Abstract: The interaction between Escherichia coli O157:H7 and the common food spoilage bacterium Pseudomonas spp. on their motility and biofilm-producing ability was investigated when cultured together. Furthermore, reverse transcription real-time quantitative polymerase chain reaction was used to analyze the changes of virulence genes (stx1, stx2, hly and eae) expression in representative strains of E. coli O157:H7 during mixed species biofilm formation. The results showed that the motility of the three tested strains of E. coli O157:H7 was lower than that of the four strains of Pseudomonas when they were cultured separately (P < 0.05). When co-cultured with E. coli O157:H7, the swimming motility of Pseudomonas was inhibited considerably. The selective culture results showed that the two species did not promote each other’s biofilm-producing ability when co-culturing for 72 h. Instead, biofilm formation by the four Pseudomonas strains was significantly inhibited by the three strains of E. coli O157:H7 (P < 0.05). Furthermore, it was found that the expression of four virulence genes of E. coli O157:H7 CICC21530 per unit volume in the mixed culture and per until area in the mixed biofilm were lower than that in the single culture and the single biofilm, respectively (P < 0.05), and the same was true for that per unit cell number, indicating that the expression of virulence genes in E. coli O157:H7 was inhibited by Pseudomonas during mixed culture and mixed biofilm formation. It was also found that the expression of the four virulence genes in the biofilm was higher than that in planktonic cells (P < 0.05), indicating that the virulence of the bacterial cells increased after biofilm formation. This study can provide a scientific basis for revealing the interaction between food-borne pathogens and spoilage bacteria during mixed biofilm formation and for risk assessment, prevention and control.

Key words: Escherichia coli O157:H7; Pseudomonas; mixed-species biofilm; interaction; virulence gene expression