Virulence Associated Morphology of Different Strains of Melissococcus plutonius, a Brood Pathogen of Honey Bees

The causative agent of European foulbrood, Melissococcus plutonius, kills young larvae of honey bees. Although, several clonal complexes have been studied, the specific role of bacterial phenotype regarding bacterial morphology and other physiological parameters are less well understood. Insights from prominent bacterial pathogens (e.g., Enterococcus faecalis) have revealed that features like chain formation and separation play a critical role in the infection process and the course of bacterial diseases. Here, we explored the diversity in cell morphology and growth behaviour of three M. plutonius strains employing electron microscopy of larvae faeces and cultured bacteria, light microscopy on cultured bacteria and expression analysis of glycan-binding protein candidate genes. We found cell morphological differences of strains from the digestive tract of infected larvae and potentially associated expressional differences of several candidate genes. A growth assay in culture medium resulted in distinct chain length profiles of tested strains. An experimental transfer of supernatant from the highly virulent (49.3) to a less virulent strain (119) consistently led to cell separation suggesting a regulatory compound released from the highly virulent strain. Our results illustrate the mechanism of cell separation and the formation of individual cocci and short chains, potentially shedding light on the underlying mechanisms.