Ralstonia insidiosa induces cell aggregation by Listeria monocytogenes

Authors: GOU, AILING - Huazhong Agricultural University; XU, YUNFENG - Northwest Agricultural & Forestry University; Mowery, Joseph; Nagy, Attila; Bauchan, Gary; Nou, Xiangwu

Submitted to: Food Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/5/2016
Publication Date: 3/7/2016
Citation: Gou, A., Xu, Y., Mowery, A.K., Nagy, A., Bauchan, G.R., Nou, X. 2016. Ralstonia insidiosa induces cell aggregation by Listeria monocytogenes. Food Control. 67:303-309.

Interpretive Summary: The formation of biofilms (bacterial communities that adhere tenaciously to surfaces) on processing equipment is a major food safety concern. We have previously demonstrated that pathogenic bacteria (E. coli) can become incorporated into biofilms during their formation by common environmental non-pathogenic strains of Ralstonia insidiosa. In this study, we evaluated the interaction between R. insidiosa and Listeria monocytogenes, an important foodborne pathogen responsible for multiple fatalities. When grown in liquid culture, R. insidiosa cells were approximately equally divided between individual cells and cell aggregates while L. monocytogenes existed only as individual cells. However, when the two bacteria were cultured together, approx 90% of L. monocytogenes cells adhered to the R. insidiosa aggregates forming large dual species aggregates. Since aggregation is a prelude to biofilm formation, our observation suggests that the interaction between R. insidiosa and L. monocytogenes may be responsible for the contamination of processing equipment by L. monocytogenes. This finding furthers our understanding of the interactions in microbial communities. This information will be usefull to other scientists, the produce industry and regulatory agencies.

Technical Abstract: 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 the Ralstonia insidiosa, an environmental bacterial species frequently isolated from fresh produce facilities, may serve as a "bridging 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 microscopic examinations 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 bridging 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.