Schematic of the Pf4 two-way trafficking and defect rescue of Pf4 cheaters within P. aeruginosa biofilm; P. aeruginosa PAO1 cells continually extrude Pf4 filamentous phages in biofilm-related conditions; there is reciprocal exchange of Pf4 between P. aeruginosa cells in a lysis-independent and pili-dependent manner; the zoomed-in area depicts the molecular details when wild-type Pf4 is transferred into cells with a defective Pf4 and repair the Pf4 functions; the blue arrows highlight the different forms of Pf4 variants detected in this study, while the gray arrows represent mechanisms previously determined; OM, outer membrane; IM, inner membrane; T4P, type IV pili; IF, infection form; RF, replication form; pV, ssDNA binding protein PA0720.