Salmonella typhi plasmid pR_ST98 was conjugated into Salmonella typhimurium and Escherichia coli to form Salmonella typhimurium/pR_ST98 and Escherichia coli/pR_ST98. These bacteria were cultured under suitable conditions to form biofilm. The planktonic bacteria were prepared. Biofilm bacteria and planktonic bacteria were adjusted to the same concentration then carried on adhesion and invasion experiments with CT26,anti-serum killing experiments and anti-macrophage phagocytosis experiments to compare virulence in different states.

Salmonella typhi plasmid pR_ST98 can advance biofilm formation of Salmonella typhimurium and Escherichia coli, and biofilm promoted all bacterium adhesion capabilities to CT26 except for Escherichia coli. The biofilm formation of Salmonella typhimurium and Escherichia coli decreased invasion capabilities to CT26 . The survival rates of Salmonella typhimurium and Escherichia coli in biofilm were higher than the bacteria in planktonic states for rabbit serum and guinea pig serum. Salmonella typhimurium survival rate within macrophages in biofilm state was significantly higher than that under planktonic state.

Plasmid pR_ST98 can promote the biofilm formation of Salmonella typhimurium and Escherichia coli. Biofilm formation promotes adhesion to cells while inhibiting invasion capability,and enhances anti-serum killing and anti-macrophage phagocytosis.