A novel epoxide antibiotic isolated from Pantoea agglomerans 48b/90 inhibits economically important plant pathogens and the human pathogen Candida albicans Microbial pathogens pose a major threat to many plants and can cause enormous losses in agriculture. Microorganisms that antagonize pathogens can offer a way to fight plant diseases that is more environmentally friendly than chemical treatment; such diseases include fire blight, which is caused by Erwinia amylovora and affects many rosaceous plants, e.g. apple and pear.Suitable strains for biocontrol agents are often plant-associated microorganisms that are forced under natural conditions to defend their ecological niches and thus adapted to compete with plant pathogens. The species Pantoea agglomerans (formerly Erwinia herbicola) comprises many strains that are promising sources for biocontrol agents. P. agglomerans are ubiquitous in nature, inhabiting plant surfaces, water, soil, animals and humans. Several Pantoea isolates are known to efficiently inhibit E. amylovora in planta. In vitro experiments have revealed some antibiotics from P. agglomerans and uncovered how they act against E. amylovora. The known antibiotics produced by P. agglomerans strains, which belong to diverse chemical classes and affect different molecular targets, exhibit both narrow and broad spectrum activity.P. agglomerans 48b/90 (Pa48b), an epiphyte from soybean leaves, attracted our attention because it strongly inhibits the growth of plant pathogens E. amylovora and Pseudomonas syringae pv. glycinea (Psg), as well as the opportunistic fungal pathogen Candida albicans. Since the mode of action of Pa48b against plant and human pathogens, is elusive, we looked for the molecular basis for the biocontrol potential of Pa48b.The epiphyte Pa48b has been isolated from soybean leaves and found to be well adapted to its niche. Pa48b produces an antibiotic with broad activity against Gram-negative bacteria e.g. Erwinia amylovora, Agrobacterium tumefaciens, Escherichia coli, several Pseudomonas syringae pathovars, Serratia marcescens, the Gram-positive Bacillus subtilis and the yeasts Candida albicans and Yarrowia lipolytica. Consequently, Pa48b is a promising biocontrol agent against various microbial plant diseases and offering possibilities of therapeutic intervention directed against Candida albicans, the causative agent of invasive mycoses that increased significantly over the past two decades. In order to characterize the compound with high activity against plant pathogens and Candida albicans, a bioassay-guided isolation approach was used. A highly polar antibiotic was obtained after anion exchange chromatography and HILIC-HPLC purification. The purified antibiotic turned out to be stable at extreme pH; in addition, it was resistant to heat and treatment with proteinase K and ?-lactamase. Its formation is associated with growth and it is temperature dependent: its rate of production is optimal between 8 °C and 12 °C.Using HR-ESI-MS and NMR experiments, the structure of the compound was identified as 2-amino-3-(oxirane-2,3-dicarboxamido)-propanoyl-valine (1). This compound has already been isolated by Shoji et al. (1989) from Serratia plymuthica CB-25. However, 1 has been neither isolated from P. agglomerans nor characterized as highly active against plant pathogens such as E. amylovora and P. syringae pathovars. In contrast to the impact of many antibiotics from P. agglomerans such as pantocin A and B or herbicolin O, the impact of 2-amino-3-(oxirane-2,3-dicarboxamido)-propanoyl-valine cannot be compensated for by supplementing the medium with amino acids or casein hydrolysate. Therefore 1 is different than most other antibiotics from P. agglomerans strains.