PINK ERWINIA AMYLOVORA: COLONY DISCOLORATION IN DIAGNOSTIC ISOLATIONS BY CO-CULTURED BACTERIA

Accurate diagnosis of fire blight from plant samples relies on characteristic colony appearance of Erwinia amylovora on isolation media. We found that diagnosis is occasionally hindered due to a pink coloration of pathogen colonies on plates with mixed cultures. When these pink colonies were streaked to purity, E. amylovora reverted to typical white mucoid colonies on King's medium B. However, E. amylovora rapidly develops a vivid pink color when streaked adjacent to, but not in contact with, phyllosphere epiphytes Erwinia persicina or Erwinia rhapontici. These bacteria had no inhibitory activity against E. amylovora in vitro. Laboratory assays with E. persicina indicate that it produces a non-volatile, color-changing metabolite, which is accumulated by E. amylovora. Cell-free extracts from E. persicina cultures caused an identical pink coloration in E. amylovora. The colorless extracts developed an intense pink color upon addition of iron. The absorption spectrum of the ferrated extract had a major peak at 556 nm and a shoulder at 510 nm characteristic of the iron (II) chelator proferrorosamine (pFR). Diffusion of pFR in agar from E. persicina was visible only in media containing iron; otherwise the pink coloration was only seen in colonies. Our findings demonstrate that although E. persicina or E. rhapontici do not inhibit growth of E. amylovora in vitro, the pathogen’s absorption of the pFR they secrete, poses a concern in diagnostics because atypical colony-color can lead to pathogen misidentification.