<i>P. aeruginosa</i> produces a small chemical inhibitor(s) in a cell density-dependent manner.

<p>(A) Virulence-inhibitory activity of strain PAO1 culture supernatants. The bacterial supernatants of different growth stages (OD₆₀₀) were used as co-culture mediums when A549 was infected against PAO1 (MOI 50). Control (CK) is the normal A549 culture without bacterial infection. (B) The physical properties of the putative virulence inhibitor(s). PAO1 supernatants (sup) at OD₆₀₀ = 1.6 were prepared as larger (>) and smaller (<) than 3 kD fractions respectively by using a Centricon-3 ultrafiltration device. A portion of PAO1 sup was boiled at 100°C for 10 min before bioassay. These sup preparations were used to prepare PAO1 inoculum and infect A549 cell lines as described above. The experiment was repeated three times and the data shown are the means of 3 replicates with standard deviation.