Improved recovery of Erwinia amylovora-stressed cells from pome fruit on RESC, a simple, rapid and differential medium

The bacterium Erwinia amylovora causes fire blight, a serious and widespread disease of several pome fruit and ornamental plants. The use of suitable detection tools is essential for preventing its dissemination and, according to the protocol of the European and Mediterranean Plant Protection Organization, the isolation and further identification of E. amylovora is the only conclusive test of its presence. However, bacterial growth on solid media can be hampered when the pathogen is suffering stressful conditions in pome fruit or in other habitats. Since copper is an essential micronutrient that, in E. amylovora, also increases the exopolysaccharide production in rich-nutrient media, we have designed a non-selective differential medium containing 1.5 mM CuSO₄ to improve the recovery of E. amylovora from plants under unfavorable conditions. In this new medium named Recovery Erwinia amylovora-Stressed Cells (RESC), its colonies were easily distinguished by a light yellow color and a high mucus production. The plating recovery of several E. amylovora strains in vitro and from naturally infected samples was significantly improved with respect to other media routinely employed, particularly when the pathogen was suffering stressful conditions. Thus, the recovery of stressed E. amylovora cells (after UV irradiation, nutrient deprivation, or the presence of copper ions in non-copper-complexing media) was significantly enhanced on RESC medium, and their culturability period extended. Therefore, RESC is a useful and valuable medium for the isolation of E. amylovora when adverse conditions in the natural environment are expected.