, over the past few decades, more than a few obligate aerobes have been found to possess anaerobic energy conservation strategies that sustain metabolic activity in the absence of growth or at very low growth rates. Similarly, studies emphasizing the aerobic prowess of certain facultative aerobes have sometimes led to underrecognition of their anaerobic capabilities. Yet an inescapable consequence of the affinity both obligate and facultative aerobes have for oxygen is that the metabolism of these organisms may drive this substrate to scarcity, making anoxic survival an essential skill. To illustrate this, we highlight the importance of anaerobic survival strategies for Pseudomonas aeruginosa and Streptomyces coelicolor, representative facultative and obligate aerobes, respectively. Included among these strategies, we describe a role for redox-active secondary metabolites (RAMs), such as phenazines made by P. aeruginosa, in enhancing substrate-level phosphorylation. Importantly, RAMs are made by diverse bacteria, often during stationary phase in the absence of oxygen, and can sustain anoxic survival. We present a hypothesis for how RAMs may enhance or even unlock energy conservation pathways that facilitate the anaerobic survival of both RAM producers and nonproducers.

Recent developments in the isolation, biological function, biosynthesis, and synthesis of phenazine natural products

Phenazines are natural products which are produced by bacteria or by archaeal Methanosarcina species. The tricyclic ring system enables redox processes, which producing organisms use for oxidation of NADH or for the generation of reactive oxygen species (ROS), giving them advantages over other microorganisms. In this review we summarize the progress in the field since 2005 regarding the isolation of new phenazine natural products, new insights in their biological function, and particularly the now almost completely understood biosynthesis. The review is complemented by a description of new synthetic methods and total syntheses of phenazines.

Redox reactions of phenazine antibiotics with ferric (hydr)oxides and molecular oxygen

Phenazines are small redox-active molecules produced by a variety of bacteria. Beyond merely serving as antibiotics, recent studies suggest that phenazines play important physiological roles, including one in iron acquisition. Here we characterize the ability of four electrochemically reduced natural phenazines--pyocyanin (PYO), phenazine-1-carboxylate (PCA), phenazine-1-carboxamide, and 1-hydroxyphenazine (1-OHPHZ)--to reductively dissolve ferrihydrite and hematite in the pH range 5-8. Generally, the reaction rate is higher for a phenazine with a lower reduction potential, with the reaction between PYO and ferrihydrite at pH 5 being an exception; the rate decreases as the pH increases; the rate is higher for poorly crystalline ferrihydrite than for highly crystalline hematite. Ferric (hydr)oxide reduction by reduced phenazines can potentially be inhibited by oxygen, where O2 competes with Fe(III) as the final oxidant The reactivity of reduced phenazines with 02 decreases in the order: PYO > 1-OHPHZ > PCA. Strikingly, reduced PYO,which isthe least reactive phenazine with ferrihydrite and hematite at pH 7, is the most reactive phenazine with O2. These results imply that different phenazines may perform different functions in environments with gradients of iron and O2.

Phenazines and other redox-active antibiotics promote microbial mineral reduction

Natural products with important therapeutic properties are known to be produced by a variety of soil bacteria, yet the ecological function of these compounds is not well understood. Here we show that phenazines and other redox-active antibiotics can promote microbial mineral reduction. Pseudomonas chlororaphis PCL1391, a root isolate that produces phenazine-1-carboxamide (PCN), is able to reductively dissolve poorly crystalline iron and manganese oxides, whereas a strain carrying a mutation in one of the phenazine-biosynthetic genes (phzB) is not; the addition of purified PCN restores this ability to the mutant strain. The small amount of PCN produced relative to the large amount of ferric iron reduced in cultures of P. chlororaphis implies that PCN is recycled multiple times; moreover, poorly crystalline iron (hydr)oxide can be reduced abiotically by reduced PCN. This ability suggests that PCN functions as an electron shuttle rather than an iron chelator, a finding that is consistent with the observation that dissolved ferric iron is undetectable in culture fluids. Multiple phenazines and the glycopeptidic antibiotic bleomycin can also stimulate mineral reduction by the dissimilatory iron-reducing bacterium Shewanella oneidensis MR1. Because diverse bacterial strains that cannot grow on iron can reduce phenazines, and because thermodynamic calculations suggest that phenazines have lower redox potentials than those of poorly crystalline iron (hydr)oxides in a range of relevant environmental pH (5 to 9), we suggest that natural products like phenazines may promote microbial mineral reduction in the environment.

Phenazines and their role in biocontrol by Pseudomonas bacteria

Various rhizosphere bacteria are potential (micro)biological pesticides which are able to protect plants against diseases and improve plant yield. Knowledge of the molecular mechanisms that govern these beneficial plant-microbe interactions enables optimization, enhancement and identification of potential synergistic effects in plant protection. The production of antifungal metabolites, induction of systemic resistance, and the ability to compete efficiently with other resident rhizobacteria are considered to be important prerequisites for the optimal performance of biocontrol agents. Intriguing aspects in the molecular mechanisms of these processes have been discovered recently. Phenazines and phloroglucinols are major determinants of biological control of soilborne plant pathogens by various strains of fluorescent Pseudomonas spp. This review focuses on the current state of knowledge on biocontrol by phenazine-producing Pseudomonas strains and the action, biosynthesis, and regulation mechanisms of the production of microbial phenazines.

Isolation of Pseudomonas fluorescens producing phenazine derivatives exclusively under strains conditions of iron deficiency

We succeeded in isolating biotypes of Pseudomonas fluorescens forming phenazine derivatives exclusively under conditions iron deficiency. A culture medium containing traces of beryllium with glucose as the only carbon source allowed these microorganisms to grow under production of a brilliant red colour. Without beryllium the colonies were brown in colour. Spectroscopic investigations showed the red pigment to be a mixture of di- and trihydroxyphenazine carboxylic acid. UV/VIS measurements of the culture medium indicated the additional presence of ironcomplexing proferro-rosamines.

Role of pyocyanin in the acquisition of iron from transferrin

Pseudomonas aeruginosa produces a blue pigment called pyocyanin. In the presence of oxidizable substrates, bacteria reduce this pigment to a colorless product, leukopyocyanin. Pyocyanin can also be nonenzymatically reduced by NADH. Leukopyocyanin formed by cell- or NADH-mediated reduction nonenzymatically reduces oxygen or Fe(III). Pyocyanin-dependent iron reduction by whole bacterial cells was measured by the formation of the ferrous-ferrozine complex. In addition, leukopyocyanin reduced chelated Fe(III) including ferric iron in complex with transferrin, the serum iron-binding protein. High-pressure liquid chromatography was used to display the reductive removal of iron from transferrin and the accumulation of iron in the ferrous-ferrozine complex. Pyocyanin stimulated the accumulation of 55Fe from [55Fe]transferrin when it was added to bacteria incubated under low-oxygen conditions. Although bacteria grown in the presence of 100 microM FeCl3 reduced pyocyanin just as rapidly as iron-limited bacteria, these cells did not accumulate iron in the presence or absence of pyocyanin. Therefore, P. aeruginosa participates indiscriminantly in the reduction of pyocyanin, but soluble or available iron generated by the pyocyanin is taken up specifically by iron-limited bacteria.