Environmental factors modulating antibiotic and siderophore biosynthesis by Pseudomonas fluorescens biocontrol strains

Understanding the environmental factors that regulate the biosynthesis of a ntimicrobial compounds by disease-suppressive strains of Pseudomonas fluore scens is an essential step toward improving the level and reliability of th eir biocontrol activity. We used liquid culture assays to identify several minerals and carbon sources which had a differential influence on the produ ction of the antibiotics 2,4-diacetylphloroglucinol (PHL), pyoluteorin (PLT ), and pyrrolnitrin and the siderophores salicylic acid and pyochelin by th e 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 b y the same factors as PHL. Production of PLT was stimulated by Zn2+, Co2+, and glycerol but was repressed by glucose. Pyrrolnitrin production was incr eased by fructose, mannitol, and a mixture of Zn2+ and NH4Mo2+. Pyochelin p roduction was increased by Co2+ fructose, mannitol, and glucose. Interestin gly, production of its precursor salicylic acid was increased by different factors, i.e., NH4Mo2+ glycerol, and glucose. The mixture of Zn2+ and NH4Mo 2+ with fructose, mannitol, or glycerol further enhanced the production of PHL and PLT compared with either the minerals or the carbon sources used al one, 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 ecolo gically and genetically diverse collection of 41 P. fluorescens biocontrol strains, the effect of certain factors was strain dependent, while others h ad a general effect. Stimulation of PHL by Zn2+ and glucose was strain depe ndent, whereas PLT production by all strains that can produce this compound was stimulated by Zn2+ and transiently repressed by glucose. Inorganic pho sphate reduced PHL production by CHA0 and seven other strains tested but to various degrees. Production of PLT but not pyrrolnitrin by CHA0 was also r educed by 100 mM phosphate. The use of 1/10-strength nutrient broth-yeast e xtract, compared with standard nutrient broth-yeast extract, amended with g lucose 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 metabo lic flow. The results of this study (i) provide insight into the biosynthet ic regulation of antimicrobial compounds, (ii) limit the number of factors for intensive study in situ, and (iii) indicate factors that can be manipul ated to improve bacterial inoculants.