PF4 and infection: PAO1-K is resistant to re-infection by Pf4, while Pf4 infection provokes lysis in the ΔPf4-K mutant; therefore, the presence of Pf4 in P. aeruginosa culture supernatants or biofilms can be in principle assayed by testing their ability to induce plaque formation when spotted onto soft-agar lawns of P. aeruginosa ΔPf4-K, but not of P. aeruginosa PAO1-K (Rice et al., 2009).