Ralstonia insidiosa induces cell aggregation of Listeria monocytogenes

Biofilm formation is an important strategy for foodborne bacterial pathogens to survive in stressful environments such as fresh produce processing facilities. Bacterial cell aggregation strongly promotes the initiation of microcolonies and the formation of biofilms on abiological surfaces. We previously showed that Ralstonia insidiosa, an environmental bacterial species frequently isolated from fresh produce facilities, may serve as a “bridge bacterium” that strongly enhanced the incorporation of several foodborne bacterial pathogens into dual species biofilms. While the R. insidiosa strain exhibited moderate cell aggregation in liquid culture, co-culturing Listeria monocytogenes with R. insidiosa resulted in significant augmentation of cell aggregation. Electron microscopy indicated that L. monocytogenes cells were initially attracted to the R. insidiosa aggregates and formed large dual species aggregates that were predominately composed of L. monocytogenes cells. The predominant presence of L. monocytogenes in the dual species aggregates was also confirmed by differential plating. These findings suggest that bridge bacteria such as R. insidiosa play critical roles in the survival of foodborne bacterial pathogens, such as L. monocytogenes and Escherichia coli, by promoting multispecies biofilm formation. The implications of such bridge bacteria on food safety need to be further evaluated.