Synergistic interactions in mixed biofilms

Mixed-species biofilms are more resistant against abiotic and biotic stress factors compared to single-species biofilms. However, the mechanisms underlying this common observation remain mostly elusive. Therefore, we studied biofilm formation of well-known opportunistic pathogens (Acinetobacter baumanii, Enterococcus faecium, Escherichia coli, Staphylococcus haemolyticus and Stenotrophomonas maltophilia) in various mechanistic approaches. Synergistic effects among different species in mixed biofilms were observed; especially when they were exposed to different stresses. By implementing a metatranscriptomic approach, changes in the global gene expression of the involved members became evident, and provided explanations for the improved survivability under nutrient limitation and exposure to disinfectants. Specific genes were linked to synergism in the mixed biofilm, i.e. those encoding proteins for vitamin B6 synthesis and iron uptake under nutrient-limited conditions. Furthermore, our study indicates that sub-lethal concentrations of an alcohol-based disinfectant significantly enhance biofilm yields of pathogenic bacteria, when they occur in mixed-species assemblages. A reduction of the dominant bacterial taxa in the mixed-species biofilm due to disinfectant pressure allowed minor taxa to bloom. The findings underline the importance of minor but antimicrobial-resistant species that serve as “protectors” for the whole assemblage due to upregulation of complementary genes involved in defence mechanisms and biofilm formation. This ultimately results in an increase in the overall yield of the mixed-species biofilm. We conclude that inter-species interactions may be crucial for the survival of certain opportunistic pathogens; especially under conditions that are typically found under hospital settings.