C12-3-oxo-AHL measurement produced by of P. aeruginosa PAO1 and by lon protease mutant derivative P. aeruginosa 18577. C12-3-oxo-AHL was extracted from spent supernatants, AHL levels were measured with P. putida SM17 (prsaL220) with a volume of extract corresponding to an amount of 5 × 108 cfu as described in the Methods section. C12-3-oxo-AHL levels is proportional to β-galactosidase activity (Miller Units). Standard deviation bars are given on the mean value of three independent cultures. Differences between PAO1 and PAO1 ID18577, analysed using T-test for independent groups were not significant (t = 2.2; p = .06; df = 6;t(6) = 2.2). The same measurement was also performed using 1 μM of synthetic C12-3-oxo-AHL (obtained from P. Williams, University of Nottingham, UK).