MECHANISMS OF PANTOEA AGGLOMERANS STRAIN E325 AS ANTAGONIST OF ERWINIA AMYLOVORA

Pantoea agglomerans strain E325, the active ingredient in Bloomtime Biological^TM, was originally isolated from apple blossoms and selected based on broad screening with detached crab apple flowers. To evaluate pH modification on 'Gala' apple stigmas as a possible mode of antagonism, exudates were extracted from flower stigmas and tested with a pH electrode. Measurement of pH in field samples resulted in only slight differences but indicated a low pH range (between 5 and 6) conducive for antibiotic activity based on laboratory assays. An extracellular product of E325 was shown under low-phosphate and low-pH conditions to inhibit Erwinia amylovora but not strains of 17 other microbial species tested so far. A minimum of 20 to 40 ng of the compound, purified using RP-HPLC, caused visible inhibition of the pathogen in plate assays. Inhibition was relatively heat stable and unaffected by amino acids, ferric ions or enzymes previously used to characterize antibiotics from other strains of P. agglomerans. Conversely, activity was critically affected by pH and phosphate buffering capacity. It was deactivated under basic conditions, and at pH 6 and 7, it diminished or disappeared at a rate that increased with the phosphate buffer concentration. The inhibitory activity was often undetectable at phosphate concentrations commonly used in tests for antibiosis. Work is in progress to develop methodology for direct evaluation of pH on flower stigmas and to further characterize the inhibitory compound produced by strain E325.