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Applied and Environmental Microbiology, February 2004, p. 921-928, Vol. 70, No. 2
0099-2240/04/$08.00+0     DOI: 10.1128/AEM.70.2.921-928.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Phenazines and Other Redox-Active Antibiotics Promote Microbial Mineral Reduction

Maria E. Hernandez,1 Andreas Kappler,2 and Dianne K. Newman2*

Department of Environmental Science and Engineering,1 Department of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 911252

Received 17 July 2003/ Accepted 3 November 2003

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.

* Corresponding author. Mailing address: Department of Geological and Planetary Sciences, Caltech, Mail Stop 100-23, Pasadena, CA 91125. Phone: (626) 395-6790. Fax: (626) 683-0621. E-mail: dkn{at}gps.caltech.edu.

Applied and Environmental Microbiology, February 2004, p. 921-928, Vol. 70, No. 2
0099-2240/04/$08.00+0     DOI: 10.1128/AEM.70.2.921-928.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.



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