Impact of different culture media and drying treatment on the isolation of <i>Melissococcus plutonius</i>, the causative agent of European foulbrood, from honey bee larval specimens

Melissococcus plutonius, the bacterium causing European foulbrood in honey bees, includes two distinct types: typical strains, which are fastidious and not easy to culture, and atypical strains, which grow more robustly and tolerate higher sodium levels. Several culture media have been used to isolate this pathogen, and growth dynamics and survival vary according to strain type and medium. The most commonly used media differs across countries, as does the ratio of typical to atypical strains isolated, implying that the choice of medium influences the type of M. plutonius that is more likely to be recovered. This study examined how two commonly used media, Basal and KSBHI, affect the isolation of typical and atypical strains from larval samples co-infected with both types. Both media supported the growth of both strains, and no isolation preference was observed, although KSBHI tended to yield more non-M. plutonius bacteria. Applying a drying treatment to larval smears reduced these secondary invaders but it shifted the isolation balance toward atypical strains. Drying resistance tests revealed that atypical strains survived longer under dry conditions than typical strains, which likely explains their increased recovery after drying. These results indicate that while both media can isolate either strain type, using both media in parallel is better for ensuring isolation of both types. Additionally, although drying reduces secondary invaders, it may bias the results toward atypical strains. These insights contribute to a more accurate diagnosis and epidemiological understanding of European foulbrood.