Purpose: This study was conducted to examine the potential factors that affected this synergistic interaction.
Methods: The ability of EcO157 attaching to solid surfaces was examined using tissue culture plates in the presence or absence of Ralstonia spp. A drip flow biofilm reactor was used to examine the development of dual-species biofilm over time. EcO157 strain EDL933 and mutants deficient in curli production were also tested for dual-species biofilm formation with Ralstonia spp. Cells in biofilms were enumerated by plating and biofilm structure was examined using confocal laser scanning microscopy (CLSM). Three replicates were conducted for each study.
Results: Within 4 h of inoculation, the presence of pre-formed Ralstonia spp. biofilms significantly enhanced the attachment of EcO157 for 0.83 log CFU/cm2 (P < 0.05) compared to that on a plain plate. This enhanced initial attachment of EcO157 was not observed when it was co-inoculated with Ralstonia spp., indicating the importance of Ralstonia spp. biofilm in this interaction. While co-culture significantly increased (P < 0.05) the incorporation of EcO157 in dual-species biofilms after 24 h, this increase was not observed when curli deficient strains (ΔcsgA and ΔcsgD) were used. Microscopic examination of dual-species biofilms showed a unique arrangement of the two strains, where EcO157 microcolones were often encapsulated by Ralstonia spp. on the solid substrate.
Significance: This study provided evidence that biofilm formation by native microflora facilitates the survival of foodborne pathogens such as EcO157 through a synergistic interaction.