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Applied and Environmental Microbiology, June 1999, p. 2429-2438, Vol. 65, No. 6
Phytopathology Group, Institute of Plant
Sciences, Swiss Federal Institute of Technology, CH-8092
Zürich, Switzerland
Received 5 August 1998/Accepted 22 March 1999
Understanding the environmental factors that regulate the
biosynthesis of antimicrobial compounds by disease-suppressive strains of Pseudomonas fluorescens is an essential step toward
improving the level and reliability of their biocontrol activity. We
used liquid culture assays to identify several minerals and carbon sources which had a differential influence on the production of the
antibiotics 2,4-diacetylphloroglucinol (PHL), pyoluteorin (PLT), and
pyrrolnitrin and the siderophores salicylic acid and pyochelin by the
model strain CHA0, which was isolated from a natural
disease-suppressive soil in Switzerland. Production of PHL was
stimulated by Zn2+, NH4Mo2+, and
glucose; the precursor compound mono-acetylphloroglucinol was
stimulated by the same factors as PHL. Production of PLT was stimulated
by Zn2+, Co2+, and glycerol but was repressed
by glucose. Pyrrolnitrin production was increased by fructose,
mannitol, and a mixture of Zn2+ and
NH4Mo2+. Pyochelin production was increased by
Co2+, fructose, mannitol, and glucose. Interestingly,
production of its precursor salicylic acid was increased by different
factors, i.e., NH4Mo2+, glycerol, and glucose.
The mixture of Zn2+ and NH4Mo2+
with fructose, mannitol, or glycerol further enhanced the production of
PHL and PLT compared with either the minerals or the carbon sources
used alone, but it did not improve siderophore production. Extending
fermentation time from 2 to 5 days increased the accumulation of PLT,
pyrrolnitrin, and pyochelin but not of PHL. When findings with CHA0
were extended to an ecologically and genetically diverse collection of
41 P. fluorescens biocontrol strains, the effect of certain
factors was strain dependent, while others had a general effect.
Stimulation of PHL by Zn2+ and glucose was strain
dependent, whereas PLT production by all strains that can produce this
compound was stimulated by Zn2+ and transiently repressed
by glucose. Inorganic phosphate reduced PHL production by CHA0 and
seven other strains tested but to various degrees. Production of PLT
but not pyrrolnitrin by CHA0 was also reduced by 100 mM phosphate. The
use of 1/10-strength nutrient broth-yeast extract, compared with
standard nutrient broth-yeast extract, amended with glucose and/or
glycerol resulted in dramatically increased accumulations of PHL (but
not PLT), pyochelin, and salicylic acid, indicating that the ratio of
carbon source to nutrient concentration played a key role in the
metabolic flow. The results of this study (i) provide insight into the
biosynthetic regulation of antimicrobial compounds, (ii) limit the
number of factors for intensive study in situ, and (iii) indicate
factors that can be manipulated to improve bacterial inoculants.
0099-2240/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Environmental Factors Modulating Antibiotic and
Siderophore Biosynthesis by Pseudomonas fluorescens
Biocontrol Strains
*
Corresponding author. Mailing address: Phytopathology
Group, Institute of Plant Sciences, Swiss Federal Institute of
Technology, Universitätstrasse 2, CH-8092 Zürich,
Switzerland. Phone: 411-632-4836. Fax: 411-632-1108. E-mail:
brion.duffy{at}ipw.agrl.ethz.ch.
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