PTGER4 and infection: However, the EP4 agonist did not cause a decrease in intracellular Y. pseudotuberculosis that is deficient in YopJ activity after 24 h, suggesting that EP4 agonist only makes a significant difference to bacterial clearance if YopJ is present in the cell, confirming our hypothesis that YopJ-mediated changes to PGE2 biosynthesis have a physiological outcome on infection.